Mineral Genesis - how minerals form and change
Introduction Environments Minerals Rocks Symbols Abundance References

The Rock Cycle

rock cycle

Introduction

This is a work in progress. Comments and contibutions (with sources) are welcome. Please mail to mingen91@mingen.hk.

The Rock Cycle

The molten magma in the Earth's mantle may cool and crystallise underground, forming plutonic igneous rocks, or it may be forced out of erupting volcanoes, then crystallise to form volcanic igneous rocks.
Either way, the igneous rocks may be eroded to form sediments which are subsequently compacted to form sedimentary rocks, or they may be heated and deformed by the pressure of overlying material to form metamorphic rocks, or they may be pushed back under the Earth's surface at converging plate boundaries, where they will be subjected to such high temperatures that they will become molten magma again.
The sedimentary rocks, formed by erosion and sedimentation, may themselves be eroded to sediment which is compacted to form new sedimentary rocks. Alternatively they may become buried so deeply that they undergo diagenesis or metamorphism, forming metamorphic rocks.
Metamorphic rocks may be formed regionally from igneous or sedimentary rocks; they also may be formed locally where intruding magma comes into contact with pre-existing rock. These are contact metamorphic rocks. All types may be eroded to form sedimentary rocks, or they may be subjected to such high temperatures that they melt into magma again, completing the cycle.

Mineral Formation

Minerals may form directly by crystallisation from molten magma, precipitation from magmatic fluids or sublimation from gases. They may also form from alteration of rocks, by weathering, metamorphic or hydrothermal processes.
When molten magma cools, minerals crystallise out in order of their melting points, highest first, as given by Bowen's Reaction Series, which has two branches.
If the magma is rich in iron and magnesium, but poor in silicon, then the order is olivine, then pyroxene, amphibole and biotite; this is the discontinuous branch. If the magma is rich in silicon but poor in iron and magnesium then there is a continuous series from calcium-rich plagioclase feldspar, anorthite, to sodium-rich plagioclase feldspar, albite. Thereafter the two branches combine, and orthoclase crystallises, followed by muscovite and finally quartz. These are the major rock- forming minerals, although other minerals may be present as minor constituents.
Excess silica SiO2 separates out as quartz, and excess alumina Al2O3 crystallises as corundum.
During crystallisation the more mafic parts of the magma are enriched in chromium, nickel, platinum and sometimes phosphorus. More silicic parts of the magma are enriched in tin, zirconium and thorium. Titanium and iron are found throughout the range of composition and are found in all types of igneous rocks.

Environments

Igneous environments excluding pegmatites and carbonatites

Igneous rocks are formed when magma cools. If it solidifies underground the resulting rocks are said to be plutonic. If the molten magma is emitted by a volcano, and subsequently solidifies, the rocks are volcanic.

Plutonic igneous environments

Bowen reaction series

Bowen reaction series

When molten magma cools, minerals crystallise out in order of their melting points, highest first, as given by Bowen's Reaction Series, which has two branches.
If the magma is rich in iron and magnesium, but poor in silicon, then the order is olivine, then pyroxene, amphibole and biotite; this is the discontinuous branch. If the magma is rich in silicon but poor in iron and magnesium then there is a continuous series from calcium-rich plagioclase feldspar, anorthite, to sodium-rich plagioclase feldspar, albite (variety oligoclase). Thereafter the two branches combine, and orthoclase (K-spar) crystallises, followed by muscovite and finally quartz. These are the major rock- forming minerals, although other minerals may be present as minor constituents.

Plutonic igneous rocks

Quartzolite

Quartzolite is a plutonic igneous rock that contains 90 - 100% quartz and 0 - 10% feldspars.
The essential constituent is quartz.
Common but not essential constituents include K-feldspars and plagioclase feldspars.

Granite

Granite is a medium- to coarse-grained silica-rich plutonic igneous rock, formed by crystallisation of a silica-rich magma in a major intrusion. It contains
80 - 100 % light coloured minerals, of which quartz is 20 - 60% and feldspars 40 - 80%. Of these feldspars 35 - 100% are K-feldspars and 0 - 65% plagioclase feldspars.
It also contains 0 - 20% dark minerals.
Essential constituents are quartz and K-feldspars (feldspars rich in potassium).
Common but not essential constituents include biotite, hornblende, muscovite and plagioclase feldspars (feldspars deficient in potassium).

Granodiorite

Granodiorite is a feldspar-rich plutonic igneous rock.
Major constituents are plagioclase feldspar, K-feldspar, quartz and mica.
Minor constituents are hornblende and augite. Oligoclase is a common constituent.
Granodiorite is the most abundant of the plutonic igneous rocks.

Syenite

Syenite is a coarse-grained plutonic igneous rock with intermediate silica content. It contains
60 - 100 % light coloured minerals of which feldspar is 80 - 100% and quartz 0 - 20% or feldspathoids 0 - 10%.
Of the feldspars, 65 - 100% are K-feldspars and 0 - 35% plagioclase feldspar feldspar (albite to anorthite).
It also contains 0 - 40% dark minerals.
The essential constituents are K-feldspars.
Common but not essential constituents include aenigmatite, amphibole, biotite, feldspathoids, hornblende, plagioclase feldspar, pyroxene, titanite and zircon (very common in nepheline syenite).

Nepheline Syenite

Nepheline syenite is a nepheline-rich syenite.
The essential constituents are K-feldspars and nepheline.
Common but not essential constituents include biotite and hornblende.

Monzonite

Monzonite is a plutonic igneous rock containing 0 to 5% quartz and 95 to 100% feldspar.
Essential constituents are K-feldspars and plagioclase feldspars.
Common but not essential minerals include amphibole and pyroxene.
Disseminated copper deposits are found frequently in monzonite.

Diorite

Diorite is a plutonic igneous rock with intermediate silica content. It contains
50 - 85% of light coloured minerals of which feldspars are 80 - 100% and quartz is 0 - 20% or feldspathoids 0 - 10%.
Of the feldspars, 65 - 100% are plagioclase feldspars and 0 - 35% are K-feldspars.
It also contains 15 - 50% dark minerals.
The essential constituent is plagioclase feldspar.
Common but not essential constituents include biotite, hornblende and quartz.

Gabbro

Gabbro is a silica-poor plutonic igneous rock. It contains
35 - 80% of light coloured minerals of which feldspars are 80 - 100% and quartz is 0 - 20% or feldspathoids 0 - 10%.
Of the feldspars, 65 - 100% are plagioclase feldspar feldspars and 0 - 35% are K-feldspars.
It also contains 20 - 65% dark minerals.
Essential constituents are plagioclase feldspars and dark minerals (mafic minerals) such as olivine and pyroxene.
Common but not essential constituents include biotite and hornblende.
Gabbro is most abundant at constructive plate margins, where tectonic plates move apart and magma wells up to fill the gap, and gabbro constitutes the lower portions of the oceanic crusts. It is also abundant in oceanic islands; these are islands without any foundation of continental rock, usually formed as the result of volcanic action.
Gabbro can also occur at destructive plate margins, where the tectonic plates are moving together, and in continental rifts, where the continental crust is thinning due to the underlying plates moving apart.
Nickel ores are associated with norites (orthopyroxene-dominated gabbros) and peridotite.

Anorthosite

Anorthosite is a plutonic igneous rock, formed by crystallisation of a silica-poor magma in a major intrusion. It contains at least 90% calcium-rich plagioclase feldspar. The remaining 10% is made up of olivine, garnet, pyroxene and iron oxides.
The essential constituent is plagioclase feldspar.
Common but not essential constituents include pyroxene.

Aplite

Aplite is a fine-grained granite consisting of only feldspar and quartz.
Essential constituents are K-feldspar, plagioclase feldspar and quartz.

Mafic and Ultramafic

Mafic rocks are rocks that are rich in dark minerals such as magnesium and iron compounds but deficient in quartz. Common rock-forming mafic minerals include olivine, pyroxene, and biotite.
Ultramafic rocks contain more than 90% mafic minerals.

Dunite

Dunite is an ultramafic plutonic igneous rock containing more than 90% of olivine.
The essential constituent is olivine.
Common but not essential constituents include magnetite and pyroxene.

Peridotite

Peridotite is an ultramafic plutonic igneous rock containing more than 40% of olivine.
The essential constituent is olivine.
Common but not essential constituents include pyroxene and chromite. Nickel ores are associated with norites (orthopyroxene-dominated gabbros) and peridotite.

Kimberlite

Kimberlite is an ultramafic igneous rock.
Essential constituents are carbonates such as calcite, together with olivine, phlogopite and pyroxene.
Common but not essential constituents include monticellite, perovskite and pyrope.

Volcanic igneous environments

Volcanic rocks develop when molten magma solidifies at the Earth's surface after eruption from a volcano. The chemical composition and mineral constitution of volcanic rocks are similar to those of the corresponding plutonic rocks.

Volcanic igneous rocks

Rhyolite

Rhyolite is a fine-grained feldspar-rich volcanic igneous rock, formed from a silica-rich magma. It contains
80 - 100% light minerals of which quartz is 20 - 60% and feldspars 40 - 80%.
Of the feldspars 35 - 100% are K-feldspars and 0 - 65% are plagioclase feldspars (albite to anorthite).
It also contains 0 - 20% dark minerals.
Essential constituents are K-feldspars, plagioclase feldspars and quartz.
Common but not essential constituents include amphibole, biotite and pyroxene. Rhyolite mainly occurs as lava domes (domes resulting from the slow extrusion of viscous lava from a volcano). Rhyolite, like granite, is most commonly associated with island arc (a chain of volcanic islands, parallel with and close to a boundary between two converging tectonic plates) and mountain-building magmatism.

Trachyte

Trachyte is a feldspar-rich volcanic igneous rock of intermediate silica content. It contains
60 - 100% light minerals of which feldspars are 80 - 100% and quartz 0 - 20% or feldspathoids 0 - 10%.
Of the feldspars 35 - 65% are K-feldspars and 35 - 65% are plagioclase feldspars (albite to anorthite).
It also contains 0 - 40 % dark minerals.
Essential constituents are albite variety oligoclase and sanidine.
Common but not essential constituents include biotite, hornblende, nepheline, pyroxene and quartz.
Trachyte is most commonly associated with ocean island (islands without any foundation of continental rock, usually formed as the result of volcanic action) and continental rift (where the continental crust is thinning due to the underlying plates moving apart) magmatism.

Andesite

Andesite is a volcanic igneous rock with intermediate silica content. It contains
60 - 85% light minerals of which 80 - 100% are feldspars and 0 - 20% are quartz or 0 - 10% are feldspathoids.
Of the feldspars 65 - 100% are plagioclase feldspars and 0 - 35% are K-feldspars.
It also contains 15 - 40% dark minerals.
Essential constituents are plagioclase feldspars.
Common but not essential constituents include biotite, hornblende, and pyroxenes.
Andesite occurs in lava flows together with basalt and trachyte.

Basalt

Basalt is a silica-poor volcanic igneous rock, formed from a silica-poor magma. It contains
30 - 60% light minerals of which feldspars are 80 - 100% and quartz 0 - 20% or feldspathoids 0 - 10%.
Of the feldspars 65 - 100% are plagioclase feldspars and 0 - 35% are K-feldspars.
It also contains 40 - 70% dark minerals.
Essential constituents are plagioclase feldspar and pyroxenes.
Common but not essential constituents include feldspathoids, olivine and quartz
In addition, many hydrothermal minerals are found filling cavities in basalt

Pegmatites

Pegmatites are particularly large-grained rocks that are formed in the final phase of crystallisation of plutonic rocks. Some portion of highly volatile components, especially water, is left over, and it contains all the elements that are too large or too small to be incorporated in the normal rock minerals (quartz, feldspar and mica). These are chiefly elements like lithium, beryllium, niobium, tantalum, cesium and rare earths.
Essential constituents are feldspar, mica and quartz.
Common but not essential constituents include albite, microcline, muscovite, topaz and tourmaline

Carbonatites

Carbonatites are igneous rocks that consist largely of the carbonate minerals calcite and dolomite; they sometimes also contain the rare-earth ore minerals parisite, and monazite, and the niobium ore mineral pyrochlore. The origin of carbonatite magma is obscure. Most carbonatites occur close to intrusions of mafic igneous rocks or to ultramafic igneous rocks such as kimberlite. Carbonatites occur in small plutonic bodies.
Carbonate minerals make up at least 50% of carbonatites.
Essential constituents are calcite and dolomite.
Common but not essential constituents include ankerite and siderite.

Sedimentary environments excluding placer deposits

Clastic sedimentary environments

Clastic sedimentary rocks form from at the earth's surface by oxidation, weathering, erosion and deposition. When granitic rocks weather at the earth's surface feldspar alters into clay minerals, particularly kaolinite, sometimes forming large deposits. Most sedimentary rocks are initially deposited as unconsolidated sediments. Consolidation sets in only gradually because of dewatering and/or because of the cementation with a binding material (clayey, calcareous or siliceous). Clastic sedimentary rocks are classified largely by grain size.

Clastic sedimentary rocks

Sandstone

Sandstone is a clastic sedimentary rock comprised mainly of sand-sized grains, between 0.0625 and 4 mm across. The grains can be quartz, feldspar or rock fragments. Sandstone is deposited by a wide range of processes including river and stream deposits (fluvial), floodplain or river delta deposits (alluvial), windborn deposits (aeolian) and under-water, sediment-laden currents (turbidity currents).
Essential constituents are feldspars and quartz.
A common constituent is calcite

Siltstone

Siltstone is a clastic sedimentary rock comprised mainly of grains sized 0.0039 to 0.0625 mm across.

Mudstone

Mudstone is a clastic sedimentary rock comprised mainly of grains sized less than 0.0039 mm across. Shale, clay and marl are types of mudstone. The majority of grains in mudstones are clay minerals such as montmorillonite and kaolinite.

Shale

Shale is a mudstone with a fissile parting. It is the most abundant clastic sedimentary rock in the Earth's crust, comprising about 70% of sedimentary rocks. It consists of a high percentage of clay minerals, substantial amounts of quartz and smaller amounts of carbonates, feldspar, fossils and organic matter. Shale is coloured red and purple by hematite and goethite, blue, green and black by ferrous iron, and grey or yellowish by calcite.
It is deposited by gentle currents on deep ocean floors, shallow sea basins and river floodplains.

Clay

Clay is a soft, cohesive, water-rich mudstone that is plastic when wet and hardens when fired. The majority of clays are largely composed of phyllosilicates, such as chlorite, kaolinite, mica, montmorillonite and muscovite variety illite.

Marl

Marl is a mudstone containing a great deal of carbonate.
The essential constituent is calcite.
Common but not essential constituents include dolomite and hematite.

Tuff

Tuff is a fine-grained pyroclastic rock, ie it is formed by the lithification of beds of volcanic ash and lava fragments. Such a rock may be classified as volcanic igneous, but here we group it with the sedimentary rocks.

Chemical sedimentary environments

Chemical sediments are created by the evaporation of sea water or saline lakes in arid regions. Typical minerals are halite, borax, colemanite and ulexite.

Chemical sedimentary rocks

Rocksalt

Rocksalt is the massive rock form of the mineral halite. It is a monomineralic chemical sedimentary rock.
The essential constituent is halite, and a common but not essential constituent is anhydrite.
Rock salt often occurs in salt domes. A salt dome is a structural dome formed when a thick bed of evaporite minerals found at depth intrudes vertically into surrounding rock strata. Salt domes contain anhydrite, gypsum, and native sulphur, in addition to halite and sylvite. Rocksalt forms from the evaporation of ocean or saline lake waters. It is rarely found at the Earth's surface, except where the climate is very arid.

Gypsum rock

Gypsum rock is a monomineralic chemical sedimentary rock with gypsum as its major constituent.
Common minor constituents are anhydrite, rocksalt, limestone and dolomite.
Gypsum rock originates by precipitation as sea water is evaporated or by the hydration of anhydrite.

Anhydrite rock

Anhydrite rock is a monomineralic rock with anhydrite as its major constituent.
Medium constituents are gypsum, calcite, dolomite, clay minerals and bitumen.
Anhydrite rock originates by precipitation from sea water or diagenetically from gypsum as a result of high temperature and thick overburden in mountain ranges.

Biogenic sedimentary environments

Biogenic sedimentary rocks form from remains of living creatures. Some rocks, such as chert, may form both as biogenic deposits and as chemical deposits.

Biogenic sedimentary rocks

Limestone

Limestone is a biogenic sedimentary rock formed in marine environments.
The essential constituent is calcite.
Common but not essential constituents include aragonite and dolomite.

Dolostone

Dolostone is a biogenic sedimentary rock rock formed in marine environments and consisting of more than 50% dolomite.
Most dolostone did not originally form as dolostone, but instead formed from the alteration of limestone as magnesium-rich water moved through it, altering its calcite and aragonite into dolomite. The replacement may be only partial, and most dolostone is a mixture of dolomite and calcite.
The main exception to this secondary dolostone is the rare primary dolostone that forms as a relatively late product of seawater evaporation.
The essential constituent is dolomite, almost always accompanied by calcite.
Common but not essential constituents include ankerite.

Diatomite

Diatomite is a biogenic sedimentary rock composed of diatom skeletons, consisting of about 90% opal. Diatoms are microscopic, single-celled algae that live in marine or fresh water, with skeletons made of silicon dioxide. The remaining 10% is made up of aluminium and iron oxides.

Chert

Chert is a chemical sedimentary rock consisting almost entirely of chalcedony; it may be biochemical, or formed by replacement.
Biochemical chert is formed when the siliceous skeletons of marine plankton are dissolved during diagenesis (rock formation), with silica being precipitated from the resulting solution.
Replacement chert, such as petrified wood, forms when other material is replaced by silica.
Chert occurs as nodules in limestone and dolostone as a replacement mineral. Flint is a variety of chert that occurs in chalk or marl. Agate is a type of chert that forms through direct precipitation of silica in voids within a rock. Chert also occurs in thin beds, when it is a primary deposit, and in beds and lenses of diatomite.
The essential constituent is chalcedony.
Common but not essential constituents include opal and quartz. dolomite and chert to talc and calcite
3CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Metamorphism of siliceous carbonate rocks causes the formation of hydrous phases such as talc and tremolite. This is a very low-grade metamorphic reaction occurring at temperature between about 150oC and 250oC.

Placer deposits

Placer deposits are accumulations of heavy and corrosion-resistant minerals in rivers and lakes. The minerals are formed elsewhere, and carried down by currents of water. Typical placer minerals are gold, platinum, garnet, ilmenite, rutile, and monazite.

Metamorphic and diagenetic environments

Changes which occur in solid rock in response to changes in pressure and temperature are categorised as diagenetic or metamorphic. There is no clear boundary between these two types of change. Processes at pressures below about 1 kbar and temperatures below about 100oC are usually categorised as diagenetic. These include changes which form sedimentary rocks from chemical or clastic sediments.
Processes at higher pressures and temperatures are usually categorised as metamorphic, and these processes proceed more quickly in the presence of circulating hydrothermal fluids, which may originate from an intruding magma, from regional metamorphic reactions, or from surface water.
Both igneous and sedimentary rocks are subject to metamorphism at sufficient temperature and pressure, and the resulting minerals are largely determined by the composition of the original, unaltered rock, called the protolith. Metamorphism can involve recrystallisation of existing minerals, the formation of new minerals through chemical reactions, and the transformation of a mineral from one polymorph to another.
Typical metamorphic minerals include andalusite, garnet, kyanite, magnetite and pyrrhotite.

Metamorphic facies

Metamorphic facies



Different minerals form from metamorphism at different temperatures and pressures. Low temperature and pressure is designated as low-grade metamorphism, and high temperature and pressure as high-grade metamorphism. The various ranges of conditions are called metamorphic facies, often named for the dominant mineral found there. The major facies, in order of increasing temperature within pressure band, are

Low Pressure

Medium Pressure

High Pressure

In the greenschist, amphibilite and granulite facies, with increasing temperature minerals firt appear in the order chlorite, biotite, garnet, staurolite, kyanite and sillimanite.

Zeolite facies

The zeolite facies is a low-grade metamorphic facies resulting from burial in thick sedimentary basins at temperature 50 to 270oC and pressure 0.3 to 7 kbar.
Zeolites are characteristic minerals.
Changes in mineralogy are most prominant in clay-rich, fine-grained clastic sediments (pelitic sediments), with transformation of clay minerals to muscovite variety illite, kaolinite and vermiculite.

Typical assemblages include:

Kaolinite and montmorillonite with laumontite, warakite, prehnite, calcite and chlorite,
and, in potassium-rich rocks,
muscovite variety illite and orthoclase variety adularia.

Generally plutonic and volcanic rocks are not affected by zeolite facies metamorphism, although vesicular basalt frequently has its vesicles filled with zeolite minerals.

Albite-epidote-hornfels facies

This is a low-grade metamorphic facies associated with contact metamorphism at temperature 250 to 500oC and pressure less than 2 kbar.
It often forms the outer portions of a thermal aureole (a region in country rock around an igneous intrusion that has experienced metamorphism due to heat from the body of magma).
Albite and epidote are characteristic minerals and hornfels is a characteristic rock.

Typical assemblages include:

Mafic protolith: actinolite, epidote, chlorite, biotite, talc and albite.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: quartz, albite, epidote, chlorite, muscovite, biotite and andalusite.
Quartzofeldspathic (rich in quartz and feldspar) protolith: microcline, albite, quartz, biotite and muscovite.
Calc-silicate protolith (calcitic and dolomitic limestones): calcite, epidote and tremolite.

Hornblende-hornfels facies

This is a medium-grade metamorphic facies associated with contact metamorphism at temperature 500 to 650oC and pressure less than 2.5 kbar.
The absence of epidote, chlorite and albite distinguishes rocks of this facies from those of the albite-epidote-hornfels facies.
Hornblende is a characteristic mineral and hornfels is a characteristic rock.

Typical assemblages include:

Mafic protolith: hornblende, tremolite, plagioclase feldspar, biotite, cordierite, anthophyllite, quartz and almandine.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: hornblende, tremolite, quartz, plagioclase feldspar, muscovite, biotite, andalusite and cordierite.
the presence of hornblende, and of muscovite in this assemblage distinguishes it from the pyroxene-hornfels facies. Biotite, cordierite and andalusite can occur as porphyroblasts, often rich in inclusions, but these minerals are not restricted to this facies.
Quartzofeldspathic (rich in quartz and feldspar) protolith:
microcline, quartz, biotite, muscovite and almandine.
Calc-silicate protolith (calcitic and dolomitic limestones): calcite, plagioclase feldspar, tremolite, grossular, diopside and quartz.

Pyroxene-hornfels facies

This is a high-grade metamorphic facies associated with contact metamorphism at temperature 650 to 800oC and pressure less than 2.5 kbar.
Rocks of this facies often form the innermost zone of a thermal aureole (a region in country rock around an igneous intrusion that has experienced metamorphism due to heat from the body of magma).
Pyroxenes are characteristic minerals and hornfels is a characteristic rock.

Typical assemblages include:

Mafic protolith: plagioclase feldspar, biotite, cordierite, orthopyroxene, quartz and diopside.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: quartz, plagioclase feldspar, microcline, andalusite or sillimanite and cordierite.
The absence of muscovite and hornblende and the presence of pyroxene in mafic rocks and sometimes in pelitic rocks distinguishes these assemblages from the hornblende-hornfels facies. In pelitic rocks biotite, cordierite and andalusite or sillimanite can occur as porphyroblasts, often rich in inclusions.
Quartzofeldspathic (rich in quartz and feldspar) protolith: microcline, quartz, biotite and plagioclase feldspar.
Calc-silicate protolith (calcitic and dolomitic limestones): calcite, plagioclase feldspar, grossular, diopside, quartz and wollastonite.

Sanidinite facies

This is a high-grade metamorphic facies associated with contact metamorphism at a temperature in excess of 800oC and pressure less than 2.5 kbar.
Sanidine is a characteristic mineral, and hydrous minerals such as biotite are absent.

Prehnite-Pumpellyite facies

This is a low-grade metamorphic facies typically resulting from subseafloor alteration at mid-ocean ridges, or from burial in thick sedimentary basins at temperature 250 to 350oC and pressure 2 to 7 kbar. A mid-ocean ridge is an underwater mountain range, formed at the boundary of two tectonic plates that are moving apart, so that magma wells up between them, forming the ridge.
Prehnite and pumpellyite are characteristic minerals and slate is a characteristic rock.

Typical assemblages include:

Mafic protolith: prehnite, pumpellyite, chlorite, albite and epidote; actinolite occurs at higher temperature and lawsonite at higher pressure.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: illite, chlorite, albite and stilpnomelane.
Quartzofeldspathic (rich in quartz and feldspar) protolith: albite, chlorite, pumpellyite, prehnite, stilpnomelane, muscovite, titanite, epidote and carbonate; actinolite replaces prehnite at higher temperature; lawsonite occurs at higher pressure.
Calc-silicate protolith (calcitic and dolomitic limestones): calcite, dolomite, prehnite, albite, chlorite and quartz.

Greenschist facies

This is a low-grade metamorphic facies associated with regional metamorphism at temperature 350 to 500oC and pressure 8 to 50 kbar.
Chlorite and biotite are characteristic greenschist facies minerals, and epidote is also common, but is not restricted to this facies.
Slate, phyllite and schist are characteristic greenschist facies rocks.

Typical assemblages include:

Mafic protolith: albite, chlorite, actinolite, epidote, titanite, quartz, muscovite and calcite; stilpnomelane occurs at low temperature and biotite and hornblende occur at higher temperature.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: muscovite, chlorite, albite, paragonite, graphite, rutile, carbonate, epidote, K-feldspar, titanite, stilpnomelane (low aluminium protoliths), pyrophyllite (high aluminium protoliths) and chloritoid (high-Al protoliths); biotite occurs at higher temperature; garnet occurs at the highest temperatures.
Quartzofeldspathic (rich in quartz and feldspar) protolith: albite, epidote, muscovite, chlorite, titanite, stilpnomelane, actinolite; biotite and garnet occur at higher temperatures.
Calc-silicate protolith (calcitic and dolomitic limestones): calcite, dolomite, graphite, quartz, muscovite, albite, K-feldspar, chlorite and zoisite.

Amphibolite facies

This is a medium grade metamorphic facies associated with regional metamorphism at temperature 500 to 750oC and pressure 8 to 70 kbar.
Garnet, staurolite, kyanite, hornblende and sillimanite are characteristic amphibolite facies minerals.
Amphibolite is the characteristic amphibolite facies rock, and schist and gneiss are also amphibolite facies rocks.

Typical assemblages include:

Mafic protolith: hornblende, oligoclase, epidote, almandine, titanite, quartz, chlorite and biotite.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: biotite, muscovite, oligoclase, almandine, cordierite (low pressure), andalusite (low pressure), kyanite (high pressure), sillimanite (moderate pressure and/or high temperature), staurolite (high temperature), graphite and titanite.
Quartzofeldspathic (rich in quartz and feldspar) protolith: oligoclase, K-feldspar, muscovite, biotite and hornblende.
Calc-silicate (calcitic and dolomitic limestones) protolith: calcite, dolomite, quartz, diopside, tremolite, forsterite, graphite, grossular, hornblende and clinozoisite.
Rocks of the amphibolite facies can undergo partial melting at 650 to 700oC in the presence of water resulting from dehydration reactions.

Granulite facies

This is a high grade metamorphic facies associated with regional metamorphism at temperature 500 to 750oC and pressure 8 to 70 kbar.
Kyanite and sillimanite are characteristic granulite facies minerals.
Granulite and gneiss are characteristic granulite facies rocks.
Amphibole and mica are both absent in granulite facies rocks, and hornblende dehydrates to form pyroxene and plagioclase feldspar. Kyanite and sillimanite are often produced from muscovite and biotite.

Typical assemblages include:

Mafic protolith: plagioclase feldspar, pyroxene, hornblende, olivine (low pressure), garnet and quartz.
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: microcline, plagioclase feldspar, scapolite, garnet, cordierite (low pressure), andalusite (low pressure), kyanite (high pressure), sillimanite (moderate to low pressure, high temperature), graphite, rutile, ilmenite, olivine, corundum, spinel and sapphirine (high temperature).
Quartzofeldspathic (rich in quartz and feldspar) protolith: microcline, plagioclase feldspar, garnet, pyroxene and hornblende.
Calc-silicate protolith (calcitic and dolomitic limestones): calcite, dolomite, quartz, diopside, scapolite, forsterite, wollastonite and graphite.

Blueschist facies

This is a low to medium grade metamorphic facies associated with subduction at temperature 200 to 500oC and pressure greater than 4 to 12 kbar (dependent on temperature).
Glaucophane and jadeite are characteristic blueschist minerals, and chlorite, epidote and kyanite are common.
Slate and schist are characteristic blueschist facies rocks.

Typical assemblages include:

Mafic protolith: glaucophane, lawsonite, jadeite (high pressure), actinolite or hornblende, albite (low pressure), titanite, pumpellyite, stilpnomelane, epidote and garnet.
Pelitic protolith: glaucophane, chlorite, muscovite, paragonite, lawsonite, epidote, kyanite, quartz (or coesite at very high pressure), chloritoid and titanite.
Quartzofeldspathic (rich in quartz and feldspar) protolith: jadeite (high pressure), lawsonite, muscovite, chlorite, titanite and glaucophane.
Calc-silicate protolith: calcite and aragonite.

Eclogite facies

This is a high-grade metamorphic facies associated with subduction at temperature in excess of 500oC and pressure greater than 12 kbar.
Omphacite and pyrope are characteristic eclogite facies minerals.
Eclogite facies rocks also occur as mantle xenoliths within mantle derived magma such as kimberlite.
At very high pressures eclogite facies rocks contain coesite, the high pressure polymorph of silica.

Typical assemblages include:

Mafic protolith: omphacite (clinopyroxene), pyrope-rich garnet, kyanite, rutile, quartz (low pressure) and coesite (high pressure).
Pelitic (clay-rich, fine-grained clastic sedimentary) protolith: omphacite-rich garnet, carpholite, muscovite, chloritoid, kyanite and quartz, or coesite at very high pressure.
Eclogite facies rocks often experience retrograde metamorphism (changes that occur during uplift and cooling of a rock) during exhumation to shallow depth.

Types of metamorphism

Contact metamorphism

Contact metamorphism results from high temperatures usually due to the proximity to a body of magma.

Contact metamorphic rocks

Hornfels

Hornfels is formed by contact metamorphism close to igneous intrusions at temperatures of 200 to 800oC and low to high pressure.
Common but not essential constituents include andalusite, cordierite, hornblende and plagioclase feldspar.
Hornfels is a characteristic rock of the hornblende-hornfels, albite-epidote-hornfels and pyroxene-hornfels facies.

Marble

Marble is formed by regional or contact metamorphism of limestone or dolostone at temperatures above 570oC and low to high pressure.
The essential constituent is calcite.
Common but not essential constituents include actinolite, diopside, dolomite and tremolite.

Skarn

Skarn is a metamorphic rock formed by the contact metamorphism of limestone when it is intruded by an igneous rock, often granite, at temperatures above 570oC and at low pressure.
The essential constituent is calcite and a common constituent is dolomite.

Regional metamorphism

Regional metamorphism is due to pressure and temperature changes over large areas during mountain building and subduction. Certain minerals known as index minerals first appear at different temperatures; with increasing temperature these minerals are chlorite, biotite, garnet, staurolite, kyanite, and sillimanite.
The index minerals generally persist into higher grade zones (higher temperature), and may survive progressive metamorphism and be present with other index minerals, for example, chlorite can frequently be found in biotite schist and garnet is often present in kyanite schist. cordierite also frequently occurs in place of garnet.

Regional metamorphic rocks

Slate

Slate is a very fine-grained, foliated rock with a pervasive fissile cleavage (splitting along flat planes) due to alignment of phyllosilicates. It is produced by the regional metamorphism of clay-rich sediments, such as shale and mudstone at about 2 kbar pressure and 500oC.
Essential constituents are feldspar, mica and quartz.
Common but not essential constituents include graphite and pyrite.

Slate is typically grey to black in colour with a dull lustre, and sometimes green. It is formed by regional metamorphism of argillaceous (clay-rich) sediments.
Slate is a characteristic rock of the prehnite-pumpellyite, greenschist and blueschist facies. With increasing metamorphic grade slate transforms into phyllite.

Serpentinite

Serpentinite is a regional metamorphic rock formed mainly from ultramafic parent rocks (protoliths) at about 5 kbar pressure and 400oC. It is a common component of oceanic crust at a convergent plate boundary where the oceanic crust is forced down beneath the continental crust. Serpentinite forms by the transformation of olivine and pyroxene in peridotite to serpentine. Relicts of olivine and pyroxene are often present in the serpentinite. Dehydration of serpentinite at high temperature produces talc, tremolite and forsterite.
The essential constituent is serpentine.

Phyllite

Phyllite is formed by regional metamorphism of argillaceous (clay-rich) sediments, such as shale and mudstone, at about 5 kbar pressure and 400oC. It is a characteristic rock of the greenschist facies, and it is also a rock of the amphibolite facies.
Essential constituents are biotite, chlorite and muscovite.
Common but not essential constituents include feldspar, graphite and quartz.

Schist

Schist is formed by regional metamorphism of a wide range of fine-grained sediments, including argillaceous (clay-rich) and arenaceous (sandy) sediments, mixed silica-rich and carbonate sediments, and igneous rocks, at about 4 to 15 kbar pressure and 450 to 700oC.
It is a characteristic rock of the greenschist and blueschist facies, and it is also a rock of the amphibolite facies.
Essential constituents are feldspar, usually albite or its variety oligoclase, mica and quartz.
Common but not essential constituents include actinolite, garnet, graphite, hornblende and kyanite.

Mica schists are derived mainly from argillaceous (clay-rich) protoliths, quartz- and feldspar-rich schists have protoliths with a more significant arenaceous (sandy) component, graphite schists typically form from carbon-rich argillaceous (clay-rich) sediments and calc-silicate schists form from clay-rich limestone or calcite-rich mudstone, and often contain diopside and wollastonite.

Gneiss

Gneiss is a metamorphic rock formed by high grade regional metamorphism of rocks containing quartz and feldspar at about 6 kbar pressure and 700oC. It is a characteristic rock of the granulite facies and it is also a rock of the amphibolite facies.
The precursor rock (original rock before metamorphism) may be granite, granodiorite, silica-rich igneous volcanic rocks, mudstone, siltstone or shale.
Common but not essential constituents include amphibole, feldspar, mica and quartz.

Amphibolite

Amphibolite is a metamorphic rock formed by regional metamorphism of silica-poor igneous rocks such as gabbro, at temperatures 500 - 750oC and pressure 8 - 70 kbar (medium-grade metamorphism).
Essential constituents are amphiboles such as hornblende, tremolite and actinolite, and plagioclase feldspar (albite to anorthite).
Common but not essential constituents include epidote, garnet, biotite, clinopyroxene, scapolite, quartz and titanite.
Amphibolite is the characteristic rock of the amphibolite facies.

Granulite

Granulite is a metamorphic rock formed by high-grade regional metamorphism of silica-poor igneous and sedimentary rocks at temperature 500 - 750oC, pressure 8 - 70 kbar.
It is the characteristic rock of the granulite facies.
Essential minerals are feldspars.
Common but not essential minerals include cordierite, amphibole, quartz and pyroxene.

Eclogite

Eclogite is formed by regional metamorphism at about 20 kbar pressure and 700oC.
Essential constituents are omphacite and pyrope.
Common but not essential constituents include kyanite, paragonite, pyroxene, quartz and rutile.

Volcanic sublimates and hot spring deposits

Minerals that crystallise directly from volcanic sublimates and hot springs are primary minerals. Sulphur is typical of volcanic sublimates, and realgar is a common mineral in hot spring deposits.

Hydrothermal environments

Hydrothermal replacement environments

Hydrothermal replacement includes the process of one mineral replacing another, such as when silica replaces wood fibres in petrified wood, of one mineral forming a pseudomorph of another, and of an ore body taking the place of an equal volume of rock. Replacement occurs when a mineralising solution dissolves the original mineral and almost simultaneously replaces it by another. Early-formed replacement minerals may themselves be replaced by later minerals. Replacement can occur in any type of rock, but especially in limestone, because of its easy solubility in acidic fluids.

Hydrothermal vein environments

The hydrothermal fluids circulating in hydrothermal veins are both water percolating down from the surface, and volatile fluid rising up from the magma. The fluid is somewhat acid, and dissolves some of the minerals in the wall rock at the top of the vein, leaving a leached zone depleted of minerals. Further down, in the oxidation zone, a variety of minerals, often sulphates, is deposited. These minerals are secondary supergene minerals, derived from the original primary minerals through chemical processes. Still further down, just below the water table, there is a reducing environment, and the minerals in solution are redeposited as secondary hypogene minerals, often sulphides. This is the enriched zone, or zone of supergene enrichment. Below this are unchanged, primary minerals.
Hydrothermal veins may be low temperature (epithermal) (100oC to 200oC), medium temperature (mesothermal) (200oC to 300oC) or high temperature (hypothermal) (300oC to 500oC), and the suite of minerals found in each type of vein is generally different.

Basaltic cavities

Many hydrothermal minerals occur filling cavities in basalt. The cavities are formed by expansion of gas in volatile-bearing magmas at low pressures. The cavities may be filled by later minerals such as carbonates, quartz, zeolites, analcime or chlorite, formed by hydrothermal processes.

Rocks

A: amphibolite, andesite, anhydrite rock, anorthosite, aplite
B: basalt
C: carbonatite, chert, clay
D: diatomite, diorite, dolostone, dunite
E: eclogite
G: gabbro, gneiss, granite, granodiorite, granulite, gypsum rock
H: hornfels
K: kimberlite
L: limestone
M: marble, marl, monzonite, mudstone
P: pegmatite, peridotite, phyllite
Q: quartzolite
R: rhyolite, rocksalt
S: sandstone, schist, serpentinite, shale, siltstone, skarn, slate, syenite
T: trachyte, tuff

Minerals

A: acanthite, actinolite, adamite, adularia (variety of orthoclase), aegirine, aenigmatite, agardite, åkermanite, albite, allophane, almandine, amazonite (variety of microcline), amblygonite, amphibole, analcime, andalusite, andesine, andradite, anglesite, anhydrite, ankerite, annite, anorthite, anthophyllite, antigorite, apatite, aphthitalite, apophyllite, aquamarine (variety of beryl), aragonite, arseniosiderite, arsenopyrite, augite, aurichalcite, autunite, axinite, azurite
B: bariopharmacosiderite, baryte, bauxite, bayldonite, beryl, beudantite, biotite, bismuth, bismuthinite, bixbyite, böhmite, borax, bornite, bradleyite, brochantite, bromargyrite, brucite, burkeite, bustamite, bytownite (variety of anorthite)
C: calcite, caledonite, carminite, carpholite, cassiterite, cavansite, celadonite, celestine, cerussite, chabazite, chalcanthite, chalcedony, chalcocite, chalcopyrite, chlorargyrite, chlorite, chloritoid, chondrodite, chromite, chrysoberyl, chrysocolla, chrysotile, cinnabar, cleavelandite (variety of albite), clinochlore (variety of chlorite), clinoenstatite, clinohumite, clinoptilolite, clinozoisite, coesite, colemanite, columbite, cookeite, copper, cordierite, corkite, corundum, covellite, crandallite, cristobalite, crocoite, cronstedtite, cummingtonite, cuprite, cyanotrichite
D: dachiardite, danburite, datolite, dawsonite, descloizite, diamond, diaspore, diopside, dioptase, dolomite, duftite, dussertite
E: edenite, emerald (variety of beryl), enargite, englishite, enstatite, epidote, epistilbite, epsomite, erionite, erythrite, ettringite, eucryptite, evansite
F: faujasite, fayalite, feldspar, feldspathoid, ferberite, ferrierite, ferro-actinolite, ferro-anthophyllite, ferrosilite, fluorapatite, fluorite, forsterite (variety of apatite),
G: gahnite, galena, garnet, garronite, gaylussite, gedrite, gehlenite, gibbsite, gismondine, glauberite, glaucochroite, glaucophane, gmelinite, goethite, gold, gonnardite, gordonite, goshenite (variety of beryl), graphite, greenalite, grossular, grunerite, gummite, gypsum, gyrolite
H: halite, halloysite, hanksite, harmotome, hedenbergite, heliodor (variety of beryl), hematite, hemimorphite, hercynite, heulandite, hornblende, hübnerite, hydrocerussite, hypersthene
I: illite (variety of muscovite), ilmenite, imogolite
J: jadeite, jarosite, johannsenite
K: K-feldspar, kalsilite, kaolinite, kernite, kintoreite, kolbeckite, kyanite
L: labradorite (variety of anorthite), laumontite, lawsonite, leadhillite, lepidocrocite, lepidolite, leucite, lévyne, linarite
M: magnesite, magnetite, malachite, manganite, marcasite, margarite (variety of beryl), mawbyite, mazzite, melanterite, melilite, merlinoite, merwinite, mesolite, metavariscite, mica, microcline, millerite, millisite, mimetite, mirabilite, molybdenite, monazite, montgomeryite, monticellite, montmorillonite, mordenite, mottramite, muscovite
N: nahcolite, natrite, natrolite, natron, nepheline, nontronite, northupite
O: offretite, okenite, oligoclase, olivine, omphacite, opal, orpiment, orthoclase, overite
P: paragonite, pargasite, paulingite, pectolite, periclase, perovskite, petalite, pharmacosiderite, phenakite, phillipsite, phlogopite, pirssonite, plagioclase feldspar, platinum, pollucite, prehnite, proustite, pseudobrookite, psilomelane, pumpellyite, pyrargyrite, pyrite, pyrochlore, pyrolusite, pyromorphite, pyrope, pyrophyllite, pyroxene, pyrrhotite
Q: quartz
R: ranciéite, realgar, red beryl (variety of beryl), rhodochrosite, rhodonite, riebeckite, rutile
S: sanidine, sapphirine, scapolite, schairerite, scheelite, schorl, scolecite, scorodite, searlesite, segnitite sellaite, sericite (variety of muscovite), serpentine, shortite, siderite, sillimanite, silver, smithsonite, sodalite, spessartine, sphalerite, spinel, spodumene, spurrite, staurolite, stephanite, stibnite, stilbite, stilpnomelane, strontianite, sulphohalite, sulphur, sylvanite
T: talc, tantalite, tephroite, tetrahedrite, thénardite, thermonatrite, thomsonite, tincalconite, titanite, tobermorite, topaz, torbernite, tourmaline, tremolite, tridymite, trona, turquoise, tychite
U: ulexite, uraninite, uvarovite
V: vanadinite, variscite, vermiculite, vesuvianite, vivianite
W: wairakite, wardite, wavellite, willemite, witherite, wollastonite, wulfenite, wüstite
X: xonotlite
Z: zeolites, zircon, zoisite

Primary and Secondary Minerals

Minerals are primary when they form directly by crystallisation from molten magma, precipitation from magmatic fluids or sublimation from gases. They are secondary when they form from alteration of primary rocks, by weathering, metamorphic or hydrothermal processes. Whether they are primary or secondary is determined by their formation, not by their composition, and the same mineral species may occur sometimes as a primary mineral and sometimes as a secondary mineral.

The terms hypogene and supergene are closely associated with, but not synonymous with, primary and secondary. Hypogene processes are processes that occur deep within the earth, and supergene processes occur near the earth’s surface. Hypogene processes tend to form deposits of primary minerals, from ascending fluids, and supergene processes tend to form secondary minerals from descending fluids.

Ascending hot aqueous solutions originating in the magma contain ions derived from the magma itself, and also from leaching of surrounding rocks. As the solutions rise the temperature and pressure fall. Eventually a point is reached where the minerals start to crystallise out. Minerals formed in this way are primary minerals. Sulphur is a common component of the fluids, and most of the common ore metals, lead, zinc, copper, silver, molybdenum and mercury, occur chiefly as sulphide and sulfosalt minerals. Examples of primary minerals formed in this way include pyrite, galena, sphalerite and chalcopyrite

Processes due to circulation of meteoric water (water derived from snow and rain) are supergene processes. The descending meteoric waters oxidise the primary sulphide ore minerals and leach metals from the oxidised ore. As the fluids descend the dissolved substances may eventually precipitate to form two zones of secondary minerals, one above and the other below the water table. Conditions above the water table are usually oxidising. Secondary minerals that are stable under these conditions include malachite, azurite, cuprite, pyromorphite, and smithsonite. Beneath the water table conditions are usually reducing, and secondary minerals that form here are dominantly sulphides, such as covellite and chalcocite. Even native metals such as copper, copper may precipitate in this environment.
This region is called the zone of supergene enrichment, because the processes here produce secondary sulphides making the metal content of the rock higher than that of the primary ore. All such processes take place at essentially atmospheric conditions, 25°C and atmospheric pressure.

Acanthite

Formula: Ag2S sulphide
Specific gravity: 7.3
Hardness: 2
Streak: Black
Colour: Lead grey
Solubility: Acanthite is slightly soluble in hydrochloric acid and sulphuric acid.
Environments:

Hydrothermal replacement environments
Hydrothermal vein environments

Acanthite is a primary silver mineral that occurs in epithermal (low temperature) hydrothermal silver ore veins. It may also be of secondary origin. At atmospheric pressure, acanthite is stable below 173°C. Above 173°C the structure changes to produce crystals of twinned acanthite, known as argentite. Argentite is unstable below 173°C, and if the temperature drops below this level it will change back to acanthite.

Common impurities: Se

Actinolite

Formula: ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5Si8O22 (OH)2
Mg/(Mg + Fe2+) is 0.5 to 0.9, inosilicate (chain silicate) amphibole
Specific gravity: 3.03 - 3.24
Hardness: 5 to 6
Streak: White
Colour: Green, green-black, grey-green, or black. Colourless, pale green to deep green in thin section.
Solubility: Insoluble in water and hydrochloric acid
Environments:

Metamorphic environments
Hydrothermal replacement environments

Actinolite is an amphibole mineral that is produced by low-grade regional or contact metamorphism of magnesium carbonate.
It may be found in schist.
Associated minerals are calcite, quartz, epidote, glaucophane, pumpellyite and lawsonite.
Actinolite is a mineral of the albite-epidote-hornfels, prehnite-pumpellyite, greenschist, amphibolite and glaucophane-bearing blueschist facies.

At the Belvidere Mountain Complex in Vermont, USA, actinolite occurs in coarse-grained amphibolite, as pods along fault-zone contacts with serpentinite and as fine-grained masses with chlorite in fine-grained amphibolite.
In the Bronx, New York City, it occurs associated with quartz and calcite.

Common impurities: Mn, Al, Na, K, Ti

Adamite

Formula: Zn2(AsO4)(OH) arsenate
Specific gravity: 4.3 - 4.5
Hardness: 3½
Streak: White
Colour: Colourless, white, yellow, green (copper adamite), and pink to violet (cobalt adamite).
Solubility: Readily soluble in hydrochloric acid and sulphuric acid.

Environments:

Hydrothermal replacement environments

Adamite is a secondary arsenate that occurs in the oxidation zone of high-temperature lode zinc- and arsenic-bearing hydrothermal mineral deposits, associated with azurite, goethite, hemimorphite, mimetite, olivinite, scorodite and smithsonite.
Adamite is abundant and widespread at the San Rafael Mine, Nevada, USA. It usually occurs alone there, but it has been found associated with wulfenite and smithsonite, or with segnitite and duftite.

Common impurities: Cu,Fe,Co

Aegirine

Formula:NaFe3+Si2O6 inosilicate (chain silicate), pyroxene group
Specific gravity: 5.50 to 3.60
Hardness: 6
Streak: Pale yellowish grey
Colour: Black, brown, green. Bright green to yellow-green in thin section.
Solubility: Aegirine is slightly soluble in hydrochloric acid.
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments

Aegirine is a pyroxene. It is a relatively rare primary mineral found chiefly in igneous rocks rich in Na and poor in SiO2 and in syenite pegmatites. It is also found as a secondary mineral in metamorphic rocks.
Aegirine may be found in granite, syenite, nepheline syenite, rhyolite and schist.
In igneous rocks it is associated with orthoclase, feldspathoids, augite and soda-rich amphibole.
In hornfels of contact metamorphic rocks it is found with quartz, spessartine and riebeckite.

Alteration

aegirine and CaO to andradite, quartz and Na2O
2NaFe3+Si2O6 + 3CaO → Ca3Fe3+2 (SiO4)3 + SiO2 + Na2O
This is a high temperature process.

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan.

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2 (Mg3Fe3+2)Si8O22(OH)2 + H2O

titanomagnetite (ilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2

jadeite, diopside, magnetite and SiO2 to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction.

Common impurities: Al,Ti,V,Mn,Mg,Ca,K,Zr,Ce

Aenigmatite

Formula: Na4[Fe2+10Ti2]O4[Si12O36] inosilicate (chain silicate)
Specific gravity: 3.74 to 3.85
Hardness: 5½ to 6
Streak: reddish brown
Colour: velvet-black
Solubility: soluble in HF
Environments:

Plutonic igneous environments
Volcanic igneous environments

Aenigmatite is a rock-forming mineral, relatively common in sodium rich alkaline rocks. It occurs in nepheline and sodalite syenites, and occasionally in alkali granite. It also occurs in phenocrysts and in the groundmass of alkaline lavas, such as trachyte.
Common associates: aegirine, arfvedsonite, augite, fayalite, hedenbergite, ilmenite and riebeckite

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4

titanomagnetite (ilmeniteilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2

Common impurities: Al,Mn,Mg,Ca,K,Cl

Agardite

Formulae
agardite-(Ce): CeCu2+6(AsO4)3(OH)6.3H2O
agardite-(La): LaCu2+6(AsO4)3(OH)6.3H2O
agardite-(Nd): NdCu2+6(AsO4)3(OH)6.3H2O
agardite-(Y): YCu2+6(AsO4)3(OH)6.3H2O
Specific gravity: Agardite-(Ce) 3.7, Agardite-(La) 3.65, Agardite-(Nd) 3.72, Agardite-(Y) 3.61 - 3.72
Hardness: 3 to 4 (all)
Streak: Greenish white (all)
Colour: Agardite-(Ce) Light green, yellowish green, Agardite-(La) Grass-green to dull green, yellowish green to intense bluish green, rarely nearly colourless, Agardite-(Nd) Grass-green, greenish, greenish-blue, Agardite-(Y) Bluish green to yellow green
Solubility: Agardite-(Y) is soluble in hydrochloric acid
Environments:

Hydrothermal environments

Agardite-(Ce) is an oxidation product formed on baryte and quartz.
Agardite-(La) was found in a fluorite mine associated with quartz, fluorite and baryte.
Agardite-(Y) occurs in the oxidation zone of copper veins or deposits. At the Bou Skour Mine, Ouazazate, Morocco it occurs in the oxidation zone associated with azurite, malachite, cuprite, native copper and quartz. At Laurium, Attica, Greece it is associated with smithsonite and aurichalcite.

Åkermanite

Formula:Ca2MgSi2O7 sorosilicate (Si2O7 groups), melilite group
Near end-member åkermanite is very rare, possibly unknown in nature.
Specific gravity: 2.90 to 2.97
Hardness: 5 to 6
Streak: White
Colour: Colourless, grey, green, brown
Melting point: 1,454oC at a pressure of 1 kbar
Environments:

Plutonic igneous environments
Metamorphic environments

Commonly found in slags. Åkermanite is a product of contact metamorphism of siliceous limestone and dolostone, and it also forms from alkalic magmas rich in calcium.
It is a common component of skarn, and a mineral of the sanidinite facies.

Alteration

As åkermanite cools from its melting point it is stable down to 1345oC, when the stable mixture is åkermanite and wollastonite. From 1240oC down to 1050oC a mixture of åkermanite, wollastonite and diopside is stable, and at lower temperatures åkermanite dissociates to form wollastonite and monticellite. The stability of åkermanite is limited to pressures of less than 14 kbar in an anhydrous environment, and less than 10.2 kbar with excess H2O.

åkermanite to wollastonite and monticellite
Ca2MgSi2O7 ⇌ CaSiO3 + CaMgSiO4
The forward reaction occurs at temperature less than 1,050oC.

åkermanite and CO2 to diopside and calcite
Ca2MgSi2O7 + CO2 ⇌ CaMgSi2O6 + CaCO3
The maximum stability limit of åkermanite in the presence of excess CO2 is about 6 kbar. Below that pressure, at relatively lower temperatures, åkermanite reacts with CO2 to form diopside and calcite according to the above reaction.

diopside and monticellite to åkermanite and forsterite
CaMgSi2O6 + 3CaMgSiO4 ⇌ 2Ca2MgSi2O7 + Mg2SiO4

monticellite and CO2 to åkermanite, forsterite and calcite
3CaMgSiO4 + CO2 ⇌ Ca2MgSi2O7 + Mg2O7 + CaCO3
The forward reaction occurs at temperature s above 890oC at and pressure above about 4.3 kbar.

monticellite and diopside to åkermanite and forsterite
3CaMgSiO4 + CaMgSi2O6 ⇌ 2Ca2MgSi27 + Mg2O7
Monticellite is stable below 890oC at pressure of about 4.3 kbar.

Albite

Formula: Na(AlSi3O8) tectosilicate (framework silicate)
albite is a plagioclase feldspar.
Andesine is a variety of albite rarely found except as grains in andesite.
Cleavelandite is a platy variety of albite, generally found in pegmatites.
Oligoclase is a variety of albite belonging to the amphibolite facies.
Specific gravity: 2.60 to 2.65
Hardness: 6 to 6½
Streak: White
Colour: Colourless, white
Melting point (albite): About 1,100oC at atmospheric pressure
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments (albite):

Pegmatites
Carbonatites
Metamorphic environments

Albite is a primary mineral crystallising at the low temperature end of the continuous arm of the Bowen reaction series. It is a plagioclase feldspar found in pegmatites, carbonatites and contact and regional metamorphic environments, where it is a secondary mineral.
Albite is a common but not essential constituent of granite and granite pegmatites.
It also may be found in metamorphic quartzite (metaquartzite), rhyolite, trachyte, hornfels, phyllite and schist.
In nepheline syenite pegmatites and carbonatites albite it is associated with acmite and nepheline.
In rhyolite and trachyte it may replace earlier microcline.
Albite is characteristic of the zeolite and albite-epidote-hornfels facies. It is also a mineral of the prehnite-pumpellyite, greenschist, amphibolite and blueschist facies.
Environments (andesine):

Plutonic igneous environments
Clastic sedimentary environments
Metamorphic environments

Andesine is widespread in igneous rocks of intermediate silica content, such as syenite and andesite. It is characteristic of the amphibolite and granulite facies. It is rarely found except as grains in andesite (where it may be associated with augite) and diorite.
Environments (oligoclase):

Plutonic igneous environments
Pegmatites
Metamorphic environments

Oligoclase is a common but not essential constituent of granodiorite.
It also may be found in granite, monzonite, gabbro, anorthosite and in gneiss with biotite and hornblende.
It is a mineral of the amphibolite and granulite facies.
Andesine and oligoclase occur towards the higher temperature range of albite and its varieties.

Alteration

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4

albite to jadeite and quartz
Na(AlSi3O8) → NaAlSi2O6 + SiO2
In quartz-jadeite rocks jadeite may be the product of the above reaction,
which occurs at high pressure ranging from above 5 kbar at 0oC to above 20 kbar at 800oC.

albite to nepheline and quartz
Na(AlSi3O8) ⇌ NaAlSiO4 + 2SiO2

albite and NaCl (aqueous) to sodalite and silica
6Na(AlSi3O8) + 2NaCl → 2Na4(Si3Al3)O12Cl + 12SiO2

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above.

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan.

albite and montmorillonite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz and H2O
8Na(AlSi3O8) + 2Ca0.5(Mg3.5Al0.5)Si8O20(OH)4.nH2O → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 15SiO2 + 6H2O
This reaction occurs in low to intermediate metatmorphism.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

analcime and quartz to albite and H2O
Na(AlSi2O6).H2O + SiO2 ⇌ Na(AlSi3O8) + H2O

anorthite, albite and H2O to jadeite, lawsonite and quartz
CaAl2 Si2O8 + NaAlSi3O8 + 2H2O → NaAlSi2O6 + CaAl2(Si2O7)(OH)2.H2 + SiO2

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2
This reaction occurs at high temperature and pressure.

diopside and albite to omphacite and quartz
CaMgSi2O6 + xNaAlSi3O8 ⇌ CaMgSi2O6.xNaAlSi2O6 + SiO2

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
+ Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies.

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies.

Jadeite and quartz to albite
NaAlSi2O6 + SiO2 ⇌ NaAlSi3O8
High pressure favours the reverse reaction.

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure.

nepheline and albite to jadeite
NaAlSiO4 + NaAlSi3O8 ⇌ 2NaAlSi2O6 The forward reaction requires high pressure, 10 to 25 kbar, and temperature 600oC to 1,000oC.

spodumene and Na+ to eucryptite, albite and Li+
2LiAlSi2O6 + Na+ → LiAlSiO4 + NaAlSi3O8 + Li+
Whether spodumene breaks down into albite or into eucryptite and albite depends largely on the presence or absence of quartz.

spodumene, quartz and Na+ to albite and Li+
LiAlSi2O6 + SiO2 + Na+ → NaAlSi3O8 + Li+
Whether spodumene breaks down into albite or into eucryptite and albite depends largely on the presence or absence of quartz.

Common impurities: Ca,K,Mg

Allophane

Formula: Al2O3(SiO2)1.3-2.0.2.5-3.0H2O phyllosilicate (sheet silicate), allophane group
Specific gravity: 1.8 to 2.78
Hardness: 3
Streak: White
Colour: White, pale blue to sky-blue, green, brown
Solubility: Gelatinises in hydrochloric acid
Environments:

Clastic sedimentary environments
Hydrothermal environments (common)

Allophane is principally a weathering product of volcanic ash, and also a hydrothermal alteration product of feldspar. It may be a precursor to halloysite, but halloysite also alters to allophane. Allophane is frequently found with admixed halloysite, imogolite, limonite, opal, gibbsite and cristobalite, and occasionally with chrysocolla or evansite.
Common impurities: Ti,Fe,Mg,Ca,Na,K

Almandine

Almandine is the commonest mineral of the garnet group.
Formula: Fe2+3Al2(SiO4)3 nesosilicate (insular SiO4 groups)
Specific gravity: 4.318
Hardness: 7 to 7½
Streak: White
Colour: Red, black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Metamorphic environments (common)

Almandine is the common garnet in metamorphic rocks, typically occuring in mica schist and gneiss, resulting from the regional metamorphism of argillaceous (clay-rich) sediments. It also occurs in contact metamorphic hornfels, and occasionally in diorite. It is stable over a wide range of pressure-temperature conditions. Metamorphic almandine is a mineral of the hornblende-hornfels, amphibolite, granulite, blueschist and eclogite facies.

Alteration

almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
Both temperature and pressure affect the equilibrium of this reaction, but temperature is more significant. This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

Amblygonite

Formula: LiAl(PO4)F phosphate
Specific gravity: 3.0 - 3.1
Hardness: 6
Streak: White
Colour: White, grey, yellow to brown, greenish, bluish
Solubility: Amblygonite is slightly soluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites

Amblygonite is a rare mineral typically found in lithium-rich granite pegmatites with spodumene, tourmaline, lepidolite and apatite.

Amphibole

Amphiboles are double chain inosilicates, such as
actinolite ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5Si8O22(OH)2 ☐Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22 (OH)2,
edenite NaCa2Mg5(Si7Al)O22(OH)2,
hornblende ☐Ca2(Fe2+4Al)(Si7Al)O22(OH)2,
pargasite NaCa2(Mg4Al)(Si6Al2)O22(OH)2 and
tremolite ☐Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22 (OH)2.
In the discontinuous branch of the Bowen reaction series amphibole is intermediate between pyroxene (higher temperature) and biotite (lower temperature).
Environments:

Plutonic igneous environments

Amphibole is typical of plutonic igneous environments.
Amphibole is a common but not essential constituent of rhyolite and gneiss.
It also may be found in granite, quartzolite, gneiss and eclogite.
Amphibole is the characteristic mineral of the amphibolite facies; it is never found in the granulite facies.

Alteration

Ca-Fe amphibole and anorthite to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2

calcium amphibole, calcite and quartz to diopside- hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies, and it is the reason why amphibole is never found in granulite facies rocks.

Analcime

Formula: Na(AlSi2O6).H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.24 - 2.29
Hardness: 5 to 5½
Streak: White
Colour: White, colourless, gray, pink, greenish, yellowish; colourless in thin section.

Solubility: Moderately soluble in hydrochloric acid
Environments:

Volcanic igneous environments
Pegmatites
Clastic sedimentary environments
Chemical sedimentary environments
Basaltic cavities

Analcime occurs as a primary mineral in some igneous rocks; it is the only zeolite that crystallises directly from molten rock. It is also the product of hydrothermal action in the filling of basalt cavities, where analcime phenocrysts crystallise in deep basaltic magma chambers at temperatures between 600oC and 640oC, and pressure between 5 and 13 kbar, in associations with prehnite, calcite and zeolites. Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that analcime crystallised at temperatures from 60oC to 300oC at depths between 72m and 1600m. Analcime may occur in mafic rocks such as aegirine-analcime-nepheline syenite. In lake beds, it may be altered from pyroclastics or clay minerals, or it may be a primary precipitate. It is authigenic (formed in place) in sandstone and siltstone.

Alteration

analcime and quartz to albite and H2O
Na(AlSi2O6).H2O + SiO2 ⇌ Na(AlSi3O8) + H2O

nepheline and H4SiO4 (silicic acid) to analcime and H2O
NaAlSiO4 + H4SiO4 ⇌ Na(AlSi2O6).H2O + H2O

Localities

Australia

In Western Tasmania analcime occurs as a late stage primary mineral with gonnardite-natrolite and as an alteration product of feldspar. It also occurs in basaltic cavities.

Iran

In the vicinity of Meshkinshahr, Ardabil Province, crystals of analcime are found embedded in small amygdules in potassium-rich basalt, associated with chabazite, mesolite and/or thomsonite. The analcime originated hydrothermally.

At Mount Kahoven, Semnan Province, orange coloured crystals of analcime are found on perched on vesicular basalt.

South Africa

At Palabora analcime is an early phase in the deepest parts of the dyke fracture zone. Crystal cores may have originally been wairakite overgrown by analcime, then later hydrothermal alteration converted wairakite to laumontite. A later generation of analcime occurs on fluorapophyllite.

Andalusite

Formula: Al2SiO5 nesosilicate (insular SiO4 groups). Polymorph (same formula, different structure) of kyanite and sillimanite.
Specific gravity: 3.13 to 3.16
Hardness: 6½ to 7½
Streak: White
Colour: Pink, brown, white, grey, violet, yellow, green and blue
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments (rarely)
Pegmatites (rarely)
Metamorphic environments (typical)
Hydrothermal replacement environments

Andalusite is found typically as a product of contact metamorphism of argillaceous (clay-rich) rocks, where it is often associated with cordierite. It also occurs occasionally in igneous rocks.
Andalusite occurs in granite, gneiss, phyllite and schist.
It is a mineral of the albite-epidote-hornfels, hornblende-hornfels, pyroxene-hornfels, amphibolite facies and granulite facies.

Alteration

Andalusite, sillimanite and kyanite are polymorphs (same formula, different structure); they are in equilibrium at a pressure of 4.2 kbar and temperature 530oC. Under conditions of higher temperature and pressure andalusite may become unstable and invert to its polymorphs sillimanite or kyanite. Andalusite is stable at lower pressure and temperature up to about 900oC; kyanite is the high pressure polymorph, and sillimanite the high temperature polymorph.

augite and andalusite to enstatite- ferrosilite and anorthite
Ca(Fe,Mg)Si2O6 +Al2SiO5 → (Mg,Fe2+)SiO3 + Ca(Al2Si2O8)

enstatite-ferrosilite and andalusite to Fe rich cordierite and spinel- hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, in an SiO2 deficient environment the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, andalusite and quartz to Fe-rich cordierite
2(Mg,Fe2+)SiO3 + 2Al2SiO5 + SiO2 → (Mg,Fe2+)2Al4Si5O18
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

pyrophyllite to andalusite, quartz and H2O
Al2Si4O10(OH)2 ⇌ Al2SiO5 + 3SiO2 + H2O
The equilibrium temperature for this reaction at 1.8 kbar pressure is 414oC, with the reaction going to the right at higher temperatures, and to the left at lower temperatures.

pyrophyllite and diaspore to andalusite and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2SiO5 + 4H2O

Andradite

Formula: Ca3Fe3+2(SiO4)3 nesosilicate (insular SiO4 groups), garnet group
Specific gravity: 3.8 - 3.9
Hardness: 6½ to 7
Streak: White
Colour: Yellow, greenish yellow to emerald-green, dark green; brown, brownish red, brownish yellow; grayish black, or black
Solubility: Andradite is slightly soluble in hydrochloric acid
Environments:

Pegmatites
Carbonatites
Metamorphic environments (typical)

Andradite typically occurs in contact or thermally metamorphosed impure calcium-rich sediments and particularly in skarn deposits often associated with such metamorphism.

Alteration

aegirine and CaO to andradite, quartz and Na2O
2NaFe3+Si2O6 + 3CaO → Ca3Fe3+2 (SiO4)3 + SiO2 + Na2O This is a high temperature process.

calcite, hematite and CO2 quartz to andradite and
3CaCO3 + Fe2O3 + 3SiO2 → Ca3Fe3+2Si3O12 + 3CO2

hematite, wüstite, quartz and calcite to andradite, hedenbergite, magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

Common impurities: Ti,Cr,Al,Mg

Anglesite

Formula: Pb(SO4) sulphate
Specific gravity: 6.37 to 6.39
Hardness: 2½ to 3
Streak: White
Colour: Colourless, white, yellow, green; colourless in transmitted light.
Solubility: Anglesite is not very soluble in water.
Environments:

Hydrothermal replacement environments

Anglesite is a common high temperature secondary mineral in the oxidation zone of hydrothermal replacement deposits rich in lead.
Lead will generally precipitate as primary galena from ore fluids rich in sulphur and lead. Removal of sulphur by precipitation of sulphides, however, may lead ultimately to an ore fluid from which galena cannot be precipitated, even with a high concentration of lead in solution. In these circumstances, anglesite, as well as cerussite and pyromorphite, could be precipitated as a primary mineral.
Anglesite is commonly associated with galena, cerussite, sphalerite, smithsonite, hemimorphite and iron oxides.

Alteration

In the oxidation zone, oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + water → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + water → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulfate is a strong oxidizing agent; it attacks galena as below.

galena, ferric sulphate, water and oxygen to anglesite, ferrous sulphate and sulphuric acid
PbS + Fe2(SO4)3 + H2O + 3O → PbSO4 + 2FeSO4 + H2SO4
Galena is oxidised to anglesite and ferric iron is reduced to ferrous iron.

galena and oxygen to anglesite
In air, at outcrops of galena,
PbS + 2O2 → PbSO4
At ordinary temperatures the equilibrium is displaced far to the right, and the apparent stability of galena is a result of the slowness of the oxidation.

Galena may also dissolve in carbonic acid from percolating rainwater to form hydrogen sulphide, which is then oxidised to form anglesite.
galena and carbonic acid to Pb2+, hydrogen sulphide and HCO3-
PbS + 2H2CO3 → Pb2+ + H2S + 2HCO3>-

hydrogen sulphide, oxygen, Pb2+ and HCO3>- to anglesite and carbonic acid
H2S + 2O2 + Pb2+ + 2HCO3>- → PbSO4 + 2H2CO3

Common impurities: Ba, Cu

Anhydrite

Formula: Ca(SO4) sulphate
Specific gravity: 2.98
Hardness: 3 - 3½
Streak: White
Colour: Colourless, white, grey, blue
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Carbonatites
Chemical sedimentary environments (typical)
Hydrothermal replacement environments

Anhydrite occurs in the oxidation zone of high temperature hydrothermal replacement deposits, in salt deposits, sediments and in some zeolite occurrences. It is often associated with gypsum but is not nearly so common. It is found in limestone and in some amygdaloidal cavities in basalt.

Alteration

anhydrite and water to gypsum
Ca(SO4) + 2H2O ⇌ Ca(SO4).2H2O
Gypsum is frequently formed by the hydration of anhydrite.

Common impurities: Sr,Ba,H2O

Ankerite

Formula: Ca(Fe2+,Mg)(CO3)2
Forms a series with dolomite and with kutnohorite
Specific gravity: 3.121
Hardness: 3½ to 4
Streak: white
Colour: brown, white to grey, yellowish-brown, greenish
Solubility: Moderately soluble in hydrochloric and sulphuric acid
Environments:

Carbonatites
Hydrothermal replacement environments

Ankerite is a common but not essential constituent of dolostone.

Alteration

ankerite-dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2

calcite, Fe2+ and Mg2+ to ankerite and Ca2+
4CaCO3 + Fe2+ + Mg2+ = 2Ca(Mg0.5Fe0.5)(CO3)2+ 2Ca2+
Ankerite is believed to be formed from calcite hydrothermally according to the above reaction.

Common impurities: Mn

Annite

Formula: KFe2+3(AlSi3O10)(OH)2 phyllosilicate (sheet silicate) mica group
Specific gravity: 3.3
Hardness: 2½ to 3
Streak: Brownish white
Colour: Black, brown
Environments:

Plutonic igneous environments
Metamorphic environments

Annite occurs in igneous and metamorphic rocks that are low in magnesium.

Alteration

almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks.

sillimanite, annite and H2O to staurolite, muscovite, SiO2 and O2
31Al2SiO5 + 4KFe2+3(AlSi3O10)(OH)2 + 6H2O → 34Fe2+2Al9si4O23(OH) + KAl2 (AlSi3O10)(OH)2 + 7 SiO2 + 1.5O2
Staurolite may occur as a product of retrograde metamorphism according to the above reaction.

staurolite, annite and O2 to hercynite, magnetite, muscovite,corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O

Common impurities: Ti,Mn,Mg,Ca,Na,K,Cl

Anorthite

Formula: Ca(Al2Si2O8) tectosilicate (framework silicate)
Anorthite is a plagioclase feldspar.
Bytownite is a variety of anorthite
Labradorite is a variety of anorthite
Specific gravity: 2.74 - 2.76, bytownite 2.61 - 2.77
Hardness: 6 to 6½
Streak: White
Colour: Anorthite is colourless, reddish grey or white. Bytownite is colourless, white, greenish, reddish or grey.
Solubility: Anorthite is slightly soluble in hydrochloric acid
Melting point: About 1,550oC at atmospheric pressure
In the continuous branch of the Bowen reaction series anorthite is the first major mineral to crystallise.
Environments (anorthite):

Metamorphic environments

Anorthite is a plagioclase feldspar found in rocks rich in dark minerals, in druses of ejected volcanic blocks and in granular limestone of contact metamorphic deposits. Anorthite also may be found in serpentinite, and hornfels.
Environments (bytownite):

Pegmatites
Metamorphic environments

Bytownite is found in granite, with hornblende and augite in gabbro, anorthosite, gneiss, granulite, and pegmatites, and lining Alpine cavities and fissures in ore veins.
Environments (labradorite):

Plutonic igneous environments
Metamorphic environments

Labradorite is found with hornblende in basalt, with hornblende and augite in gabbro.

Anorthite is a mineral of the amphibolite facies.

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4

Ca-Fe amphibole and anorthite to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

anorthite to calcite and kaolinite in the early Earth's atmosphere
CO2 + H2O + anorthite → calcite + kaolinite
CO2 + 2H2O + CaAl2Si2O8 → CaCO3 + Al2Si2O5(OH)4

anorthite, H2O and CO2kaolinite and calcite
2CaAl2 Si2O8 + 4H2O + 2CO2 ⇌ Al4Si4O10(OH)8 + 2CaCO3
calcite is found as a low-temperature, late-stage alteraation product according to the above reaction.

anorthite, H2SO4 and H2 to gypsum and kaolinite
CaAl2 Si2O8 + H2SO4 + 3H2O → CaSO4.2H2O + Al2Si2O5(OH)4

anorthite, albite and H2O to jadeite, lawsonite and quartz
CaAl2 Si2O8 + NaAlSi3O8 + 2H2O → NaAlSi2O6 + CaAl2(Si2O7)(OH)2.H2 + SiO2

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies.

calcium amphibole, calcite and quartz to diopside- hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

anorthite variety labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure.

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2
This reaction occurs at high temperature and pressure.

augite and andalusite to enstatite- ferrosilite and anorthite
Ca(Fe,Mg)Si2O6 + Al2SiO5 → (Mg,Fe2+)SiO3 + Ca(Al2Si2O8)

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure.

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies.

epidote and chlorite to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) + Mg5Al2Si3O18(OH)8 → Ca2Mg5Si8O22(OH)2 + 10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the greenschist facies to the amphibolite facies.

epidote and quartz to anorthite, grossular and H2O
4Ca2Al3(SiO4)3(OH) + SiO2 → 5CaAl2Si2O8 + Ca3Al2(SiO4)3 + 2H2O
This reaction occurs as the degree of metamorphism increases

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8) = Mg2Si2O6 + CaMgSi2O6 + MgAl2O4

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

Al-rich hornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure.

Common impurities in anorthite: Ti,Fe,Na,K

Anthophyllite

Formula: ☐Mg2Mg5Si8O22(OH)2 inosilicate (chain silicate) amphibole
Anthophyllite is a dimorph of cummingtonite. Specific gravity: 2.8 - 3.2
Hardness: 5½
Streak: White
Colour: Brownish, yellowish grey
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Metamorphic environments

Anthophyllite is a metamorphic product of magnesium-rich rocks such as ultramafic igneous rocks and impure dolomite shale. It is common in cordierite-bearing gneiss and schist. It may also form as a retrograde (retrograde metamorphism refers to changes that occur during uplift and cooling of a rock) product rimming relict orthopyroxenes such as enstatite, and olivine.
Anthophyllite may be found in gneiss and schist.
It is a mineral of the hornblende-hornfels facies.

Alteration

talc to anthophyllite, quartz and H2O
7Mg3Si4O10(OH)2 → 3☐Mg2Mg5Si8O22(OH)2 + 4SiO2 + 4H2O
This reaction occurs when the degree of metamorphism increases

talc and forsterite to anthophyllite and H2O
9Mg3Si4O10(OH)2 + 4Mg2SiO4 = 5Mg2Mg5Si8O22(OH)2 + 4H2O
This reaction occurs at a higher metamorphic grade than that forming forsterite and talc from serpentine.

Common impurities: Ti,Al,Mn,Ca,Na

Antigorite

Formula: Mg3Si2O5(OH)4 phyllosilicate (sheet silicate), serpentine group
Specific gravity: 2.0 - 2.6
Hardness: 2½ - 4
Streak: White
Colour: Green, grey to black, white, brownish
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Environments:

Metamorphic environments

Antigorite forms in reactions at temperatures that can exceed 600°C during metamorphism, and it is the serpentine group mineral stable at the highest temperatures. It is common in regional and contact metamorphosed serpentinite.

Alteration

antigorite to forsterite, talc and H2O
5Mg3Si2O5(OH)4 = 6Mg2SiO4 + Mg3Si4O10(OH)2 + 9H2O
This reaction may occur in olivine-diopside- antigorite schist within the aureole of the tonalite in the southern Bergell Alps, Italy, at a higher grade of metamorphism than that which produces forsterite and tremolite.

antigorite and calcite to forsterite, diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 → 4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist at about 3 kbar pressure and 400 to 500oC

antigorite and magnesite to forsterite, CO2 and H2O
Mg3Si2O5(OH)4 + MgCO3 → 2Mg2SiO4 + CO2 + 2H2O

See also results for serpentine, which is a group of minerals including antigorite, chrysotile and lizardite, all of which share the same formula, although they have slightly different structures.

Common impurities: Ni,Al,Mn

Antlerite

Formula: Cu2+3(SO4)(OH)4 anhydrous sulphate containing hydroxyl
Pseudomorphs of cuprite after fibrous antlerite have been observed
Specific gravity: 3.9
Hardness: 3½
Streak: Light green
Colour: Emerald-green to blackish-green, also light green
Solubility: Soluble in dilute sulphuric acid
Environments:

Hydrothermal vein environments

Antlerite is a secondary mineral occurring in the oxidised zone of copper deposits in arid regions.
It is metastable at ambient temperature with respect to brochantite.

Apatite

The apatite group comprises three minerals:
Fluorapatite: Ca5(PO4)3F
Hydroxylapatite: Ca5(PO4)3(OH)
Chlorapatite: Ca5(PO4)3Cl
All of these are anhydrous phosphates containing hydroxyl or halogen
Fluorapatite is the commonest mineral in the apatite group.
Properties of fluorapatite:
Specific gravity: 3.1 - 3.25
Hardness: 5
Streak: White
Colour: Colourless, yellow, green, blue and purple. Manganoan varieties are dark green and blue-green.
Solubility: Apatite is slightly soluble in sulphuric acid and moderately soluble in hydrochloric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Chemical sedimentary environments
Metamorphic environments
Hydrothermal vein environments

Apatite is a primary and secondary mineral, widely distributed with important concentrations in carbonatites. It is the most common rock-forming phosphate and a major mineral in sedimentary phosphorites. It occurs the oxidation zone of hypothermal (high temperature) veins and in Alpine cleft-type veins.
Apatite is a common constituent of marble, skarn and magnetite deposits.
Apatite may also be found in quartzolite, granite, syenite, diorite, rhyolite, trachyte, andesite and basalt. Common impurities: OH,Cl,transition elements,La,Ce,Pr,Nd,Sm,Eu,Gd,Dy,Y,Er,Mn

Aphthitalite

Formula: K3Na(SO4)2 anhydrous normal sulphate
Specific gravity: 2.66 to 2.71
Hardness: 3
Streak: White
Colour: Colourless (rare), white, grey, bluish, greenish, reddish; colourless in transmitted light
Solubility: Soluble in water
Environments:

Chemical sedimentary environments
Volcanic sublimates and hot spring deposits

Aphthitalite occurs in two different environments. It is formed in volcanoes as incrustations in fumeroles, where it may deposit from volcanic gas during cooling from about 800 down to 400oC, together with thénardite. It also occurs in lacustrine (lake) salt deposits.

Apophyllite

Apophyllite refers to a three minerals:
Fluorapophyllite-(K) KCa4Si8O20F.8H2O
Fluorapophyllite-(Na) NaCa4Si8O20F.8H2O and
Hydroxyapophyllite-(K) KCa4Si8O20(OH,F).8H2O
They are all phyllosilicates (sheet silicates)
Specific gravity: 2.3 - 2.4
Hardness: 4½ - 5
Streak: White
Colour: Colourless, white, yellow, green, brown, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:

Basaltic cavities

Apophyllite occurs as a secondary mineral lining cavities in basalt and related rocks, associated with zeolites, calcite, datolite and pectolite.

Aragonite

Formula: Ca(CO3) carbonate
Specific gravity: 2.947
Hardness: 3½ to 4
Streak: White
Colour: colourless to white or grey, often stained blue, green, red or violet by impurities; colourless in transmitted light.
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Clastic sedimentary environments
Chemical sedimentary environments
Hot spring deposits
Hydrothermal replacement environments

Aragonite is found in the oxidised zones of ore deposits and in evaporites, hot spring deposits and limestone caves. It is also found in some metamorphic and igneous rocks. It occurs with siderite in iron deposits and with calcite, dolomite and other magnesium minerals in altered serpentinite, dunite and peridotite.
Aragonite is a common constituent of limestone.
It is a mineral of the blueschist facies.

Alteration

Aragonite and calcite are both forms of calcium carbonate. Aragonite is less stable than calcite under atmospheric conditions, but the alteration of aragonite to calcite is a very slow reaction, so calcite and aragonite can co-exist in new rocks, but aragonite is a rare mineral in old rocks and shells. At extremely high pressure aragonite is the stable mineral.

Calcite and aragonite are precipitated according to the following reactions:
Carbon dioxide in the atmosphere is dissolved in rainwater forming weak carbonic acid:
H2O + CO2 → H2CO3
Carbonic acid dissolves limestone forming calcium bicarbonate
H2CO3 + CaCO3 → Ca(HCO3)2
This solution percolates into caves where calcium carbonate may be precipitated with the release of liquid water and gaseous carbon dioxide:
Ca(HCO3)2 ⇆ CaCO3 (solid) + H2O (liquid) + CO2 (gas)
The net effect of these changes could be written as the reversible reaction
CaCO3 (solid) + H2CO3 (in solution) ⇌ Ca2+ + 2(HCO3)-
The forward reaction, solution of calcium carbonate, occurs in acid environments, and the reverse reaction, precipitation of calcium carbonate, occurs in strongly basic (alkaline) environments.

Common impurities: Sr,Pb,Zn

Arseniosiderite

Formula: Ca2Fe3+3O2(AsO4)3.3H2O

Arsenopyrite

Formula: FeAsS supharsenide
Specific gravity: 5.9 - 6.2
Hardness: 5½ - 6
Streak: Black
Colour: Tin white to steel white
Solubility: Arsenopyrite is moderately soluble in nitric acid but insoluble in hydrochloric and sulphuric acids
Environments:

Pegmatites (sparingly)
Hydrothermal replacement environments
Hydrothermal vein environments

Arsenopyrite is the most common arsenic-containing mineral. It occurs with tin and tungsten ores in high-temperature hydrothermal deposits, associated with silver and copper ores, galena, sphalerite, pyrite and chalcopyrite. It is frequently associated with gold. It is found in contact metamorphic deposits, and disseminated in limestone.

Common impurities: Ag,Au,Co,Sn,Ni,Sb,Bi,Cu,Pb

Augite

Formula: (Ca,Mg,Fe)2Si2O6 Inosilicate (chain silicate), pyroxene group
Specific gravity: 3.19 - 3.56
Hardness: 5½ to 6
Streak: Greenish gray, light to dark brown
Colour: Black, brown, green
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments (typical)
Pegmatites
Carbonatites
Metamorphic environments

Augite is a primary and secondary mineral and it is the most common pyroxene. It is an important rock-forming mineral, found chiefly in dark coloured igneous rocks. The melting point of pyroxenes is about 1,000oC.

Augite is a common but not essential constituent of gabbro, anorthosite, peridotite, kimberlite, rhyolite, trachyte, andesite and basalt.
It also may be found in granite.

Alteration

ankerite, dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2

augite and CO2 to hypersthene, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe)SiO3 + CaCO3 + SiO2

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2
This reaction occurs at high temperature and pressure.

augite and andalusite to enstatite- ferrosilite and anorthite
Ca(Fe,Mg)Si2O6 +Al2SiO5 → (Mg,Fe2+)SiO3 + Ca(Al2Si2O8)

Fe-rich dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2

hypersthene, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

Common impurities: Ti,Cr,Na,Mn,K

Aurichalcite

Formula: (Zn,Cu)5(CO3)2(OH)6 carbonate
Specific gravity: 3.6 - 4.3
Hardness: 2
Streak: White to light blue
Colour: Light blue, bluish and greenish blue
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal replacement environments
Hydrothermal vein environments

Aurichalcite is a secondary mineral occuring in the oxidation zone of hypothermal (high temperature) veins in copper and zinc deposits.

Common impurities: Ca

Autunite

Formula: Ca(UO2)2(PO4)2.10-12H2 hydrated normal phosphate
Specific gravity: 3.2
Hardness: 2 - 2½
Streak: White to yellowish
Colour: Yellow, due to the uranyl ion UO2+2
Solubility: Autunite is moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal vein environments

Autunite is a secondary mineral found chiefly in the zone of oxidation and weathering derived from the alteration of uraninite or other uranium minerals in hypothermal (high temperature) veins.

Common impurities: Ba,Mg

Axinite

Axinite represents three minerals:
Axinite-(Fe) Ca2(Fe2+Al2BSi4O15(OH)
Axinite-(Mg) Ca2MgAl2BSi4O15(OH) and
Axinite-(Mn) Ca2(Mn2+Al2BSi4O15(OH)
All of these are sorosilicates (Si2O7 groups) with additional (OH) Specific gravity: 3.3
Hardness: 6½ - 7
Streak: White
Colour: Brown, grey, violet, blue, greenish
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments

Axinite occurs in cavities in granite, in pegmatites, and in the contact zones surrounding granitic intrusions.

Common impurities for Axinite-(Mg): Ti,V,Mn,Zn,K,H2O;
Common impurities for Axinite-(Mn): Ti,Fe,Mg,Na,K, H2O

Azurite

Formula: Cu3(CO3)2(OH)2 anhydrous carbonate containing hydroxyl
Specific gravity: 3.77
Hardness: 3½ to 4
Streak: Light blue
Colour: Deep blue
Solubility: Azurite is moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Carbonatites
Hydrothermal replacement environments

Azurite is less common than malachite but has the same origin and associations. It is a secondary mineral found largely in the oxidation portions of high temperature copper deposits.

It may be found in gneiss.

Alteration

Azurite is formed by the action of carbonated water on copper-containing minerals, or from copper-containing solutions, such as CuSO4 or CuCl2 reacting with limestone.

azurite and H2O to malachite and CO2
2Cu3(CO3)2(OH)2 + H2O → 3Cu2(CO3(OH)2 + CO2
Azurite is unstable under atmospheric conditions, and slowly converts to the more stable malachite according to the above reaction.

Bariopharmacosiderite

Formula: Ba0.5Fe3+4(AsO4)3(OH)4.5H2O

Baryte

Formula: Ba(SO4) sulphate
Specific gravity: 4.48
Hardness: 3 - 3½
Streak: White
Colour: Colourless, white, yellowish, reddish, blue
Solubility: Insoluble in water, hydrochloric and nitric acid; soluble in sulphuric acid if heated
Environments:

Carbonatites (typical)
Clastic sedimentary environments
Hydrothermal replacement environments
Hydrothermal vein environments (typical)

Baryte is a common and widely distributed mineral. It is a typical mineral in epithermal (low temperature) and mesothermal hydrothermal veins in limestone with calcite and associated with ores of silver, lead, copper, cobalt, manganese and antimony. It is also found as residual masses in clay overlying limestone. Also as concretions in sandstone. and other sedimentary rocks. In places acts as a cement in sandstone but it may also be found as lenses or replacement deposits in sedimentary rocks, both of primary and secondary origin.

Alteration

calcite, Ba2+, H2S and O2 to baryte, Ca2+, CO2 and H2O
CaCO3 + Ba2+ + H2S + 2O2 = BaSO4 + Ca2+ + CO2 and H2O
Baryte may be precipitated by the action of ore fluid and groundwater on calcite.

Localities

Iran

At Mount Kahoven, Semnan Province, snow white crystals of baryte are found on quartz coloured deep red by hematite. The lack of hematite staining the baryte shows that it, the baryte, was formed later than the quartz.

Bastnäsite

Formula:
Bastnäsite-(Ce): Ce(CO3)F the most common member of the bastnäsite group
Bastnäsite-(La): La(CO3)F
Bastnäsite-(Nd): Nd(CO3)F
Bastnäsite-(Y): Y(CO3)F
All of these are anhydrous carbonates containing halogen
Bastnäsite forms oriented overgrowths and alteration pseudomorphs after fluocerite. Crystal axes of both species oriented in parallel position
Specific gravity: 4.9 to 5.2
Hardness: 4 to 5
Streak: White
Colour: Yellow, reddish-brown; colourless to light yellow in transmitted light; bastnäsite-(Nd) may be pale purplish pink to colourless
Solubility: Bastnäsite-(Ce) is soluble in strong, hot acids
Environments:

Pegmatites
Metamorphic environments

Bastnäsite-(Ce) occurs in contact metamorphic amphibole skarn; bastnäsite-(Nd) occurs in the outer rim zone in crystals of bastnäsite-(Ce), found in cavities in yttrian fluorite, and in granite pegmatites

Bauxite

Bauxite is not a mineral, but a mixture of iron and aluminium oxides and hydroxides which is the principal ore of aluminium. It is almost always a residual soil rather than a sediment. Major constituents are gibbsite, böhmite and diaspore.

Bayldonite

Formula: Cu3PbO(AsO3OH)2(OH)2

Beryl

Formula: Be3Al2Si6O18 cyclosilicate (ring silicate)
Specific gravity: 2.63 - 2.92
Hardness: 7½ to 8
Streak: White
Colour: Pure beryl is colourless, but it occurs in many different colours due to impurities. Goshenite is the colourless, gemmy variety of beryl. The blue-green colour of aquamarine is caused by trivalent iron Fe3+ and divalent iron Fe2+ in different positions in the crystal lattice. The golden yellow colour of heliodor is caused by trivalent iron Fe3+; the green of emerald is caused by chromium and/or vanadium; the pink of morganite is caused by small amounts of manganese and the deep red of red beryl is also caused by trivalent manganese Mn3+.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites (typical)
Metamorphic environments

Beryl, although containing the rare element Be, is rather common and widely distributed. It occurs usually in granitic rocks, or in pegmatites. It is also found in mica schist of regional metamorphic rocks.

In pegmatites, associations include quartz, microcline, albite, muscovite, biotite, members of the columbite-tantalite series, cookeite (Al,Li)3Al2(Si,Al)4OSub>10(OH)8, tourmaline, lepidolite, topaz and spessartine.
In medium-temperature metamorphic deposits beryl is associated with topaz, cassiterite and ferberite- hübnerite MnWO4;
in Alpine and hydrothermal veins it is found with quartz and feldspar.

Red beryl is found in topaz rhyolites with topaz, high-temperature quartz and bixbyite Mn3+2O3.

Localities

Australia

At Tom's quarry, South Australia, beryl occurs intergrown with childrenite Fe2+Al(PO4)(OH)2.H2O, variscite Al(PO4).2H2O and strontium and iron bearing crandallite.

Canada

At Airy Creek, British Columbia, aquamarine (a variety of beryl) occurs in a granite pegmatite dyke that cuts high-grade metamorphic gneiss.

At Lened, Tungsten, Northwest Territories, Canada, emerald (a variety of beryl) occurs in a series of vuggy quartz/carbonate veins within a calc-silicate skarn. The colour is probably due to traces of vanadium.

At the Little Nahanni Pegmatite Group, Tungsten, Northwest Territories, Canada, goshenite (a variety of beryl) occurs within a series of lithium-bearing pegmatites.

At Mountain River, Mackenzie Mountains, Northwest Territories, Canada, emerald occurs in quartz-plagioclase-carbonate veins hosted in shale, siltstone and sandstone. The colour is due to chromium Cr and vanadium V.

At Port Joli, Nova Scotia, Canada, beryl occurs embedded in pegmatite within a biotite granite.

The Taylor pegmatite, Ontario, Canada, intrudes altered ultramafic rocks, which are the likely source of chromium which causes the green colour of the emerald which is found there.

China

At Pingwu County, Sichuan, China, beryl is associated with tin and tungsten minerals.

USA

In New York City beryl crystals are found in pegmatites that cut schist and gneiss, and also frozen in a smoky quartz matrix.

The consolidated Quarry at Maine, USA, is in a simple granite pegmatite, enriched in lithium in some zones. These have cavities that contain albite, muscovite and beryl, together with other minerals. The paragenesis for the beryl alteration is
beryl → beryllonite NaBe(PO4) → moraesite Be2(PO4) (OH).4H2O → hydroxylherderite CaBe(PO4)(OH) → fluorapatite → greifensteinite Ca2Be4Fe2+5(PO4)6(OH)4.6H2O.
Late-stage low-temperature aqueous fluids likely caused partial dissolution of primary beryl resulting in the formation of hydroxylherderite and other secondary beryllium phosphates.

At Yucca Valley, California, aquamarine occurs in pockets in pegmatite associated with cleavelandite and smoky quartz.

In the Sierrita Mountains, Arizona, aquamarine occurs in pegmatites embedded in white quartz enclosed by feldspar, with quartz and mica or in contact with biotite.

At Stoneham, Maine, beryl occurs mostly in solid pegmatites, and only rarely in pockets. When it is in quartz it is aquamarine, but the beryl occurring in feldspar is common beryl.

Alteration

At high temperature and pressure beryl commonly alters to different secondary minerals, depending on the pH.
At pH 2 to 3 (strongly acid) quartz is the dominant alteration product.
At pH 4 to 5 bertrandite Be4Si2O7(OH)2, euclase BeAlSiO4(OH) or phenakite are formed.
Near the neutral pH of 7 bertrandite Be4Si2O7(OH)2 or bavenite Ca4Be2+xAl2-xSi9O26-x(OH)2+x, with x between 0 and 1, are produced.
At pH of 8 to 9 (alkaline) bavenite, milarite KCa2(Be2AlSi12)O30.H2O (not to be confused with millerite) or bityite CaLiAl2(Si2BeAl)O10(OH)2 are produced.
At pH 10 to 11 (strongly alkaline) epididymite or eudidymite, both Na2Be2Si6O15.H2O, are produced.
At high temperature and pressure beryl becomes unstable and breaks down into chrysoberyl, phenakite and quartz.

Common impurities: Fe,Mn,Mg,Ca,Cr,Na,Li,Cs,O,H,OH,H2O,K,Rb

Beudantite

Formula: PbFe3+3(AsO4)(SO4)(OH)6 compound arsenate
Beudantite Group > Alunite Supergroup
Specific gravity: 4.48
Hardness: 3½ to 4½
Streak: Black to grey
Colour: Dark green, brown, black
Solubility: Soluble in HCl.
Beudantite is a secondary mineral occurring in the oxidised zones of polymetallic deposits. Some beudantite may contain minor antimony Sb replacing iron Fe.

Localities

Germany

At the Clara Mine, in the Black Forest, beudantite is common, lining cavities in baryte or quartz, and associated with segnitite, kintoreite, corkite and dusserite.

United Kingdom

At Burdell Gill, Cumbria, beudantite is uncommon, but it sometimes occurs on baryte or quartz associated with mimetite and carminite.

At Roughton Gill, Cumbria, beudantite occurs as crusts on quartz, and also in cellular quartz veinstone, with baryte and lepidocrocite. It is sometimes associated with carminite.

At Sandbed, Cumbria, beudantitee occurs as a encrustation on mimetite with scorodite and pharmacosiderite. A mineral intermediate between beudantite and segnitite occurs in cavities in quartz, apparantly formed by the oxidation of primary arsenopyrite.

At Short Grain, Cumbria, beudantite occurs on quartz or baryte associated with mimetite, arseniosiderite and carminite. It sometimes forms pseudomorphs after mimetite. Other associates include supergene baryte and bariopharmacosiderite.

At Silver Gill, Cumbria, beudantite occurs coating mimetite crystals.

USA

At the San Rafael Mine, Nevada, beudantite occurs in boxwork limonite associated with segnitite, mimetite and adamite, and also within quartz-lined vugs.

Common impurities: Al,P

Biotite

Biotite refers to dark mica minerals, such as phlogopite KMg3(AlSi3O10)(OH)2 phyllosilicate (sheet silicate)
In the discontinuous branch of the Bowen reaction series biotite is intermediate between amphibole (higher temperature) and orthoclase (lower temperature, after joining with the continuous branch).
Environments:

Plutonic igneous environments (typical)
Volcanic igneous environments
Pegmatites
Metamorphic environments

Solubility: Slightly soluble in sulphuric acid

In metamorphic environments biotite is formed over a wide range of temperature and pressure conditions, in both regionally metamorphosed and contact metamorphosed rocks.
Typical associations are biotite with chlorite and biotite with muscovite.
Biotite is a primary and secondary mineral.
Biotite is an essential constituent of phyllite.
It is a common constituent of granite, diorite, rhyolite, andesite and gneiss.
It also may be found in syenite, gabbro, trachyte, basalt, limestone, dolostone and hornfels.
Biotite is characteristic of the greenschist facies and it is also a mineral of the albite-epidote-hornfels, hornblende-hornfels, pyroxene-hornfels and amphibolite facies.
It never occurs in the sanidinite facies

Alteration

biotite and SiO2 to enstatite- ferrosilite, orthoclase and H2O
K(Mg,Fe)3(AlSi3O10)(OH)2 + 3SiO2 → 3(Mg,Fe2+)SiO3 + KAlSi3O8 + H2O
enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction.

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌ K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage.

Bismuth

Formula: Bi native element
Specific gravity: 9.7 - 9.8
Hardness: 2 - 2½
Streak: Lead grey
Colour: Silver white
Solubility: Slightly soluble in hydrochloric acid, readily soluble in sulphuric and nitric acid
Environments:

Pegmatites
Hydrothermal replacement environments
Hydrothermal vein environments

Bismuth occurs in pegmatites and as an unaltered primary mineral in hypothermal, (high temperature) tin ore veins.

Common impurities: Fe,Te,As,S,Sb

Bismuthinite

Formula: Bi2S3 sulphide
Specific gravity: 6.8 - 7.2
Hardness: 2
Streak: Grey
Colour: Lead grey to yellowish white
Solubility: Insoluble in hydrochloric acid and sulphuric acid; readily soluble in nitric acid
Environments:

Pegmatites
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Bismuthinite occurs as an unaltered primary mineral in hypothermal (high temperature) hydrothermal veins in tin and silver-cobalt deposits, and more rarely in contact metamorphic deposits and pegmatites.

Common impurities: Pb,Cu,Fe,As,Sb,Se,Te

Bixbyite

Formula: Mn3+2O3 simple oxide
Specific gravity: 4.945
Hardness: 6 to 6½
Streak: Black
Colour: Black
Environments:

Volcanic igneous environments
Metamorphic environments
Hydrothermal environments

Localities

Sweden

At Langban bixbyite occurs in metamorphosed manganese ores.

USA

At the Black Range tin district, New Mexico, it occurs in rhyolite.

At Topaz Mountain, Utah, bixbyite occurs in cavities in rhyolite associated with topaz, spessartine, red beryl and hematite, also with fluorite.

Common impurities: Al,Mg,Si,Ti

Böhmite

Formula: AlO(OH) oxide containing hydroxyl
Specific gravity: 3.03
Hardness: 3½
Streak: White
Colour: White to grey
Environments:

Pegmatites
Metamorphic environments

Böhmite is widely distributed in bauxite. It also may exsolve from corundum.

Borax

Formula: Na2B4O5(OH)4.8H2O hydrated borate containing hydroxyl
Specific gravity: 1.7 - 1.8
Hardness: 2 - 2½
Streak: White
Colour: White, yellowish, seldom bluish, greenish
Solubility: Soluble in water.
Melting point: 878 °C. The melt dissolves numerous metal oxides. Loses 5 mol of water when heated to 100°C, another 4 mol when heated to 150 °C, and the last mol at 400 °C. Rapidly dehydrates in air to tincalconite.
Environments:

Chemical sedimentary environments (typical)
Volcanic sublimates and hot spring deposits

Borax is the most widesppread of the borate minerals. It is formed on the evaporation of enclosed lakes and as an efflorescence on the surface in arid regions. Salt lake beds, hot spring deposits and volcanic vents.

Bornite

Formula: Cu5FeS4 sulphide
Specific gravity:4.9 - 5.3
Hardness: 3
Streak: Dark grey
Colour: Reddish silver grey on fresh break
Solubility: Moderately soluble in nitric acid
Environments:

Pegmatites
Carbonatites
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Bornite is a widely occurring copper ore usually found associated with chalcocite, chalcopyrite, covellite, pyrrhotite, pyrite and other sulphides, especially in the enriched zone of mesothermal (moderate temperature) and hypothermal (high temperature) veins. It is less frequently found as secondary deposits in the oxidation zone of copper veins. It occurs disseminated in ultramafic rocks, in contact and regional metamorphic deposits and in pegmatites.

Alteration

Chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite, covellite and brochantite.

chalcopyrite and chalcocite to bornite
CuFe3+S2 + 2Cu2S = Cu5FeS4

Common impurities: Ag,Ge,Bi,In,Pb

Bournonite

Formula: CuPbSbS3 sulphosalt
Bournonite-Seligmannite Series.
Specific gravity: 5.83
Hardness: 2½ to 3
Streak: Steel grey
Colour: Steel grey
Solubility: Slightly soluble in nitric acid
Environments:

Hydrothermal vein environments

Bournonite is found in mesothermal veins, both as a primary mineral and in the enriched zone

Bradleyite

Formula: Na3Mg(PO4)(CO3) compound phosphate

Brochantite

Formula: Cu4(SO4)(OH)6 sulphate with hydroxyl
Specific gravity: 3.97
Hardness: 3½ - 4
Streak: Green to light green
Colour: Emerald green
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal replacement environments

Brochantite occurs as a secondary mineral in the oxidation zone of high temperature copper ores.

Alteration

Chalcopyrite CuFeS2 (primary) readily alters to the secondary minerals bornite Cu5FeS4, covellite CuS and brochantite Cu4SO4(OH)6.

Bromargyrite

Formula: AgBr anhydrous halide

Brucite

Formula: Mg(OH)2 hydroxide
Specific gravity: 2.03
Hardness: 2½
Streak: White
Colour: White, grey, greenish, bluish, yellow to brown
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Carbonatites
Metamorphic environments

Brucite is found in limestone, marble, dolostone and slate, as an alteration product of serpentine. At Palabora, Limpopo Province, South Africa, it is found in carbonatite.
It is associated with serpentine, dolomite, magnesite and chromite.

Alteration

brucite to periclase and H2O
Mg(OH)2 ⇌ MgO + H2O

forsterite and H2O to serpentine and brucite
2Mg2SiO4 + 3H2O ⇌ Mg3Si2O5(OH)4 + Mg(OH)2
The forward reaction is highly exothermic. Stable equilibrium occurs at 350°C for pressure 0.5 kbar, 380°C for pressure 2.0 kbar, 400°C for 3.5 kbar, 420°C for 5.0 kbar and 430°C for 6.5 kbar.

Burkeite

Formula: Na4(SO4)(CO3) compound sulphate
Specific gravity: 2.57
Hardness: 3Æ
Streak: White
Colour: White, light buff, grayish; colourless in transmitted light
Solubility: Soluble in cold water
Environment:

Chemical sedimentary environments

Burkeite occurs in continental evaporite deposits. At Searles Lake, California, USA, it is associated with gaylussite, trona, sulphohalite, borax and northupite. At the Konya Basin, Central Anatolia, Turkey, burkeite occurs with halite and trona.

Bustamite

Formula: CaMn2+Si2O6 inosilicate (chain silicate) wollastonite group
Specific gravity: 3.32 to 3.43
Hardness: 5½ to 6½
Streak: White
Colour: Pale to medium pink; brownish red; colourless to yellowish pink in transmitted light. Pink colour fades on exposure to sunlight
Solubility: Partly soluble in hydrochloric acid
Environments:

In manganese ores formed by metamorphism of manganese-bearing sediments with attendant metasomatism. Type locality - Carbonate-silicate hosted zinc deposit Bustamite typically results from metamorphism of manganese-bearing sediments. At Franklin, New Jersey, the oldest rocks are Precambrian gneisses of mixed sedimentary and volcanic origin. Franklin Marble was deposited within these rocks, along with sediments containing zinc, manganese and iron minerals. These sediments were metamorphosed later in the Precambrian, then the rocks were uplifted from the late Precambrian into the Cambrian and quartzite was deposited on the eroded surface. In Cambrian-Ordovician time the quartzite was in turn overlain by limestone, and the rocks have been subject to uplift and erosion up to the present time.
Common associates: calcite, diopside, glaucochroite, grossular, johannsenite, rhodonite and wollastonite

Alteration

bustamite, tephroite and calcite to glaucochroite and CO2
CaMn2+Si2O6 + Mn2+2(SiO4) + 2CaCO3 ⇌ 3CaMn2+(SiO4) + 2CO2

Calcite

Formula: CaCO3 carbonate
Specific gravity: 2.7102(2)
Hardness: 3
Streak: White
Colour: Colourless, white, yellow, red, orange, blue, green, pink, purple
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Carbonatites (essential)
Chemical sedimentary environments (typical)
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Calcite is a common and widespread mineral. It occurs in limestone, marble and chalk in all of which it is essentially the only mineral present. It is an important constituent of calcareous marl and calcareous sandstone. Water carrying calcium carbonate in solution and evaporating in limestone caves often deposits calcite as stalagtites, stalagmites and incrustations. Both hot and cold calcium-bearing spring water may form cellular deposits of calcite known as travertine or tufa around their mouths. Calcite occurs as a primary mineral in some igneous rocks such as nepheline syenite, carbonatites and pegmatites. It is a late crystallisation product in cavities in lavas, and it is also a common mineral in the oxidation zone of hypothermal (high temperature), mesothermal (moderate temperature) and epithermal (low temperature) hydrothermal veins associated with sulphide ores.
Carbonates such as calcite are essential constituents of kimberlite.
Calcite is an essential constituent of limestone, marl and skarn.
It is a common but not essential constituent of sandstone.
It also may be found in dolostone.

Calcite may occur in all metamorphic facies with the exception of the very high-grade eclogite facies.

Alteration

Calcite and aragonite are precipitated according to the following reactions:
Carbon dioxide in the atmosphere is dissolved in rainwater forming weak carbonic acid:
H2O + CO2 → H2CO3
Carbonic acid dissolves limestone forming calcium bicarbonate
H2CO3 + CaCO3 → Ca(HCO3)2
This solution percolates into caves where calcium carbonate may be precipitated as calcite with the release of liquid water and gaseous carbon dioxide:
Ca(HCO3)2 ⇆ CaCO3 (solid) + H2O (liquid) + CO2 (gas)
The net effect of these changes could be written as the reversible reaction
CaCO3 (solid) + H2CO3 (in solution) ⇌ Ca2+ + 2(HCO3)-
The forward reaction, solution of calcium carbonate, occurs in acid environments, and the reverse reaction, precipitation of calcium carbonate, occurs in strongly basic (alkaline) environments.

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+[Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O

diopside and calcite
Ca2MgSi2O7 + CO2 ⇌ CaMgSi2O6 + CaCO3
The maximum stability limit of åkermanite in the presence of excess CO2 is about 6 kbar. Below that pressure, at relatively lower temperatures, åkermanite reacts with CO2 to form diopside and calcite according to the reaction:

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above.

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2 In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

anorthite to calcite and kaolinite in the early Earth's atmosphere
CO2 + H2O + anorthite → calcite + kaolinite
CO2 + 2H2O + CaAl2Si2O8 → CaCO3 + Al2Si2O5(OH)4

anorthite, H2O and CO2kaolinite and calcite
2CaAl2 Si2O8 + 4H2O + 2CO2 ⇌ Al4Si4O10(OH)8 + 2CaCO3
Calcite is found as a low-temperature, late-stage alteraation product according to the above reaction.

antigorite and calcite to forsterite, diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 → 4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist at about 3 kbar pressure and 400 to 500oC

aragonite or calcite and Mg2+ (from Mg-rich fluid) to dolomite and Ca2+
2CaCO3 + Mg2+ ⇌ CaMg(CO3)2 + Ca2+

augite and CO2 to hypersthene, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe)SiO3 + CaCO3 + SiO2

bustamite, tephroite and calcite to glaucochroite and CO2
CaMn2+Si2O6 + Mn2+2(SiO4) + 2CaCO3 ⇌ 3CaMn2+(SiO4) + 2CO2

calcite, Ba2+, H2S and O2 to baryte, Ca2+, CO2 and H2O
CaCO3 + Ba2+ + H2S + 2O2 = BaSO4 + Ca2+ + CO2 and H2O
Baryte may be precipitated by the action of ore fluid and groundwater on calcite.

calcite, Fe2+ and Mg2+ to ankerite and Ca2+
4CaCO3 + Fe2+ + Mg2+ = 2Ca(Mg0.5Fe0.5)(CO3)2+ 2Ca2+
Ankerite is believed to be formed from calcite hydrothermally according to the above reaction.

calcite, hematite and quartz to andradite and CO2
3CaCO3 + Fe2O3 + 3SiO2 → Ca3Fe3+2Si3O12 + 3CO2

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

diopside, CO2 and H2O to tremolite, calcite and SiO2
5CaMgSi2O6 + 3CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2
Diopside is produced by the metamorphism of siliceous dolostone, and if water is introduced at a later stage tremolite may be produced from the above reaction, or by the reaction of diopside with dolomite.

dolomite and chert to talc and calcite
3CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Metamorphism of siliceous carbonate rocks causes the formation of hydrous phases such as talc and tremolite. This is a very low-grade metamorphic reaction occurring at temperature between about 150oC and 250oC.

diopside and dolomite to forsterite, calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 → 2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC.

diopside, dolomite, CO2 and H2O to actinolite and calcite
4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3
Diopside is produced by the metamorphism of siliceous dolostone, and if water is introduced at a later stage tremolite may be produced from the above reaction, or by the reaction of diopside with CO2 and H2O.

diopside, dolomite and H2O ⇌ , calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌ 4Mg2SiO4.Mg(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, associated with calcite occurs in a narrow zone in the central parts of the nodules due to the above reaction

diopside, forsterite and calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 + 2CO2
This reaction requires a high temperature.

diopside-hedenbergite and CO2 to enstatite- ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2

dolomite, K-feldspar and H2O to phlogopite, calcite and CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O = KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of phlogopite according to the above equation.

dolomite and quartz to forsterite, calcite and CO2
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3 + 2CO2 In siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.

dolomite, quartz and H2O to tremolite, calcite and CO2
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
This is a metamorphic reaction in dolomitic limestone.

dolomite and tremolite to forsterite, calcite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 → 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
Dolomite can be metamorphosed to talc and calcite, then at higher temperatures the talc and calcite react to form tremolite. In turn tremolite reacts with dolomite to form forsterite, according to the above equation.

enstatite and calcite to forsterite, diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 + 2CaMgSi2O6 + 2CO2
enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

enstatite, calcite and quartz to diopside and CO2
3Mg2Si2O6 + 2CaCO3 + 2SiO2 ⇌ + 2CaMgSi2O6 + 2CO2 enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

ferro-actinolite, calcite and quartz to hedenbergite, CO2 and H2O
Ca2Fe2+5Si8O22(OH)2 + 3CaCO3 + 2SiO2 ⇌ 5CaFe2+Si2O6 + 3CO2 + H2O In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, according to the above reaction, probably due to the instability of ferro-actinolite with rising temperature.

forsterite, calcite and SiO2 to diopside and CO2
Mg2SiO4 + 2CaCO3 + 3SiO2 → 2CaMgSi2O6 + 2CO2
In high temperature environments with excess SiO2 diopside may form accoring to the above reaction.

forsterite, calcite and SiO2 to monticellite and CO2
Mg2SiO4 + 2CaCO3 + SiO2 → 2CaMg(SiO4) + 2CO2

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 ⇌ 3CaMg(SiO4) + 2 CO2
This reaction occurs during contact metamorphism of magnesian limestone.

forsterite and dolomite to calcite and hydroxylclinohumite
4Mg2SiO4 + CaMg(CO3)2 + H2O → Mg9(SiO4)4(OH)2 + CaCO3 + CO2
This is probably the reaction responsible for a forsterite- clinohumite assemblage in silica-rich in the aureole of the Alta granodiorite in Utah, USA.

forsterite, dolomite and H2O to calcite, hydroxylclinohumite and CO2
4Mg2SiO4 + CaMg(CO3)2 + H2O → Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite assemblage in the silica-rich dolomite in the aureole of the Alta granodiorite in Utah, USA, is probably due to the above reaction.


grossular, diopside, monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2 A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises.

hematite, wüstite, quartz and calcite to andradite, hedenbergite magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical. kaolinite, dolomite, quartz and H2O to chlorite, calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2 + 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 + 5CO2
Chlorite often forms in this way from reactions between clay minerals such as kaolinite and carbonates such as dolomite.

laumontite and calcite to prehnite, quartz, H2O and CO2
CaAl2Si4O12.4H2O + CaCO3 → Ca2Al(Si3Al)O10(OH)2 + SiO2 + 3H2O + CO2
Prehnite and pumpellyite form from the Ca zeolites in the presence of calcite, as in the above equation.

monticellite and CO2 to åkermanite, forsterite and calcite
3CaMgSiO4 + CO2 ⇌ Ca2MgSi27 + Mg2O7 + CaCO3
The forward reaction occurs at temperature s above 890oC at and pressure above about 4.3 kbar.

monticellite and spurrite to merwinite and calcite
2CaMg(SiO4) + Ca5(SiO4)2(CO3) ⇌ 2Ca3Mg(SiO4)2 + CaCO3

phlogopite, calcite and quartz to diopside, microcline, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter according to the above reaction.

quartz and calcite to wollastonite and CO2
3SiO2 + 3CaCO3 ⇌ Ca3Si3O9 + 3CO2 (gaseous)
This is a contact metamorphic change occurring at temperatures from about 600°C such as in the immediate border zone of an igneous intrusion into limestone. High pressure inhibits the forward reaction by suppressing the formation of gaseous CO2 talc and calcite to dolomite and quartz
talc + calcite + CO2dolomite + quartz + H2O
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 ⇌ 3CaMg(CO3)2 + 4SiO2 + H2O

talc and calcite to tremolite dolomite, CO2 and H2O
2Mg3Si4O10(OH)2 + 3CaCO3 +4SiO2 → Ca2Mg5Si8O22(OH)2 + CaMg(CO3)2 + CO2 +H2O
This is a low-grade metamorphic change, occurring at temperature between about 250oC and 450oC.

talc, calcite and quartz to tremolite, CO2 and H2O
5Mg3Si4O10(OH)2 + 6CaCO3 +4SiO2 → 3Ca2Mg5Si8O22(OH)2 + 6CO2 +2H2O
Metamorphism of siliceous carbonate rock causes the formation of hydrous phases such as talc and tremolite.

tremolite and calcite to diopside, dolomite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 ⇌ 4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O
The forward reaction is a diopside-forming metamorphic reaction.

tremolite, calcite and quartz to diopside, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2 → 5CaMgSi2O6 + 3CO2 + H2O
This is a medium-grade metamorphic change occurring at temperatute between about 450oC and 600oC.

tremolite and dolomite to forsterite, calcite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 → 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O

tremolite, dolomite and H2O ⇆ hydroxylclinohumite, calcite and CO2
Ca2Mg5Si8O22(OH)2 + 13CaMg(CO3)2 + H2O ⇆ 2(4Mg2SiO4.Mg(OH)2) + 15CaCO3 + 11CO2

Common impurities: Mn,Fe,Zn,Co,Ba,Sr,Pb,Mg,Cu,Al,Ni,V,Cr,Mo

Caledonite

Formula: Cu2Pb5(SO4)3(CO3)(OH)6 compound sulphate
Specific gravity: 5.75 to 5.77
Hardness: 2½ to 3
Streak: Greenish-blue to bluish-white, paler than the sample
Colour: Dark blue to bluish-green; light bluish green in transmitted light
Solubility: Soluble in nitric acid with effervescence

Hydrothermal replacement environments

Caledonite is an uncommon secondary mineral in the oxidised portions of Pb–Cu deposits. Alteration to cerussite has been observed.

Localities

Australia

At Pilbara, Western Australia, caledonite occurs associated with linarite, cerussite, brochantite and malachite. It sometimes forms a massive coating on cerussite.

United Kingdom

Caledonite occurs at many localities in Caldbeck, Cumbria, England.
- At Balliway Rigg it occurs in a quartz-galena matrix associated with leadhillite and its polymorph susannite, and the rare mineral mattheddleite Pb5(SiO4)1.5(SO4)1.5Cl; it also occurs in cavities in vein quartz with linarite, leadhillite and susannite.
- At Brae Fell Mine it occurs associated with linarite, or with leadhillite and cerussite.
- At Driggith Mine it occurs with leadhillite and linarite, or with linarite in oxidation rims surrounding galena.
- At Red Gill Mine it occurs in cavities in quartz associated with leadhillite, linarite, cerussite, anglesite and mattheddleite, Pb5(SiO4)1.5(SO4)1.5Cl which is very rare worldwide, but not uncommon here.

USA

In the Otto Mountains, Baker, California, caledonite is sometimes associated with linarite.

Cancrinite

Formula: Na,Ca,☐)8(Al6Si6)O24 (CO3,SO4)2.2H2O tectosilicate
Specific gravity:
Hardness: 5 to 6
Streak: White
Colour: Grey-green, white, yellow, blue, orange, reddish
Solubility: It is unusual among the silicate minerals in that it is moderately soluble in hydrochloric acid due to the carbonate ions in its structure.
Environments:

Plutonic igneous environments
Pegmatites
Hydrothermal replacement environments

Cancrinite is a primary mineral in some alkaline igneous rocks, including pegmatites in nepheline syenites. It is also an alteration product of nepheline in igneous rocks

Common impurities: Ti,Fe,Mg,K,Cl,S

Carminite

Formula: PbFe3+2(AsO4)2(OH)2

Carpholite

Formula: Mn2+Al2Si2O6(OH)4 inosilicate (chain silicate)
Specific gravity: 3.0
Hardness: 5 -5½
Streak: White
Colour: Straw-yellow
Environments:

Metamorphic environments
Hydrothermal vein environments

Carpholite occurs in hydrothermal veins in tin deposits. It is a mineral of the eclogite facies.

Cassiterite

Formula: SnO2 simple oxide, rutile group
Specific gravity: 6.98 - 7.01
Hardness: 6 to 7
Streak: Brownish white, white, greyish
Colour: Black, yellow, brown, red or white.
Solubility: Slightly soluble in hydrochloric and nitric acid; insoluble in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Clastic sedimentary environments
Placer deposits
Metamorphic environments
Volcanic sublimates and hot spring deposits
Hydrothermal replacement environments
Hydrothermal vein environments

Cassiterite is widely distributed. It occurs as a primary mineral in igneous rocks and pegmatites but it is more commonly found as an unaltered primary mineral in hypothermal (high temperature) hydrothermal veins of tin deposits in or near granitic rocks. Because of its durability it is also found frequently in placer deposits.
Cassiterite is often associated with ferberite, molybdenite and arsenopyrite.

Alteration

Cassiterite may be formed from magmatic gases at high temperature and low pressure according to the equation:
SnCl4 (gaseous) + 2H2O (gaseous) → SnO2 ( solid cassiterite) + 4HCl (gaseous)

Common impurities: Fe,Ta,Nb,Zn,W,Mn,Sc,Ge,In,Ga

Cavansite

Formula: Ca(V4+O)(Si4O10).4H2O phyllosilicate (sheet silicate)
Specific gravity: 2.21 to 2.31
Hardness: 3 to 4
Streak: Bluish white
Colour: Blue to greenish blue
Solubility: Slightly soluble in acids
Environments:

Volcanic igneous environments
Clastic sedimentary environments

Cavansite occurs in zeolites in basalt, and also in Tuff. At Lake Owyhee State Park cavansite occurs in fractures in tuff, associated with zeolites and calcite. Near Pua, India, it occurs in silica-rich breccia.

Celadonite

Formula: KMgFe3+Si4O10(OH)2 phyllosilicate (sheet silicate), mica group
Specific gravity: 2.95 to 3.05
Hardness: 2
Streak: Greenish white
Colour: Blue-green, olive green, apple green
Environments:

Metamorphic environments
Basaltic cavities

Celadonite occurs as vesicle lining and coatings in altered volcanic rocks of intermediate to basic compositions, under low grade metamorphism.
Common impurities: Mn,Ca,Na

Celestine

Formula: Sr(SO4) sulphate
Specific gravity: 3.9 - 4.0
Hardness: 3 - 3½
Streak: White
Colour: Colourless, white, blue, reddish, greenish, brownish
Solubility: Slightly soluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Chemical sedimentary environments
Hydrothermal replacement environments (occasionally)
Basaltic cavities

Celestine is usually found disseminated through limestone or sandstone, or lining cavities in such rocks. It is also found occasionally in hydrothermal replacement environments and in vesicles in vocanic rocks.
It is often associated with calcite, dolomite, gypsum, halite, sulphur and fluorite. Celestine occurs either as a primary precipitate from aqueous solutions or, more usually, by the interaction of gypsum or anhydrite with Sr-rich waters. Beds of celestine are therefore commonly found immediately above or below gypsum or anhydrite deposits.

Cerussite

Formula: Pb(CO3) carbonate
Specific gravity: 6.4 - 6.6
Hardness: 3 - 3½
Streak: White
Colour: Colourless, white, grey, yellow, brown, blackish (from inclusions of galena)
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Environments:

Carbonatites
Hydrothermal replacement environments

Cerussite is generally a secondary mineral that occurs in the oxidation zone of high temperature lead-zinc deposits. It also occurs as alteration pseudomorphs after anglesite, phosgenite, leadhillite, caledonite, hydrocerussite, bournonite, linarite, pyromorphite and vanadinite. and also as substitution pseudomorphs after calcite and sphalerite.

Lead will generally precipitate as primary galena from ore fluids rich in sulphur and lead. Removal of sulphur by precipitation of sulphides, however, may lead ultimately to an ore fluid from which galena cannot be precipitated, even with a high concentration of lead in solution. In these circumstances, cerussite, as well as pyromorphite and anglesite, could be precipitated as a primary mineral.

Alteration

In the oxidation zone of epithermal veins primary galena PbS alters to secondary cerussite PbCO3 or anglesite depending on the acidity. Cerussite forms in more basic (alkaline) environments than anglesite.

Formation of cerussite
Galena may dissolve in carbonic acid from percolating rainwater to form lead ions, Pb2+.
PbS + 2H2CO3 → Pb2+ + H2S + 2HCO3>-
These lead ions may then combine with carbonate ions CO32- to form cerussite, which is virtually insoluble in water and weak acids.
Pb2+ + CO3>2- → PbCO3

cerussite and aqueous H2AsO4-, Cl- and H+ to mimetite and aqueous H2CO3
5PbCO3 + 3H2AsO4- + Cl- + 7H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
or
5PbCO3 + 3HAsO42- + Cl- + 4H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
cerussite and mimetite can co-exist only under basic conditions at rather high PCO2.

Chabazite

The chabazite series comprises five mineral species:
Chabazite-Ca: Ca2[Al4Si8O24]. 13H2O
Chabazite-K: (K2NaCa0.5)[Al4Si8 O24].11H2O
Chabazite-Mg: (Mg0.7K0.5 Na0.1)[Al3Si9O24].10H2O
Chabazite-Na: (Na3K)[Al4Si8O24].11H2 O
Chabazite-Sr: (Sr,Ca)2[Al4Si8O24]. 11H2O
These are all tectosilicates (framework silicates), zeolite group
Specific gravity: 2.08
Hardness: 4½
Streak: White
Colour: Colourless, white, yellow, orange, brown
Solubility: Moderately soluble in hydrochloric acid
Environments:

Pegmatites
Basaltic cavities

Chabazite is found, usually with other zeolites, in cavities in basalt and pegmatites.
It is a low temperature mineral. Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that chabazite crystallised at temperatures from 55oC to 75oC at depths between 50m and 400m.

At Willy Wally Gully, New South Wales, Australia, it typically occurs alone in vesicles in basalt, but occasionally it is associated with apatite, lévyne-Ca Ca3(Si12Al6)O36.18H2O/offretite KCaMg(Si13Al5)O36.15H2O, phillipsite, aragonite and/or calcite. It is only rarely associated with more than one other zeolite in the same vesicle.
At Rollinsons Quarry, Victoria, Australia, late forming chabazite-Ca was found in some cavities in basalt associated with augite and the K-feldspar anorthoclase (Na,K)AlSi3O8.
At a road cut prehnite occurrence on the Russell-Hermon Road, St Lawrence County, New York, chabazite-Ca occurs rarely, associated with quartz and calcite.
At Palabora, South Africa, chabazite-Ca is found associated with analcime, fluorapophyllite and heulandite.

Chalcedony

Chalcedony is a variety of quartz.
Formula: SiO2 tectosilicate (framework silicate)
Specific gravity: 2.6
Hardness: 6½; - 7
Streak: White
Colour: Colourless, white, gray, blue, any colour due to embedded minerals; multicoloured specimens not uncommon.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Clastic sedimentary environments
Chemical sedimentary environments
Metamorphic environments
Hydrothermal vein environments

Chalcedony is deposited from aqueous solutions, and is frequently found lining or filling cavities in volcanic rocks, as crusts in epithermal (low temperature) to mesothermal (medium temperature) hydrothermal veins, as a constituent of silica-rich marine sedimentary rocks, and as nodular concretions and layers in limestone and marl.

Chalcanthite

Formula: Cu(SO4).5H2O sulphate
Specific gravity: 2.2 - 2.3
Hardness: 2Æ
Streak: Blue
Colour: Blue
Solubility: Readily soluble in water, hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal replacement environments

Chalcanthite is a secondary mineral that is found in the oxidation zone of copper deposits, usually in arid regions where it is not dissolved by ground water. It is formed by the oxidation of chalcopyrite and other copper sulphates. It is often found as a post-mining deposit on mine walls, where it crystallises from mine waters.

Common impurities: Fe,Mg,Co

Chalcocite

Formula: Cu2S sulphide
Specific gravity: 5.7 - 5.8
Hardness: 2½ - 3
Streak: Blackish to dark grey
Colour: Dark lead grey to blackish
Solubility: Moderately soluble in nitric acid
Environments:

Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Chalcocite may occur as a primary mineral in veins with bornite, chalcopyrite, enargite and pyrite, but its principal occurrence is as a secondary, supergene mineral in enriched zones of mesothermal (moderate temperature) and hypothermal (high temperature) sulphide deposits. Under surface conditions the primary copper sulphides are oxidised; the soluble sulphates so formed move downwards, reacting with the primary minerals to form chalcocite, enriching the ore in copper. The water table is the lower limit of the zone of oxidation and a chalcocite selfet may form here.

Alteration

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + water → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + water → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulfate is a strong oxidizing agent; covellite is formed from chalcocite by the reaction below.
chalcocite and ferrous sulphate to copper sulphate, ferrous sulphate and covellite
Cu2S + Fe2(SO4)3 → CuSO4 + 2FeSO4 + CuS

chalcopyrite and chalcocite to bornite
CuFe3+S2 + 2Cu2S = Cu5FeS4

Common impurities: Fe

Chalcopyrite

Formula: CuFeS2 sulphide
Specific gravity: 4.1 - 4.3
Hardness: 3½ - 4
Streak: Greenish black
Colour: Brass yellow, often with an iridescent tarnish
Solubility: Moderately soluble in nitric acid
Environments:

Plutonic igneous environments
Carbonatites
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Chalcopyrite is the most widely occurring copper mineral. It is a Primary mineral. In hypothermal (high temperature) and mesothermal (moderate temperature) veins and replacement deposits it occurs associated with galena, sphalerite and dolomite. It may contain gold or silver. Chalcopyrite is often present in large bodies of pyrite. Primary chalcopyrite readily alters to the secondary minerals bornite, covellite and brochantite, and also malachite, azurite, langite and numerous other secondary copper minerals.
It occurs in carbonatite at Palabora, South Africa.

Alteration

Chalcopyrite may alter to covellite.

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + water → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + water → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulfate is a strong oxidizing agent; it oxidises chalcopyrite according to the reaction below.

chalcopyrite and ferric sulphate to copper sulphate, ferrous sulphate and sulphur
CuFeS2 + 2Fe2(SO4)3 → CuSO4 + 5FeSO4 + 2S

chalcopyrite and chalcocite to bornite
CuFe3+S2 + 2Cu2S = Cu5FeS4

Common impurities: Ag,Au,In,Tl,Se,Te

Chlorargyrite

Formula: AgCl chloride
Specific gravity: 5.5 - 5.6
Hardness: 1½
Streak: White to grey
Colour: Colourless, white, yellowish, brownish, grey, black
Melts at 455°
Environments:

Hydrothermal replacement environments
Hydrothermal vein environments

Chlorargyrite is a secondary mineral found in the oxidation zone of silver deposits, especially in arid regions, and in epithermal (low temperature) veins. It is found associated with native silver, cerussite and other secondary minerals.

At Broken Hill, New South Wales, Australia, chlorargyrite occurs in vuggy coronodite Pb(Mn4+6Mn3+2)O16 with goethite, sometimes associated with smithsonite. It is also found with cerussite, dolomite and malachite. At deeper levels it is a late-stage mineral in the arsenate zones, associated with carminite PbFe3+2(AsO4)2(OH)2 and members of the segnitite to beudantite series.

Chlorargyrite and bromargyrite AgBr occur sparingly at the San Rafael Mine, Nevada, USA, associated with mimetite, anglesite and bayldonite Cu3PbO(AsO3OH)2(OH)2.

Common impurities: I

Chlorite

Chlorite refers to a group of minerals, the most common members being
clinochlore: Mg5Al(AlSi3O10)(OH)8 and
chamosite: (Fe2+,Mg,Al,Fe3+)6(Si,Al)4 O10(OH,O)8
phyllosilicates (sheet silicates) chlorite group
Specific gravity: 2.6 - 3.3
Hardness: 2
Streak: Green, more rarely brown
Colour: Dark green to brown
Solubility: Slightly soluble in sulphuric acid
Environments:

Metamorphic environments (typical)
Hydrothermal environments
Basaltic cavities

Chlorite is a common mineral in metamorphic rocks, such as chlorite schist; it is also formed by hydrothermal alteration of igneous rocks often associated with quartz and siderite; it is found in cavities in igneous volcanic rocks and mixed with clay minerals in argillaceous sediments. At Palabora, South Africa, it occurs in mineralised cavities in carbonatite.
Chlorite is an essential constituent of phyllite.
It also may be found in hornfels.
Commonly associated minerals are actinolite and epidote, and an assemblage of quartz, albite, sericite and garnet.
Chlorite is characteristic of the greenschist facies; it is also a mineral of the zeolite, albite-epidote-hornfels, prehnite-pumpellyite, amphibolite and blueschist facies.

Alteration

Chlorite forms as an alteration product of Mg-Fe silicates such as pyroxene, amphibole, biotite and garnet.

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above.

Ca-Fe amphibole, anorthite and H2O to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases

chlorite and quartz to enstatite- ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+2)2Al4Si5O18 + 4H2O
enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation.

epidote and chlorite to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) + Mg5Al2Si3O18(OH)8 → Ca2Mg5Si8O22(OH)2 + 10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the greenschist facies to the amphibolite facies.

kaolinite, dolomite, quartz and H2O to chlorite, calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2 + 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 + 5CO2
Chlorite often forms in this way from reactions between clay minerals such as kaolinite and carbonates such as dolomite.

Common impurities: clinochlore: Cr,Ca; chamosite: Mn,Ca,Na,K

Chloritoid

Formula: Fe2+Al2O(SiO4)(OH)2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.4 - 3.8
Hardness: 6½
Streak: Colourless, green, grey
Colour: Dark green to green-gray or nearly black.
Solubility: Insoluble in water; soluble in sulphuric acid with decomposition
Environments:

Metamorphic environments

Chloritoid is a relatively common constituent of low to medium-grade regionally metamorphosed clay-rich rocks, particularly those rich in aluminium and ferric iron, and poor in calcium, magnesium, potassium and sodium, and it sometimes occurs due to contact metamorphism of marble.
Chloritoid may be found in phyllite, schist and marble.
In regional metamorphic environments it generally occurs as large crystals in a finer-grained matrix in association with muscovite, chlorite, staurolite, garnet and kyanite.
In contact metamorphism of marble, it may be associated with corundum and quartz, and also it sometimes occurs withcorundum in emery deposits.
In metamorphic environments chloritoid first appears at the beginning of the greenschist facies, and it is also a mineral of the blueschist and eclogite facies.

Alteration

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O

Chondrodite

Formula: Mg5(SiO4)2F2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.1 - 3.2
Hardness: 6 - 6½
Streak: White
Colour: Yellow, brown, red, rarely green
Solubility: Slightly soluble in hydrochloric acid
Environments:

Carbonatites (rarely)
Metamorphic environments (typically)

Chondrodite occurs in contact or regionally metamorphosed dolomitic limestone or dolostone, associated with diopside, spinel and phlogopite.
It also may be found in serpentinite associated with magnetite, and in skarn associated with wollastonite, forsterite and monticellite.
At Palabora, Limpopo Province, South Africa, chondrodite occurs atypically in carbonatite, associated with calcite, magnetite, clinochlore and less commonly with iowaite Mg6Fe3+2(OH)16Cl2.4H2O.

Common impurities: Ti,Al,Mn

Chromite

Formula: Fe2+Cr2O4 chromate
Specific gravity: 4.5 - 4.8
Hardness: 5½
Streak: Brown
Colour: Brownish black to iron black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Clastic sedimentary environments
Placer deposits

Chromite is a common constituent of peridotite and other mafic rocks and of serpentine derived from them. It is one of the first minerals to separate from a cooling magma.

Alteration

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4 This reaction occurs at fairly low temperature and pressure.

Common impurities: Mg,Mn,Zn,Al,Ti

Chrysoberyl

Formula: BeAl2O4 multiple oxide
Specific gravity: 3.75
Hardness: 8½
Streak: White
Colour: Green shades, emerald-green, greenish white, yellowish green, greenish brown, yellow, blue
Environments:

Pegmatites
Metamorphic environments
Hydrothermal vein environments

Chrysoberyl is normally found in pegmatites, and rarely in some fluorite-rich veins. The colour-change variety alexandrite is usually found in mica schist.

Localities

Brazil

At Carnaiba the variety alexandrite occurs in schist associated with emerald.

Switzerland

Near St Gothard chrysoberyl occurs in dolostone with corundum.

USA

At many localities in Maine chrysoberyl occurs with columbite, tourmaline, gahnite and beryl.

Common impurities: Fe,Cr,Ti

Chrysocolla

Formula: (Cu2-xAlx)H2-xSi2O5 (OH)4.nH2O phyllosilicate (sheet silicate)
Specific gravity: 2.0 - 2.2
Hardness: 2 - 4
Streak: Greenish white
Colour: Light blue, blue, greenish blue
Solubility: Slightly soluble in hydrochloric, sulphuric and nitric acid. Insoluble in water.
Environments:

Hydrothermal replacement environments

Chrysocolla is a secondary mineral that forms in the oxidation zone of all types of hydrothermal replacement deposits, often encrusting or replacing earlier secondary minerals. It is associated with malachite, azurite, cuprite and native copper.

Chrysotile

Formula: Mg3Si2O5(OH)4 phyllosilicate (sheet silicate), serpentine group
Specific gravity: 2.0 - 2.6
Hardness: 2½ - 4
Streak: White
Colour: Green, grey to black, white, brownish
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid forming an insoluble silica gel
Environments:

Metamorphic environments

Chrysotile is a major constituent of serpentinite, that is usually formed at very low temperature from peridotite by low-grade metamorphism. It occurs with other serpentine minerals such as antigorite and lizardite.

Alteration

See results for serpentine, which is a group of minerals including antigorite, chrysotile and lizardite, all of which share the same formula, although they have slightly different structures.

Cinnabar

Formula: HgS sulphide
Specific gravity: 8.1
Hardness: 2 2½
Streak: Red
Colour: Red
Solubility: Insolubla in hydrochloric acid, sulphuric and nitric acid
Environments:

Metamorphic environments
Volcanic sublimates and hot spring deposits
Hydrothermal vein environments

Cinnabar occurs in the oxidation zone of epithermal (low temperature) hydrothermal veins, at points where volcanic gases issue from the surrounding rocks, and also in hot springs, associated with baryte, native mercury, pyrite, marcasite, opal, quartz, realgar, stibnite, and sulphides of copper. Cinnabar is the most important ore of mercury but is found in quantity at comparatively few locations.

Clinoenstatite

Formula: Mg2Si2O6 inosilicate (chain silicate) pyroxene
Specific gravity:
Hardness: 5 to 6
Streak: White
Colour: Colourless, greenish yellow, yellowish brown, green-brown
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments

Alteration

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4
The reaction can proceed in either direction, depending on the ambient conditions.

Common impurities: Ti,Al,Cr,Fe,Mn,Ca,Na

Clinohumite

Formula: Mg9(SiO4)4F2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.17 to 3.35
Hardness: 6
Streak: White
Colour: Brown, yellow, white, orange, reddish brown.
Environments:

Metamorphic environments

Clinohumite is a product of contact metamorphism.
It may be found in dolostone, kimberlite, limestone and peridotite. Titanium is a minor constituent of clinohumite in most occurrences. Clinohumite is stable throughout the upper mantle to depths of at least 410 km.

Common impurities: Fe,Ti,Al,Mn,Ca,(OH)

Clinoptilolite

Formula:
Clinoptilolite-Ca: Ca3(Si30Al6)O72.20H2O
Clinoptilolite-K: K6(Si30Al6)O72.20H2O
Clinoptilolite-Na: Na6(Si30Al6)O72.20H2O

All are tectosilicates (framework silicates), zeolite group
Specific gravity: 2.1 to 2.2
Hardness: 3½ to 4
Streak: White
Colour: White, Reddish white, red
Environments:

Volcanic igneous environments

Clinoptilolite is most commonly formed as a devitrification product of silicic volcanic glass from tuff (consolidated pyroclastic rock). It also occurs in cavities in rhyolites, andesites, and basalts.

Clinozoisite

Formula: Ca2Al3[Si2O7][SiO4]O(OH) sorosilicate (Si2O7 groups) epidote group
Specific gravity:
Hardness: 7
Streak: Greyish white
Colour: Colourless, green, grey, light green, yellow-green, pink
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Environments:

Metamorphic environments

Clinozoisite is a mineral of the amphibolite facies.

Alteration

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

lawsonite and jadeite to clinozoisite, paragonite, quartz and H2O
4CaAl2(Si2O7)(OH)2.H2 + NaAlSi2O6 ⇌ 2Ca2Al3[Si2o7][SiO4]O(OH) + NaAl2(Si3Al)O10(OH)2 + SiO2 +6H2
Clinozoisite and paragonite may have been derived from lawsonite by the above reaction.

Common impurities: Ti,Fe,Mn,Mg

Coesite

Formula: SiO2 A high-pressure modification of SiO2 tectosilicate (framework silicate)
Specific gravity: 2.92
Hardness: 7½ - 8
Streak: White
Colour: Colourless
Environments:

Metamorphic environments

Coesite may be found in eclogite and kimberlite.
It is a mineral of the blueschist and eclogite facies.

Alteration

Coesite is the high pressure polymorph of quartz. It is formed at very high pressures, from 20 to 40 kbar, depending on temperature.

Common impurities: Al,Fe,Na

Colemanite

Formula: CaB3O4(OH)3.H2O borate
Specific gravity: 2.4
Hardness: 4½
Streak: White
Colour: Colourless, white
Solubility: Slightly soluble in water
Environments:

Chemical sedimentary environments

Colemanite occurs in borax lake bed deposits and sediments, in arid alkali environments. Ulexite and borax are usually associated, and colemanite is believed to have originated by their alteration.

Columbite

There are three columbites:
columbite-(Fe): Fe2+Nb2O6,
columbite-(Mg): MgNb2O6, and
columbite-(Mn): Mn2+Nb2O6
All of these are multiple oxides containing niobium, and members of the columbite group

Specific gravity: 5.3 - 8.1
Hardness: 6
Streak: Brown to black
Colour: Brownish black to black
Solubility: Insoluble in hydrochloric and nitric acid; slightly soluble in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites

Columbite occurs in granitic rocks and pegmatites, associated with quartz, feldspars, mica, tourmaline, beryl, spodumene, cassiterite, hübnerite-ferberite, microlite and monazite.

Cookeite

Formula: (Al,Li)3Al2(Si,Al)4O10(OH)8 phyllosilicate (sheet silicate) chlorite group
Specific gravity: 2.58 to 2.69
Hardness: 2½ to 3½
Streak: White
Colour: White, yellowish green, pink, brown. Colourless when pure.
Solubility: Insoluble in common acids; soluble in HF
Environments:

Pegmatites
Hydrothermal environments
Hydrothermal vein environments

Cookeite occurs as late stage mineralisation in gem-pocket-bearing granite pegmatites associated with cleavelandite, elbaite, lepidolite and spodumene. It has also been found as pseudomorphs of spodumene, petalite and elbaite. Cookeite also occurs in some simple quartz-crystal lined veins, and in hydrothermally altered sedimentary rocks with kaolinite, diaspore, böhmite, illite, sandstone and bauxite.

Alteration

Cookeite has been synthesised at 260 to 480oC, 1-14 kbar pressure.
It can be formed by the alteration of kaolinite and diaspore by lithium-bearing solutions, releasing quartz.

Common impurities: Fe,Mn,Mg,Ca,Na,K

Copper

Formula: Cu native element
Specific gravity: 8.93
Hardness: 2½ - 3
Streak: Copper red
Colour: Copper red
Solubility: Slightly soluble in hydrochloric acid; moderately soluble in sulphuric acid; readily soluble in nitric acid
Environments:

Hydrothermal replacement environments
Basaltic cavities

Small amounts of native copper have been found at many localities in the oxidation zones of copper deposits, associated with cuprite, malachite and azurite. These minerals may be carried in solution down to the enriched zone, where secondary copper may be redeposited. Deposits of native copper are also found in cavities in basalt lavas, resulting from the reaction of hydrothermal solutions with iron oxide minerals.

Cordierite

Formula: Mg2Al4Si5O18 cyclosilicate (ring silicate)
Specific gravity: 2.5 - 2.8
Hardness: 7 - 7½
Streak: White
Colour: Grey, blue to violet, yellow to brown, greenish, also colourless
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Metamorphic environments (typical)

Cordierite is a common constituent of contact and regionally metamorphosed argillaceous (clay-rich) rocks. It is especially common in hornfels, and it is also found in regionally metamorphosed cordierite-garnet- sillimanite gneiss and schist.
Cordierite may be found in granite, gabbro, phyllite, schist and gneiss. It is also found in river gravel.
Common associations are sillimanite and spinel, or spinel, plagioclase feldspar and orthopyroxene. Cordierite associated with anthophyllite has been described from several localities.
Cordierite is a mineral of the hornblende-hornfels, pyroxene-hornfels, amphibolite and granulite facies.

Alteration

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases

chlorite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
Enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation.

Fe rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel- hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, in an SiO2 deficient environment the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, andalusite and SiO2 to Fe-rich cordierite
2(Mg,Fe2+)SiO3 + 2Al2SiO5 + SiO2 → (Mg,Fe2+)2Al4Si5O18
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

gedrite-ferro-gedrite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al4Si6O22(OH)2 + 2Si2 → 3(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 + H2O In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction.

talc and kyanite to cordierite, corundum and H2O
2Mg3Si4O10(OH)2 + 7Al2OSiO4 ⇌ 3Mg2Al4Si5O18 + Al2O3 + 2H2O

Common impurities: Mn,Fe,Ti,Ca,Na,K

Corkite

Formula: PbFe3+3(SO4)(PO4)(OH)6

Corundum

Formula: Al2O3 oxide
Specific gravity: 3.98 - 4.1
Hardness: 9
Streak: White
Colour: Colourless, blue, red, pink, yellow, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Clastic sedimentary environments
Placer deposits
Metamorphic environments

Corundum is common as an accessory mineral in contact and regionally metamorphosed rocks, such as limestone, mica-schist and gneiss. It is also found as an original constituent of silica deficient igneous rocks such as syenite and nepheline syenite, and in pegmatites. It may be found in large masses in the zone separating peridotite from the adjacent country rock. It is found frequently as a detrital material in sediments, preserved through its hardness and chemical inertness. It also may be found in eclogite and gneiss.
Commonly associated minerals are chlorite, mica, olivine, serpentine, magnetite, spinel, kyanite and diaspore. Quartz never occurs with corundum.
It is a mineral of the granulite facies.

Alteration

muscovite to corundum, K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 + K(AlSi3O8) + H2O
This reaction takes place above temperatures ranging from 600oC at atmospheric pressure to about 720oC at pressure above 4 kbar.

staurolite, annite and O2 to hercynite, magnetite, muscovite, corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O

talc and kyanite to cordierite, corundum and H2O
2Mg3Si4O10(OH)2 + 7Al2OSiO4 ⇌ 3Mg2Al4Si5O18 + Al2O3 + 2H2O

Covellite

Formula: CuS sulphide
Specific gravity: 4.68
Hardness: 1½ - 2
Streak: Blue-black
Colour: Blue-black
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Environments:

Metamorphic environments
Hydrothermal replacement environments
Volcanic sublimates and hot spring deposits (very rarely)

Covellite is not an abundant mineral; it is usually found as a secondary a href="#copper"target="_self">copper mineral in copper deposits, more rarely as a primary mineral, and only very rarely as a volcanic sublimate. It is found in the enrichment zone of most copper deposits, usually as a coating, associated with other copper minerals, principally chalcocite, chalcopyrite, bornite and enargite, and is derived from them by alteration.

Alteration

Covellite may occur as an alteration product of chalcopyrite.

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + water → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + water → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulfate is a strong oxidizing agent and attacks both chalcocite and covellite.

chalcocite and ferrous sulphate to copper sulphate, ferrous sulphate and covellite
Cu2S + Fe2(SO4)3 → CuSO4 + 2FeSO4 + CuS

covellite and ferric sulphate to ferrous sulphate, copper sulphate and sulphur
CuS + Fe2(SO4)3 → 2FeSO4 + CuSO4 + S
Covellite is further oxidised according to the above reaction to form sulphur.

Common impurities: Fe,Se,Ag,Pb

Crandallite

Formula: CaAl3(PO4)(PO3OH)(OH)6 hydrated phosphate containing hydroxyl
Specific gravity: 2.92
Hardness: 5
Streak: White
Colour: Yellow, white, gray; colourless in transmitted light.
Solubility: Soluble with difficulty in acids.
Environments:

Pegmatites
Sedimentary environments

Crandallite is a secondary mineral that occurs in iron-rich, silica-poor phosphate deposits and as an alteration product in pegmatites.

Localities

USA

Crandallite at the Bone Valley Formation, Florida, is associated with clay, millisite and wavellite.

At the Brooklyn Mine, Utah, it is associated with quartz and baryte.

At fairfield, Utah, crandallite is associated with millisite, wardite, variscite, gordonite, englishite, montgomeryite, overite, kolbeckite and apatite.

Alteration

Crandallite may be found as a pseudomorph after gordonite.

Common impurities: Sr and Ba substituting for Ca and Fe3+ for Al.

Cristobalite

Formula: SiO2 tectosilicate (framework silicate)
Cristobalite is a polymorph of quartz that exists both as α and as β phases. β-cristobalite is the stable form of SiO2 from 1,460oC to the melting point, 1,728oC. It exists as a metastable phase below 1,470oC because the transition to tridymite proceeds very slowly. The transformation from β to α-cristobalite occurs at 268oC for pure cristobalite, but may be as low as 175oC if a high level of impurities exists.
Specific gravity: 2.2 to 2.33
Hardness: 6½
Streak: White
Colour: Colorless, white, also blue grey, brown, grey, yellow
Environments:

Volcanic igneous environments
Metamorphic environments

Cristobalite occurs both as α and as β forms as a component of opal. Crystalline material occurs in silica-rich volcanic rocks. It is characteristic in vesicles in rhyolite, andesite and trachyte, where it is associated with tridymite, quartz, sanidine, pyroxene, fayalite and magnetite, and also in basalt. It is also found in thermally metamorphosed sandstone.
Common impurities: Fe,Ca,Al,K,Na,Ti,Mn,Mg,P

Crocoite

Formula: Pb(CrO4) chromate
Specific gravity: 5.9 - 6.0
Hardness: 2½; - 3
Streak: Orange
Colour: Red to orange
Solubility: Slightly soluble in hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal replacement environments

Crocoite is a rare mineral found in the oxidation zone of high temperature lead deposits in those regions where lead veins have traversed rocks containing chromite. Associated with pyromorphite, cerussite and wulfenite.

Common impurities: Zn,S

Cronstedtite

Formula:(Fe2+,Fe3+)3(Si,Fe3+)2O5(OH)4 pyllosilicate (sheet silicate) serpentine group

Formula:
Specific gravity:
Hardness: 3½
Streak: Dark olive green
Colour: Black, dark brown-black, green-black
Environments:

Hydrothermal vein environments

Cronstedtite is a low temperature hydrothermal product in ore veins, commonly associated with clinochlore, pyrite, quartz, siderite and sphalerite

Alteration

fayalite, H2O and O2 to cronstedtite and magnetite
6Fe2+2(SiO4) + 6H2O + ½O2 = 3Fe3Si2O5(OH)4 + Fe2+Fe3+2O4

Common impurities: Al,Ca

Cummingtonite

Formula: ☐Mg2Mg5Si8O22(OH)2 inosilicate (chain silicate) amphibole
Cummingtonite is a dimorph of anthophyllite.
Hardness: 5 to 6
Streak: not determined
Colour: Translucent dark green, brown, grey, colourless
Environments:

Metamorphic environments

Cummingtonite is found in both contact and regional metamorphic rocks

Alteration

cummingtonite-grunerite and H2O to serpentine and SiO2
6(Fe,Mg)7Si8O22(OH)2 + 22H2O ⇌ 7(Fe,Mg)6Si4O10(OH)8 + 20SiO2

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
(Fe,Mg)7Si8O22(OH)2 + (Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)SiO3 + H2O

enstatite and H2O to forsterite and cummingtonite
9MgSiO3 + H2O = Mg2SiO4 + Mg2Mg5Si8O22(OH)2
Cummingtonite may be formed by retrograde metamorphism according to the above reaction.

enstatite-ferrosilite, SiO2 and H2O to cummingtonite-grunerite
7(Mg,Fe2+)SiO3 + SiO2 + H2O ⇌ (Fe,Mg)7Si8O22(OH)2

Common impurities: Mn,Ca,Al,Ti,Na,K

Cuprite

Formula: Cu2O oxide
Specific gravity: 6.15
Hardness: 3½ - 4
Streak: Brownish red
Colour: Red
Solubility: Moderately soluble in hydrochloric acid; slightly soluble in sulphuric and nitric acid
Environments:

Hydrothermal replacement environments

Cuprite is a secondary mineral found in the oxidation portions of high temperature copper deposits, associated with limonite and secondary copper minerals such as copper, malachite, azurite and chrysocolla.
It may be found as an oxidation product coating native copper, or as an alteration product of chalcopyrite. Cuprite crystals are found rimmed with green malachite at the Cotopaxi Mine, Colorado, overgrown with goethite at the Chino Mine, New Mexico and pseudomorphed by shattuckite at the Mashamba West Mine, Democratic Republic of Congo.

Cyanotrichite

Formula: Cu4Al2(SO4)(OH)12(H2O)2 sulphate
Specific gravity: 3.7 - 3.9
Hardness: 3½ to 4
Streak: Blue
Colour: Sky blue
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Hydrothermal replacement environments

Cyanotrichite is a secondary copper mineral found sparsely in the oxidation zones of hypothermal (high temperature) copper-bearing ore bodies rich in aluminium and sulphate. Associated minerals include brochantite, spangolite, chalcophyllite, olivenite, tyrolite, parnauite, azurite and malachite.

Dachiardite

Dachiardite is a series between three minerals:
Dachiardite-Ca: Ca2(Si20Al4)O48.13H2O
Dachiardite-K: K4(Si20Al4)O48.13H2O
Dachiardite-Na: Na4(Si20Al4)O48.13H2O
These minerals are tectosilicates (framework silicates), zeolite group
Specific gravity: 2.14 to 2.21
Hardness: 4 to 4½
Streak: White
Colour: Colourless, white, pink, orange-red
Solubility: Decomposed by common acids
Environments:

Volcanic igneous environments
Pegmatites
Hydrothermal environments


Dachiardite is a hydrothermal zeolite that generally occurs in silica-rich environments, in late stages of pegmatites and in Si-rich volcanic rocks, but also in basalt and in hydrothermally altered tuffaceous sediments. It may be associated with mordenite, ferrierite and heulandite, which, like dachiardite, are all zeolites. At Yellowstone Park drill holes show temperatures of formation of 100 to 200oC.

Danburite

Formula: CaB2Si2O8 tectosilicate (framework silicate)
Specific gravity: 2.9 - 3.0
Hardness: 7 - 7½
Streak: White
Colour: Colourless, yellowish to dark brown, pink
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Chemical sedimentary environments
Metamorphic environments
Hydrothermal vein environments

Danburite occurs in hypothermal (high temperature) veins, where it is associated with quartz, cassiterite, fluorite and orthoclase, and in contact metamorphic rocks, where it is associated with andradite, wollastonite and sulphides.
Danburite occurs in granite and metamorphosed carbonates and evaporites, in skarn, in marine salt deposits and in alpine crevices.

Common impurities:Fe,Mn,Al,Mg,Sr,Na

Datolite

Formula: CaB(SiO4)(OH) nesosilicate (insular SiO4 groups)
Specific gravity: 2.9 - 3.0
Hardness: 5 - 5½
Streak: White
Colour: Colourless, white, yellow, greenish, seldom grey, reddish
Solubility: Slightly soluble in hydrochloric acid

Environments:

Basaltic cavities

Datolite is a secondary mineral usually found in cavities in basalt lavas and similar rocks, associated with zeolites, prehnite, apophyllite and calcite. It also occurs in limestone skarn, serpentinite, schist, and ore veins.

Common impurities: Mn,Mg,Al,Fe

Dawsonite

Formula: NaAl(CO3)(OH)2 anhydrous carbonate containing hydroxyl
Specific gravity: 2.44
Hardness: 3
Streak: White
Colour: Colourless, white
Solubility: Soluble in acids with effervescence
Environments

Hydrothermal environments

Dawsonite is a low temperature hydrothermal carbonate formed decomposing aluminous silicates.

Localities

Algeria

In Tenes, Chlef Province, it occurs with baryte.

Canada

At McGill University campus, Quebec, Dawsonite is found as coatings on joint surfaces of a feldspar dyke associated with calcite, dolomite, pyrite, galena and manganese oxides.

Germany

In Solingen, North Rhine-Westphalia, it occurs in veins associated with late Tertiary basalt.

Italy

At Santa Fiora, Tuscany, dawsonite is associated with calcite, dolomite, pyrite, fluorite and cinnabar.

Descloizite

Formula: PbZn(VO4)(OH) anhydrous vanadate containing hydroxyl
Specific gravity: 6.2
Hardness: 3 to 3½
Streak: Orange to brownish-red
Colour: Brownish red, red-orange, reddish brown to blackish brown, nearly black
Solubility: Readily soluble in acids
Environments

Hydrothermal environments

Descloizite is a secondary mineral often found in the oxidation zones of metal ore deposits in association with mottramite in ore bodies containing vanadinite, pyromorphite, mimetite and cerussite. It has been found as pseudomorphs after vanadinite.

Localities

Namibia

At Berg Aukas Mine and at Tsumeb, descloizite occure with mottramite in sandy pockets in limestone and dolomite.

USA

At the Mammoth Mine, Arizona, descloizite occurs with mottramite

Common impurities: Cu

Diamond

Formula: C native element
Specific gravity: 3.5 - 3.53
Hardness: 10
Streak: none
Colour: Colourless, yellow, green, pink, blue, black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Placers

Most commonly diamond is found as placers in alluvial deposits where it accumulates because of its inert chemical nature, its great hardness (it is the hardest natural substance known) and its fairly high specific gravity. The rock in which it is found in situ is kimberlite. It is formed deep in the mantle, and is only brought to the surface via kimberlite pipes. It is also found in alluvial deposits, along with quartz, corundum and zircon. With decreasing pressure the diamonds dissolve back into the rock. To occur and survive in a metastable state at the surface they must arrive from depth quickly.
Diamond may be found in kimberlite, and also occasionally in eclogite.

Alteration

Coexistence of diamond and carbonate minerals:
The coexistence of diamond and carbonate minerals in mantle eclogite is explained by the reaction:
dolomite + coesitediopside + diamond + oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2

Diaspore

Formula: AlO(OH) oxide containing hydroxyl
Specific gravity:
Hardness: 6½ to 7
Streak: White
Colour: White, brown, colourless, pale yellow, greyish, greenish grey, lilac, pinkish
Solubility: Insoluble in hydrochloric, sulphuric and nitric acids

Pegmatites
Clastic sedimentary environments
Metamorphic environments

Diaspore occurs most commonly in metamorphic bauxite deposits associated with gibbsite and böhmite. It is found in
- nepheline syenite pegmatites at Ovre Åro, Norway
- emery schist in the Russian Urals
- marble at Campolonga, Switzerland
- a metamorphic bauxite deposit in Mugla Province, Turkey

At the deposit in Turkey, medium to high grade metamorphism changes syenite and nepheline syenite to gneiss, limestone to marble, and mudstone to schist. The diaspore occurs in hydrothermally mineralised fractures formed below 515oC in a metabauxite deposit in marble, associated with calcite, muscovite and chloritoid on a goethite-rich matrix.

Alteration

kaolinite to pyrophyllite, diaspore and H2O
2Al2Si2O5(OH)4 → Al2Si4O10(OH)2 + 2AlO(OH) + 2H2O
In the absence of quartz, kaolinite breaks down on heating according to the above reaction.

pyrophyllite and diaspore to andalusite and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2SiO5 + 4H2O

kaolinite to diaspore, SiO2 and H2O
Al2Si2O5(OH)4 ⇌ 2AlO(OH) + 2SiO2 (aqueous) + H2O
At 1 kbar pressure kaolinite is stable at temperatures less than 300oC; it can be in equilibrium with quartz and water in solutions both saturated and undersaturated with quartz. Diaspore is stable at temperatures less than 400oC but only in solutions undersaturated with quartz. High temperature and low quartz saturation favours the forward reaction.

Common impurities: Fe,Mn,Cr,Si

Diopside

Formula: CaMgSi2O6 inosilicate (chain silicate), one of the most common members of the pyroxene group.
Specific gravity: 3.22 - 3.38
Hardness: 5½ to 6½
Streak: White
Colour: Green, brown, colourless
Melting point: About 1,400oC at atmospheric pressure
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Carbonatites
Metamorphic environments

Diopside is a common metamorphic mineral formed by the metamorphism of siliceous, magnesium-rich limestone or dolostone. It may be found in granite, skarn, marble, eclogite and kimberlite,
It often occurs in marble associated with spinel, phlogopite, tremolite and grossular.
In hornfels of contact and regional metamorphic rocks diopside is found in association with phlogopite, chondrodite and actinolite.
In carbonatites it occurs in association with dolomite, fluorite and andradite.
Other associations include tremolite, scapolite, vesuvianite, garnet and titanite. It is a mineral of the hornblende-hornfels, pyroxene-hornfels, amphibolite and granulite facies.

Alteration

During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite

calcite
Ca2MgSi2O7 + CO2 ⇌ CaMgSi2O6 + CaCO3
The maximum stability limit of åkermanite in the presence of excess CO2 is about 6 kbar. Below that pressure, at relatively lower temperatures, åkermanite reacts with CO2 to form diopside and calcite according to the above reaction.

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan.

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2

antigorite and calcite to forsterite, diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 → 4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist at about 3 kbar pressure and 400 to 500oC

Fe-rich cordierite and diopside-hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2

diopside, CO2 and H2O to tremolite, calcite and SiO2
5CaMgSi2O6 + 3CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2
Diopside is produced by the metamorphism of siliceous dolostone, and if water is introduced at a later stage tremolite may be produced from the above reaction, or by the reaction of diopside with dolomite.

diopside and albite to omphacite and quartz
CaMgSi2O6 + xNaAlSi3O8 ⇌ CaMgSi2O6.xNaAlSi2O6 + SiO2

diopside and dolomite to forsterite, calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 → 2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC.

diopside, dolomite, CO2 and H2O to tremolite and calcite
4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3
2 and H2O.

diopside, dolomite and H2O ⇌ , calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌ 4Mg2SiO4.Mg(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, associated with calcite occurs in a narrow zone in the central parts of the nodules due to the above reaction

diopside, forsterite and calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 + 2CO2
This reaction requires a high temperature.

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2

dolomite and coesite to diopside and diamond and oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2
The coexistence of diamond and carbonate minerals in mantle eclogite is explained by the above reaction.

dolomite and quartz to diopside and CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
In the metamorphism of siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.

dolomite, tremolite and forsterite to diopside and enstatite
tremolite + forsterite ⇌ diopside + enstatite + H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌ 2CaMgSi2O6 + H2O
With increasing temperature tremolite and forsterite react to form diopside and enstatite. At a pressure of 4 kbar this change occurs at 840oC and above, according to the above reaction.

enstatite and calcite to forsterite, diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 + 2CaMgSi2O6 + 2CO2 Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

enstatite, calcite and quartz to diopside and CO2
3Mg2Si2O6 + 2CaCO3 + 2SiO2 ⇌ + 2CaMgSi2O6 + 2CO2 Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure.


enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies.

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

forsterite and åkermanite to diopside and monticellite
Mg2SiO4 + 2Ca2MgSi2O7 → CaMgSi2O6 + 3CaMg(SiO4)

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4
The reaction can proceed in either direction, depending on the ambient conditions.

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8)= Mg2Si2O6 + CaMgSi2O6 + MgAl2O4

forsterite, calcite and SiO2 to diopside and CO2
Mg2SiO4 + 2CaCO3 + 3SiO2 → 2CaMgSi2O6 + 2CO2
In high temperature environments with excess SiO2 diopside may form accoring to the above reaction.

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 = 3CaMg(SiO4) + 2CO2

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 ⇌ 3CaMg(SiO4) + 2 CO2
This reaction occurs during contact metamorphism of magnesian limestone.

grossular, diopside,monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2
A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises.

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical. jadeite, diopside, magnetite and SiO2 to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction.

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure.

monticellite and diopside to åkermanite and forsterite
3CaMgSiO4 + CaMgSi2O6 ⇌ 2Ca2MgSi27 + Mg2O7
Monticellite is stable below 890oC at pressure of about 4.3 kbar.

phlogopite, calcite and quartz to diopside, microcline, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter according to the above reaction.

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 ⇌ Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O + H2O
In lower grade assemblages associated with contact and regional metamorphism serpentine may form tremolite and forsterite according to the above reaction.

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4 This reaction occurs at fairly low temperature and pressure.

tremolite to diopside, enstatite and quartz
Ca2Mg5Si8O22(OH)2 ⇌ 2CaMgSi2O6 + 3MgSiO3 + SiO2 + H2O
At 8 kbar pressure the equilibrium temperature is about 930oC, with the forward reaction favoured by higher temperatures.

tremolite and calcite to diopside, dolomite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 ⇌ 4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O
The forward reaction is a diopside-forming metamorphic reaction.

tremolite, calcite and quartz to diopside, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2 → 5CaMgSi2O6 + 3CO2 + H2O
This is a medium-grade metamorphic change occurring at temperatute between about 450oC and 600oC.

Common impurities: Fe,V,Cr,Mn,Zn,Al,Ti,Na,K

Dioptase

Formula: CuSiO3.H2O cyclosilicate (ring silicate)
Specific gravity: 3.3
Hardness: 5
Streak: Green
Colour: Emerald green
Solubility: Moderately soluble in hydrochloric acid
Environments:

Hydrothermal replacement environments

Dioptase occurs in the oxidation zone of high temperature hydrothermal deposits.

Dolomite

Formula: CaMg(CO3)2 carbonate
Specific gravity: 2.84 - 2.86
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, gray, reddish-white, brownish-white, or pink; colourless in transmitted light
Solubility: Readily soluble in hydrochloric acid
Environments:

Carbonatites (essential)
Clastic sedimentary environments
Chemical sedimentary environments
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Dolomite overwhelmingly occurs in dolostone or in marble formed from the contact metamorphism of dolostone. It is a mineral of the prehnite-pumpellyite, greenschist and amphibolite facies, granulite. It also occurs in hypothermal (high temperature), mesothermal (moderate temperature) and epithermal (low temperature) veins, chiefly in lead and zinc veins in limestone, associated with quartz, fluorite, calcite, baryte, magnesite, sphalerite, siderite, galena, pyrite and chalcopyrite.
Few dolomites are primary in origin. The main exception to this is primary dolomite that forms in evaporites as a relatively late product of seawater evaporation. These primary dolomites, however, are rare.
Sedimentary dolomite results from alteration of calcite and aragonite. In sedimentary dolostone, dolomite is often associated with calcite, aragonite, gypsum, anhydrite and halite.
Although uncommon, when dolomite occurs in altered ultramafic igneous rocks, such as serpentinite, it may be associated with magnesite, serpentine and talc.
Dolomite is an essential constituent of dolostone.
It is a common but not esential constituent of limestone and skarn.
It also may be found in marble and serpentinite.
It is a mineral of the prehnite-pumpellyite, greenschist, amphibolite and granulite facies.

Alteration

ankerite, dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2

aragonite or calcite and Mg2+ (from Mg-rich fluid) to dolomite and Ca2+
2CaCO3 + Mg2+ ⇌ CaMg(CO3)2 + Ca2+ diopside and dolomite to forsterite, calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 → 2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC.

diopside, dolomite, CO2 and H2O to tremolite and calcite
4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O = Ca2Mg5Si8O22(OH)2 + 3CaCO3 Diopside is produced by the metamorphism of siliceous dolostone, and if water is introduced at a later stage tremolite may be produced from the above reaction, or by the reaction of diopside with CO2 and H2O.

diopside, dolomite and H2O ⇌ , calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌ 4Mg2SiO4.Mg(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, associated with calcite occurs in a narrow zone in the central parts of the nodules due to the above reaction

dolomite and chert to talc and calcite
3CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
Metamorphism of siliceous carbonate rocks causes the formation of hydrous phases such as talc and tremolite. This is a very low-grade metamorphic reaction occurring at temperature between about 150oC and 250oC.

dolomite and coesite to diopside, diamond and oxygen
MgCa(CO3)2 + 2SiO2 → CaMgSi2O6 + 2C + 2O2
The coexistence of diamond and carbonate minerals in mantle eclogite is explained by the above reaction.

dolomite, K-feldspar and H2O to phlogopite, calcite and CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O = KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of phlogopite according to the above equation.

dolomite and quartz to diopside and CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
In siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.

dolomite and quartz to forsterite, calcite and CO2
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3 + 2CO2 In siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.

dolomite and quartz to diopside and CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
This reaction is the result of metamorphism of siliceous, Mg-rich limestones or dolostones. dolomite, quartz and H2O to tremolite, calcite and CO2
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
This is a metamorphic reaction in dolomitic limestone

dolomite and tremolite to forsterite, calcite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 → 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
Dolomite can be metamorphosed to talc and calcite, then at higher temperatures the talc and calcite react to form tremolite. In turn tremolite reacts with dolomite to form forsterite, according to the above equation.

forsterite, dolomite and H2O to calcite, hydroxylclinohumite and CO2
A forsterite-clinohumite assemblage in the silica-rich dolomite in the aureole of the Alta granodiorite in Utah, USA, is probably due to the reaction:
4Mg2SiO4 + CaMg(CO3)2 + H2O → Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite assemblage in the silica-rich dolomite in the aureole of the Alta granodiorite in Utah, USA, is probably due to the above reaction.

kaolinite, dolomite, quartz and H2O to chlorite, calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2 + 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 + 5CO2
Chlorite often forms in this way from reactions between clay minerals such as kaolinite and carbonates such as dolomite.

talc and calcite to dolomite and quartz + H2O
talc + calcite + CO2 ⇌ dolomite + quartz
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 ⇌ 3CaMg(CO3)2 + 4SiO2 + H2O

talc and calcite to tremolite dolomite, CO2 and H2O
2Mg3Si4O10(OH)2 + 3CaCO3 +4SiO2 → Ca2Mg5Si8O22(OH)2 + CaMg(CO3)2 + CO2 +H2O
This is a low-grade metamorphic change, occurring at temperature between about 250oC and 450oC.

tremolite and calcite to diopside, dolomite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 ⇌ 4CaMgSi2O6 + CaMg(CO3)2 + CO2 + H2O
The forward reaction is a diopside-forming metamorphic reaction.

tremolite and dolomite to forsterite, calcite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 → 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O

tremolite, dolomite and H2O ⇆ hydroxylclinohumite, calcite and CO2
Ca2Mg5Si8O22(OH)2 + 13CaMg(CO3)2 + H2O ⇆ 2(4Mg2SiO4.Mg(OH)2) + 15CaCO3 + 11CO2

Common impurities: Fe,Mn,Co,Pb,Zn

Duftite

Formula: PbCu(AsO4)(OH) anhydrous arsenate containing hydroxyl
Adelite-Descloizite Group. Conichalcite-Duftite Series. Duftite-Mottramite Series.
Specific gravity: 6.12
Hardness: 4½
Streak: Pale green, white
Colour: Olive-green, grey-green; light apple-green in transmitted light.
Environments:

Hydrothermal environments

Duftite is a secondary mineral that occurs in the oxidised zone of hydrothermal metal ore deposits. At the type locality it is associated with azurite

Localities

United Kingdom

At Arm O'Grain, Cumbria, mottramite and duftite are almost indistinguishable, due to much arsenate substitution for vanadate in mottramite. It occurs with mimetite.

At Driggith and Sandbed Mines, Cumbria, duftite is associated with mottramite.

At Short Grain, Cumbria, duftite occurs rarely in vugs in quartz and baryte, commonly overgrown by mimetite.

USA

At the San Rafael Mine, Nevada, duftite occurs in pods of limonite associated with adamite, segnitite, wulfenite and mimetite.

Dussertite

Formula: BaFe3+3(AsO4)(AsO3OH)(OH)6

Edenite

Formula: NaCa2Mg5(Si7Al)O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.00 to 3.06 (Webmin)
Hardness: 5 to 6
Streak: White
Colour: Green, brown, white
Environments:

Plutonic igneous environments
Metamorphic environments

Edenite occure in plutonic igneous rocks and in medium-grade metamorphic rocks such as marble.

Common impurities: Ti,Mn,K,P

Enargite

Formula: Cu3AsS4 sulphosalt
Specific gravity: 4.4
Hardness: 3½
Streak: Black
Colour: Steel grey to iron black
Solubility: Slightly soluble in nitric acid
Environments:

Hydrothermal replacement environments
Hydrothermal vein environments

Enargite is a comparatively rare mineral, found in vein and replacement deposits formed at moderate temperatures associated with pyrite, sphalerite, bornite, galena, tetrahedrite, covellite and chalcocite. Enargite occurs in arsenic containing copper ore veins.

Common impurities: Sb,Fe,Pb,Zn,Ag,Ge

Englishite

Formula: K3Na2Ca10Al15(OH)7(PO4)21.26H2O hydrated phosphate with hydroxyl
Specific gravity: 2.68
Hardness: about 3
Streak: White
Colour: Colourless to white, grayish green, colourless in transmitted light
Environments:

Pegmatites
Hydrothermal environments

Englishite occurs in variscite nodules at the type locality Clay Canyon, Utah, USA, associated with montgomeryite, wardite, millisite and crandallite. It is also found in the Tip Top pegmatite, South Dakota, USA, associated with crandallite and other phosphates.

Enstatite

Formula: Mg2Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.2 - 3.9
Hardness: 5 to 6
Streak: Grey to white
Colour: Green, brown, white
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Metamorphic environments

Enstatite is a widespread mineral of the pyroxene group. It usually occurs in magnesium- and iron-rich igneous rocks and in both iron and stony meteorites.
In igneous rocks, a high silica melt will give augite, hornblende and enstatite or biotite.
Metamorphic enstatite is a mineral of the granulite facies.

Alteration

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 5Mg2Si2O6 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs as the metamorphic grade decreases from the granulite to the amphibolite facies.

augite and andalusite to enstatite- ferrosilite and anorthite
2Ca(Fe,Mg)Si2O6 + 2Al2SiO5 → (Mg,Fe2+)2Si2O6 + 2Ca(Al2Si2O8)

biotite and SiO2 to enstatite-ferrosilite, orthoclase and H2O
2K(Mg,Fe)3(AlSi3O10)(OH)2 + 6SiO2 → 3(Mg,Fe2+)2Si2O6 + 2KAlSi3O8 + 2H2O
Enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction.

chlorite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → (Mg,Fe2+)2Si2O6 + (Mg,Fe2+2)2Al4Si5O18 + 4H2O
Enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation.

Fe-rich cordierite and diopside-hedenbergite to enstatite-ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 2(Mg,Fe2+)2Si2O6 + 2Ca(Al2Si2O8) + SiO2

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
2(Fe,Mg)7Si8O22(OH)2 + 2(Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)2Si2O6 + 2H2O

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
2Ca(Mg,Fe)Si2O6 + 2CO2 → (Mg,Fe2+)2Si2O6 + 2CaCO3 + 2SiO2

enstatite and H2O to forsterite and cummingtonite
9Mg2Si2O6 + 2H2O = 2Mg2SiO4 + 2Mg2Mg5Si8O22(OH)2
Cummingtonite may be formed by retrograde metamorphism according to the above reaction.

enstatite and calcite to forsterite, diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

enstatite, calcite and quartz to diopside and CO2
3Mg2Si2O6 + 2CaCO3 + 2SiO2 ⇌ + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure.

enstatite-ferrosilite and H2O to serpentine and quartz
3(Mg,Fe2+)2Si2O6 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel-hercynite
5(Mg,Fe2+)2Si2O6 + 10Al2SiO5 → 4(Mg,Fe2+)2Al4Si5O18 + 2(Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, in an SiO2 deficient environment the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, andalusite and SiO2 to Fe-rich cordierite
(Mg,Fe2+)2Si2O6 + 2Al2SiO5 + SiO2 → (Mg,Fe2+)2Al4Si5O18
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)2Si2O6 + 2Ca(Mg,Fe2+)Si2O6 + 2NaAlSi3O8 + 2Ca(Al2Si2O8) + 2H2O ⇌ 2NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 8SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies.

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
(Mg,Fe2+)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)2Si2O6 + 2K(AlSi3O8) + 2H2O ⇌ 2K(Mg,Fe)3(AlSi3O10)(OH)2+ 6SiO2
The forward reaction leads to an amphibolite facies assemblage.

enstatite-ferrosilite, quartz and H2O to cummingtonite- grunerite
7(Mg,Fe2+)2Si2O6 + 2SiO2 + 2H2O ⇌ 2(Fe,Mg)7Si8O22(OH)2

forsterite and CO2 to enstatite and magnesite
2Mg2SiO4 + 2CO2 ⇌ Mg2Si2O6 + 2MgCO3

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8)= Mg2Si2O6 + CaMgSi2O6 + MgAl2O4

forsterite and SiO2 to enstatite
Mg2SiO4 + SiO2 → Mg2Si2O6
Forsterite is not stable in the presence of free SiO2 and will react with it to form enstatite according to the above reaction.

forsterite, enstatite and H2O to serpentine
2Mg2SiO4 + Mg2Si2O6 + 4H2O → 2Mg3Si2O5(OH)4
Serpentine is not stable in the presence of carbon dioxide, and may further react with it to form talc and magnesite.

forsterite and talc to enstatite and H2O
2Mg2SiO4 + 2Mg3Si4O10(OH)2 ⇌ 5Mg2Si2O6 + 2H2O

gedrite-ferro-gedrite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
2(Mg,Fe2+)5Al4Si6O22(OH)2 + 4Si2 → 3(Mg,Fe2+)2Si2O6 + 2(Mg,Fe2+)2Al4Si5O18 + 2H2O
In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction.

olivine and CO2 to enstatite-ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3

olivine and SiO2 to enstatite-ferrosilite
(Mg,Fe)2SiO4 + SiO2 → (Mg,Fe2+)2Si2O6

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure.

tremolite to diopside, enstatite, quartz and H2O
2Ca2Mg5Si8O22(OH)2 ⇌ 4CaMgSi2O6 + 3Mg2Si2O6 + 2SiO2 + 2H2O
At 8 kbar pressure the equilibrium temperature is about 930oC, with the forward reaction favoured by higher temperatures.

tremolite and forsterite to diopside, enstatite and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌ 2CaMgSi2O6 + H2O
With increasing temperature tremolite and forsterite react to form diopside and enstatite. At a pressure of 4 kbar this change occurs at 840oC and above, according to the above reaction.

Common impurities: Fe,Ca,Al,Co,Ni,Mn,Ti,Cr,Na,K

Epidote

Formula: Ca2(Al2Fe3+)[Si2O7] [SiO4]O(OH) sorosilicate (Si2O7 groups) epidote group
Specific gravity: 3.38 - 3.49
Hardness: 6
Streak: White
Colour: Yellowish-green, green, brownish-green, black
Solubility: Slightly soluble in hydrochloric acid; insoluble in sulphuric and nitric acid
Environments:

Pegmatites
Metamorphic environments (typical)
Basaltic cavities

Epidote is a widespread mineral, found in veins and joint fillings in some granitic rocks, in pegmatites, and in contact and regional metamorphic environments. It is a low temperature mineral formed by metamorphism of limestone with calcium-rich garnet, diopside, vesuvianite and calcite.
Epidote may be found in gneiss and hornfels.
It is characteristic of the albite-epidote-hornfels facies and it is also a mineral of the prehnite-pumpellyite, greenschist, amphibolite and blueschist facies.

Alteration

Epidote forms as a reaction product of plagioclase feldspar, pyroxene and amphibole.

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O

Ca-Fe amphibole, anorthite and H2O to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2

epidote and chlorite to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) + Mg5Al2Si3O18(OH)8 → Ca2Mg5Si8O22(OH)2 + 10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the greenschist facies to the amphibolite facies.

epidote and quartz to anorthite, grossular and H2O
4Ca2Al3(SiO4)3(OH) + SiO2 → 5CaAl2Si2O8 + Ca3Al2(SiO4)3 + 2H2O
This reaction occurs as the degree of metamorphism increases

Common impurities: Al,Mg,Mn

Epistilbite

Formula: Ca3[Si18Al6O48].16H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.22 to 2.28
Hardness: 4 to 4½
Streak: White
Colour: Colourless, white, orange, red

Environments:

Volcanic Igneous environments
Metamorphic environments

Epistilbite generally is found in silica-rich basalt and olivine basalt, and also in gneiss. No diagenetic epistilbite has been reported.
In Iceland epistilbite occurs in geothermal wells in basalt at 80 to 160oC.

Common impurities: Fe,Mg,Na,K

Epsomite

Formula: Mg(SO4.7H2O sulphate
Specific gravity: 1.68
Hardness: 2 - 2½
Streak: White
Colour: White, sometimes greenish, reddish, yellowish
Solubility: Effloresces in dry air. Very soluble in water.
Environments:

Volcanic sublimates and hot spring deposits
Hydrothermal replacement environments

Epsomite occurs as an efflorescence on the rocks in mine workings and on the walls of caves. It occurs as deposits in salt springs and ore bodies, deposits in salt lakes and in arid regions in blueschists facies rocks.

Common impurities: Ni,Fe,Co,Mn,Zn

Erionite

The erionites are:
Erionite-Ca, Ca5[Si26Al10O72].30H2O
Erionite-K, K10[Si26Al10O72].30H2O
Erionite-Na, Na10[Si26Al10O72].30H2O
They are all tectosilicates (framework silicates) and members of the zeolite group.
Specific gravity: 2.09 to 2.13
Hardness: 3½ to 4
Streak: White
Colour: Colourless, white, green, grey, orange
Environments:

Volcanic igneous environments
Sedimentary environments
Basaltic cavities

Erionite is common in altered silicic tuff, especially in saline lake deposits, where it crystallises when the pH reaches about 8.5 (moderately alkaline). In this environment trivalent iron may substitute for aluminium in the structure.
Hydrothermal erionite occurs in cavities in volcanic rocks. It is often intergrown with offretite.
Epitaxial overgrowths of erionite on lévyne have been reported .

Localities

Austria

At Pauliberg, Burgenland, erionite needles are found on primary magnetite and hematite in vesicular basalt.

In the basalt at Kollnitz, Carinthia, erionite is found on celadonite associated with pyrite, calcite and phillipsite.

Canada

At Chase Creek, British Columbia, pure erionite without any offretite intergrowth occurs in vesicular basalt, rarely covered by paulingite, harmotome, heulandite and clay.

At Pass Valley, British Columbia, erionite intergrown with clay and associated with clinoptilolite occurs in vesicles in rhyolite.

At Twig Creek, British Columbia, erionite crystals capped by offretite line vesicles in basalt.

Denmark

Erionite covered with chabazite occurs in basalt in the Faroe Islands.

France

Near Araules, Auvergne-Rhône-Alpes, offretite intergrown with small amounts of erionite is found in basalt, associated with phillipsite, chabazite and calcite.

At Mont Semiol, Auvergne-Rhône-Alpes, erionite is found as terminations on offretite crystals in olivine basalt, associateed with mazzite, phillipsite, chabazite, calcite and siderite.

Georgia

At Shurdo, Georgia, erionite occurs in volcanic rock associated with heulandite, chabazite, mordenite, stilbite and opal.

Germany

At Sasbach, Baden-Württemberg, offretite-erionite occurs in vesicles in nepheline-olivine- augite basalt, associated with faujasite and phillipsite.

At Teichelberg, Bavaria, offretite-erionite occurs in cavities in basalt associated with gismondine, phillipsite, calcite and chabazite.

At Wiesau, Bavaria, erionite-offretite occurs in basalt with thomsonite and montmorillonite.

At the Zeilberg Quarry, Bavaria, erionite is found in basalt with dachiardite, clinoptilolite, phillipsite, natrolite, heulandite, thomsonite, laumontite, scolecite, stilbite, gmelinite, chabazite, gismondine, apophyllite, calcite, gyrolite, okenite, garronite and tobermorite.

Near Gedern, Hessen, erionite is found in the extremities of offretite needles in vesicular basalt, associated with phillipsite, chabazite, montmorillonite and, rarely, gismondine.

At Geilhausen, Hessen, offretite covered by erionite occurs in vesicles in olivine basalt, associated with chabazite and phillipsite.

At Hungen, Hessen, erionite-offretite occurs with chabazite, faujasite, ferrierite, lévyne, phillipsite and thomsonite.

At the Ortenberg Quarry, Hessen, erionite is found, rarely, in vesicles in a sandstone xenolith embedded in olivine basalt, with paulingite, clinoptilolite, nontronite, phillipsite, calcite, dachiardite, merlinoite, apophyllite and chabazite.

Italy

At Nuoro, Sardinia, erionite is found as overgrowths on lévyne.

Japan

Erionite on lévyne crystals is found in vesicular basalts at Chojabaru, Nagasaki Prefecture, associated with chabazite and stilbite.

At Narushima, Nagasaki Prefecture, erionite occurs with heulandite in the glassy margins of a rhyolite dyke.

Along the seashore of Maze, Niigata Prefecture, erionite is found associated with chabazite.

New Zealand

At Waitakere, North Island, erionite is found surrounded by lévyne.

At Moeraki, South Island, erionite occurs in altered vesicular basalt associated with chabazite, heulandite and phillipsite.

Russia

In lavas along the Nidym River, Siberia, erionite is associated with heulandite.

Tanzania

At Lake Natron, Arusha Region, eriorite is associated with phillipsite, mordenite, analcime and chabazite.

In the Olduvai Gorge, Arusha Region, erionite is abundant in sedimentary beds associated with phillipsite, clinoptilolite, chabazite and analcime.

United Kingdom

At Giant's Causeway, County Antrim, Northern Ireland, erionite occurs in basalt associated with heulandite and phillipsite.

Erionite is often intergrown with offretite on lévyne at many localities in County Antrim, Northern Ireland.

At the Storr, Isle of Skye, Scotland, erionite occurs in vesicles in basalt with massive garronite.

United States

Near Ajo, Arizona, erionite occurs in vesicular basalt associated with phillipsite, heulandite and calcite on the primary minerals hematite, augite, kaolinite, enstatite, pseudobrookite and rancieite.

At Malpais Hill, Arizona, in vesicular olivine basalt, small vugs are lined with celadonite and montmorillonite covered with offretite intergrown with erionite.

At Thum Butte, Arizona, erionite occurs in vesicular olivine basalt associated with quartz, mordenite, clinoptilolite, phillipsite and opal.

Near Freedom, Idaho, erionite occurs in vesicular basalt with chalcedony and opal.

At Reese River, Nevada, erionite occurs in lacustrine (deposited in a lake) mudstone associated with clinoptilolite, chabazite and phillipsite.

Near Aneroid Lake, Oregon, erionite occurs in vesicular basalt, frequently covered with heulandite.

Along the Beaver Divide, Wyoming, erionite and clinoptilolite occur in altered tuff.

At the Durkee fire-opal mine, Oregon, some erionite fibres are found covered by common opal and, rarely, fire opal.

At Yaquina Head, Oregon, erionite occurs in vesicular basalt associated with clinoptilolite, mordenite, phillipsite, pyrite and, rarely, dachiardite and baryte.

Erythrite

Formula: Co3(AsO4)2.8H2O arsenate, vivianite group
Specific gravity: 3.07
Hardness: 2
Streak: Pink
Colour: Red
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Environments:

Hydrothermal replacement environments

Erythrite is a rare secondary mineral that occurs in the oxidation zone of some Ni-Co-As mineral deposits as an alteration product of cobalt arsenides. It is rarely present in large amounts and usually forms as crusts or fine aggregates filling cracks.

Common impurities: Ni,Fe,Zn

Ettringite

Formula: Ca6Al2(SO4)3(OH)12.26H2O hydrated sulphate containing hydroxyl
Specific gravity: 1.77
Hardness: 2 to 2½
Streak: White
Colour: Colourless to white, yellow to light brown, colourless in transmitted light
Solubility: Partially decomposed by water, giving an alkaline solution. Easily soluble in dilute acids.
Environments:

Volcanic igneous environments
Metamorphic environments

Localities

Germany

Near ettringen, Rhine, ettringite occurs in cavities in metamorphosed limestone inclusions in basaltic rocks rich in leucite and nepheline.

USA

At the Lucky Cuss Mine, Arizona, ettringite is an alteration product of calcium-aluminium silicates.

At Franklin, New Jersey, ettringite occurs with andradite and manganese-rich biotite.

Eucryptite

Formula: LiAlSiO4 inosilicate, phenakite group
Specific gravity: 2.657 - 2.666
Hardness: 6½
Streak: white
Colour: Colourless, white, pale tan, pale grey
Solubility: gelatinises in acids
Environments:

Pegmatites

Eucryptite occurs in lithium-rich pegmatites, often as graphic intergrowths with albite derived from alteration of spodumene

Alteration

Eucryptite is a secondary mineral derived from spodumene and associated with albite, spodumene, petalite, amblygonite, lepidolite and quartz.
The co-occurrence of the lithium aluminium silicates spodumene LiAlSi2O6, petalite LiAlSi4O10 and eucryptite LiAlSiO4 is not common, but it does occur in some pegmatites in northern Portugal. The spodumene precipitates early from the magma, petalite later, and eucryptite is hydrothermal and secondary. On alteration spodumene is mainly replaced by albite and muscovite, and petalite by K-feldspar and eucryptite.

spodumene and Na+ to eucryptite, albite and Li+
2LiAlSi2O6 + Na+ → LiAlSiO4 + NaAlSi3O8 + Li+

Common impurities: Na,K

Evansite

Formula: Al3(PO4)(OH)6.8H2O hydrated phosphate containing hydroxyl
Specific gravity: 1.8 to 2.2
Hardness: 3 to 4
Streak: White, lightly tinted
Colour: Colourless, milk-white, lightly tinted blue, green or yellow at times; brown, reddish-brown, or red due to wüstite inclusions; colourless to brown in transmitted light
Solubility: Easily soluble in acids
Environments:

Metamorphic environments

Evansite is a secondary phosphate mineral that forms as a derivative of guano, usually in the form of cave fillings in graphite-containing deposits, gneiss and coal strata. Associated minerals include variscite, allophane and limonite.
Common impurities: Cu,Pb

Faujasite

Faujasite is three mineral species:
Faujasite-Ca, (Ca,Na,Mg)2(Si,Al)12O24.15H2O
Faujasite-Mg, (Mg,Na,K,Ca)2(Si,Al)12O24.15H2O
Faujasite-Na, (Na,Ca,Mg)2(Si,Al)12O24.15H2O
They are tectosilicates (framework silicates), zeolite group
Specific gravity: 1.92 to 1.93
Hardness: 4½ to 5
Streak: White
Colour: Colourless, white, pale brown
Environments:

Volcanic igneous environments
Sedimentary environments

Faujasite usually occurs in basaltic volcanics, and also as an alteration product of volcanic tuff.

Fayalite

Formula: Fe2+2(SiO4) nesosilicate (insular SiO4 groups), olivine group
Properties of fayalite:
Specific gravity: 4.392
Hardness: 6½
Streak: White
Colour: Green, yellow, brown
Solubility: Insoluble in water and nitric acid; soluble in hydrochloric acid forming an insoluble silica gel
Environments:

Volcanic igneous environments

Alteration

fayalite and H2O to magnetite, SiO2 and H2
3Fe2+2(SiO4) + 2H2O &38594; Fe2+Fe3+2O4 + 3SiO2 + 2H2
This reaction is highly exothermic.

fayalite, H2O and O2 to cronstedtite and magnetite
6Fe2+2(SiO4) + 6H2O + ½O2 = 3Fe3Si2O5(OH)4 + Fe2+Fe3+2O4

fayalite, oxygen and H2O to hematite and silicic acid
2Fe2SiO4 + O2 + 4H2O → 2Fe2O3 + 2H4SiO4
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above.

fayalite, SiO2 and H2O to grunerite
7Fe2+2(SiO4) + 9SiO2 + 2H2O → 2Fe2+2Fe2+5Si8O22(OH)2
Fayalite may undergo partial regression to grunerite according to the above reaction.

ferrosilite to fayalite and SiO2
Fe2Si2O6 ⇌ Fe2SiO4 + SiO2
Ferrosilite is unstable at pressure less than about 15 kbar

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

siderite and SiO2 to fayalite and CO2
2Fe(CO3) + SiO2 = Fe2+2(SiO4) + 2CO2

Common impurities: Mn

Feldspar

Feldspars comprise a group of rock-forming tectosilicate minerals that make up as much as 60% of the Earth's crust. They are divided into two groups, K-feldspar, which are rich in potassium, and plagioclase feldspar feldspars that contain no potassium.
The major K-feldspar are orthoclase, sanidine, microcline and anorthoclase.
The major plagioclase feldspars are albite, oligoclase, andesine, labradorite and anorthite.
Feldspars are primary minerals; they are essential constituents of granite, syenite, diorite, gabbro, rhyolite, trachyte, andesite, basalt and , sandstone.
Feldspars are common constituents of phyllite and gneiss, and medium constituents of quartzolite.
K-feldspar are minerals of the greenschist and amphibolite facies.

Feldspathoid

Feldspathoids form a family of tectosilicate minerals which resemble feldspars but have a different structure and much lower silica content. They occur in rare and unusual types of igneous rocks, and are not found in rocks containing primary quartz.
Feldspathoids are common constituents of basalt.
They also may be found in diorite and gabbro.

Ferberite

Formula: Fe2+(WO4) anhydrous tungstate
A complete solid solution exists between ferberite and hübnerite.
Specific gravity: 7.51
Hardness: 4 to 4½
Streak: Black to brownish-black
Colour: Black
Solubility: Easily fusible. Slowly decomposed by hot concentrated sulphuric or hydrochloric acid. Decomposed by aqua regia with the separation of tungstic oxide.
Environments:

Pegmatites
Clastic sedimentary environments
Metamorphic environments
Hydrothermal vein environments

Ferberite occurs in hydrothermal veins, medium temperature metamorphic rocks and granitic pegmatites immediately associated with granitic intrusive rocks; it also occurs in alluvial and residual deposits.
In high temperature (hypothermal) hydrothermal veins it is associated with cassiterite, arsenopyrite, apatite, tourmaline, topaz, fluorite, specular hematite, molybdenite and bismuth. In moderate temperature (mesothermal) veins it is associated with cassiterite and sulphides, scheelite, bismuthinite and siderite.

Localities

Australia

At Rumsby's mine, New South Wales, ferberite is the main ore mineral, associated with a granite intrusion, and it is commmonly intergrown with bismuth, arsenopyrite and fluorite. Some ferberite has altered to secondary scheelite.

Common impurities: Nb,Ta,Sc,Sn

Ferrierite

Ferrierite three minerals:
Ferrierite-K, (K,Na)5(Si31Al5)O72.18H2O
Ferrierite-Mg, [Mg2(K,Na)2Ca0.5](Si29Al7)O72.18H2O
Ferrierite-Na, (Na,K)5(Si31Al5)O72.18H2O
They are tectosilicate (framework silicates), zeolite group
Specific gravity: 2.06 to 2.23
Hardness: 3 to 3½
Streak: White
Colour: Colourless, white, pink, orange, red due to Fe3+
Environments:

Metamorphic environments
Sedimentary environments
Basaltic cavities

Ferrierite occurs in cavity fillings in basaltic rocks and as a diagenetic mineral in rhyolite tuff, as a result of hydration reactions. It also occurs in metamorphic rocks.

Ferro-actinolite

Formula: ☐Ca2(Mg2.5-0.0Fe2+2.5-5.0)Si8O22 (OH)2
Mg/(Mg+Fe2+) < 0.5, inosilicate (chain silicate)
Specific gravity: 3.24 - 3.48
Hardness: 5 - 6
Streak: greenish white
Colour: black-green, dark green
Environments

Metamorphic environments

Ferro-actinolite occurs in iron-rich greenschist and blueschist facies metamorphic rocks and in metamorphosed iron formations. In replecement metamorphic environments it is associated with hedenbergite. In contact metamorphic environments it is associated with andradite and ilvaite. In iron formations it is associated with cummingtonite, quartz, magnetite, riebeckite, biotite and hematite.

Alteration

ferro-actinolite to hedenbergite, grunerite, quartz and H2O
7Ca2Fe2+5Si8O22(OH)2 → 14CaFe2+Si2O6 + 3Fe2+2Fe2+5 Si8O22(OH)2 + 4SiO2 + 4H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, probably due to the instability of ferro-actinolite with rising temperature. The association of hedenbergite and grunerite has been widely described, and its formation may be due to the above reaction.

ferro-actinolite, calcite and quartz to hedenbergite, CO2 and H2O
Ca2Fe2+5Si8O22(OH)2 + 3CaCO3 + 2SiO2 ⇌ 5CaFe2+Si2O6 + 3CO2 + H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, according to the above reaction, probably due to the instability of ferro-actinolite with rising temperature.

Common impurities: Ti,Al,Mn,Na,K,F,P

Ferro-anthophyllite

Formula: Fe2+2Fe2+2Si8O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity:
Hardness: 5½ to 6
Streak: White to greyish white
Colour: Clove-brown to dark brown, grey to white, pale green

Ferro-gedrite

Formula: ☐Fe2+2(Fe2+3Al2) (Si6Al2)O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity:
Hardness: 5½ to 6
Streak: White
Colour: Pale greenish-grey to brown
Solubility:
Environments:

Metamorphic environments

Ferro-gedrite is formed by contact metamorphism

Alteration

gedrite-ferro-gedrite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
2(Mg,Fe2+)5Al4Si6O22(OH)2 + 4Si2 → 3(Mg,Fe2+)2Si2O6 + 2(Mg,Fe2+)2Al4Si5O18 + 2H2O
In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction.

Common impurities: Ti,Mn,Ca,Na,K,F

Ferrosilite

Formula: Fe2+2Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity:
Hardness: 5 to 6
Streak: Pale grey brown
Colour: Dark brown to black

Plutonic igneous environments
Volcanic igneous environments
Metamorphic environments

Ferrosilite is found in basic and ultrabasic igneous rocks and also in high-grade metamorphic rocks, such as the granulite facies. Ferrosilite is common in metamorphosed iron formation in association with grunerite.

Alteration

augite and andalusite to enstatite-ferrosilite and anorthite
2Ca(Fe,Mg)Si2O6 + 2Al2SiO5 → (Mg,Fe2+)2Si2O6 + 2Ca(Al2Si2O8)

biotite and SiO2 to enstatite-ferrosilite, orthoclase and H2O
K(Mg,Fe)3(AlSi3O10)(OH)2 + 3SiO2 → 3(Mg,Fe2+)SiO3 + KAlSi3O8 + H2O Enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction.

chlorite and quartz to enstatite- ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+2)2Al4Si5O18 + 4H2O
Enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation.

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
(Fe,Mg)7Si8O22(OH)2 + (Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)SiO3 + H2O

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2

enstatite-ferrosilite and H2O to serpentine and quartz
6(Mg,Fe2+)SiO3 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel- hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, andalusite and SiO2 to Fe-rich cordierite
2(Mg,Fe2+)SiO3 + 2Al2SiO5 + SiO2 → (Mg,Fe2+)2Al4Si5O18
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, diopside- hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies.

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌ K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage.

enstatite-ferrosilite, SiO2 and H2O to cummingtonite- grunerite
7(Mg,Fe2+)SiO3 + SiO2 + H2O ⇌ (Fe,Mg)7Si8O22(OH)2

ferrosilite to fayalite and SiO2
Fe2Si2O6 ⇌ Fe2SiO4 + SiO2
Ferrosilite is unstable at pressure less than about 15 kbar

gedrite- and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al4Si6O22(OH)2 + 2Si2 → 3(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 + H2O In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction.

olivine and CO2 to enstatite-ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3

olivine and SiO2 to enstatite-ferrosilite
(Mg,Fe)2SiO4 + SiO2 → 2(Mg,Fe2+)SiO3

Common impurities: Ca,Na,K,Al,Co,Ni,Mn,Ti,Cr

Fluorite

Formula: CaF2 fluoride
Specific gravity: 3.18
Hardness: 4
Streak: White
Colour: Colourless, white, purple, green, blue, yellow, pink, red
Solubility: Slightly soluble in hydrochloric acid; moderately soluble in sulphuric acid; slightly soluble in water
Melting point 1403°C, boiling point 2500°C.
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Clastic sedimentary environments
Volcanic sublimates and hot spring deposits
Hydrothermal replacement environments
Hydrothermal vein environments

Fluorite is a common and widely distributed mineral; it mainly occurs as a pore-filling mineral in limestone and dolostone, and in mesothermal (moderate temperature) and hypothermal (high temperature) hydrothermal vein deposits associated with lead and silver ores. Less often it may be found as a primary mineral in igneous rocks and pegmatites, where it is a late-stage mineral following deposition of beryl, topaz and tourmaline. In carbonatites it is associated with albite and pyrite. It also may be precipitated at hot springs.

Fluorite may be found in granite, quartzolite and dolostone.

When fluorite occurs as a cavity fill in carbonate rocks it is usually associated with calcite, dolomite, anhydrite, gypsum and sulphur.
In hydrothermal vein deposits, fluorite may be found with calcite, dolomite, baryte, galena and sphalerite.

Pseudomorphs of quartz after fluorite are common. Fluorite also forms pseudomorphs after calcite, baryte and galena.

At Rumsby's mine, New South Wales, Australia, indications are that fluorite was formed at temperatures between 451 and 462oC.

Common impurities: Y,Ce,Si,Al,Fe,Mg,Eu,Sm,O,ORG,Cl,TR

Forsterite

Formula: Mg2SiO4 nesosilicate (insular SiO4 groups), olivine group
Specific gravity: 3.222
Hardness: 6½ to 7
Streak: White
Colour: Yellow, green, brown, black
Melting point: About 1,900oC at atmospheric pressure
Solubility: Insoluble in water and sulphuric acid; soluble in hydrochloric and nitric acid forming an insoluble silica gel
Environments:

Volcanic igneous environments
Metamorphic environments

Forsterite is a primary mineral occurring in mafic and ultramafic igneous rocks, and also in metamorphosed impure dolostone, and it is also found as glassy grains in stony and stony-iron meteorites. It occurs in the mantle above a depth of about 400 km. If the melt is low in silica, but above average in magnesium and iron, olivine will crystallise out.
Forsterite may be found in dolostone and skarn.
It is often associated with pyroxene, plagioclase feldspar, magnetite, corundum, chromite and serpentine.
Forsterite is a mineral of the amphibolite and granulite facies.

Alteration

During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite

antigorite to forsterite, talc and H2O
5Mg3Si2O5(OH)4 = 6Mg2SiO4 + Mg3Si4O10(OH)2 + 9H2O
This reaction may occur in olivine-diopside- antigorite schist within the aureole of the tonalite in the southern Bergell Alps, Italy, at a higher grade of metamorphism than that which produces forsterite and tremolite.

antigorite and calcite to forsterite, diopside, CO2 and H2O
3Mg3Si2O5(OH)4 + CaCO3 → 4Mg2SiO4 + CaMgSi2O6 + CO2 +6 H2O
This reaction has been found to occur in antigorite schist at about 3 kbar pressure and 400 to 500oC

antigorite and magnesite to forsterite, CO2 and H2O
Mg3Si2O5(OH)4 + MgCO3 → 2Mg2SiO4 + CO2 + 2H2O

diopside and dolomite to forsterite, calcite and CO2
CaMgSi2O6 + 3CaMg(CO3)2 → 2Mg2SiO4 + 4CaCO3 + 2CO2
This is a high-grade metamorphic change occurring at temperature in excess of 600oC.

diopside, forsterite and calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 + 2CO2
This reaction requires a high temperature.

dolomite and quartz to forsterite, calcite and CO2
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3 + 2CO2 In siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.

dolomite and tremolite to forsterite, calcite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 → 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O
dolomite can be metamorphosed to talc and calcite, then at higher temperatures the talc and calcite react to form tremolite. In turn tremolite reacts with dolomite to form forsterite, according to the above equation.

enstatite and H2O to forsterite and cummingtonite
9MgSiO3 + H2O = Mg2SiO4 + Mg2Mg5Si8O22(OH)2
Cummingtonite may be formed by retrograde metamorphism according to the above reaction.

enstatite and calcite to forsterite, diopside and CO2
3Mg2Si2O6 + 2CaCO3 ⇌ 2Mg2SiO4 + 2CaMgSi2O6 + 2CO2 Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

Al-rich enstatite and Al-rich diopside to forsterite, enstatite, diopside and anorthite
Mg9Al2Si9O30 + Ca5Mg4Al2Si9O30 ⇌ 2Mg2SiO4 + 3Mg2Si2O6 + 3CaMgSi2O6 + 2Ca(Al2Si2O8)
This reaction occurs at fairly low temperature and pressure.

forsterite and CO2 to enstatite and magnesite
Mg2SiO4 + CO2 ⇌ MgSiO3 + MgCO3

forsterite and H2O to serpentine and brucite
2Mg2SiO4 + 3H2O ⇌ Mg3Si2O5(OH)4 + Mg(OH)2
The forward reaction is highly exothermic. Stable equilibrium occurs at 350°C for pressure 0.5 kbar, 380°C for pressure 2.0 kbar, 400°C for 3.5 kbar, 420°C for 5.0 kbar and 430°C for 6.5 kbar.

forsterite, SiO3 and H2O to serpentine
3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5 (OH)4
This reaction is highly exothermic.

forsterite and åkermanite to diopside and monticellite
Mg2SiO4 + 2Ca2MgSi2O7 → CaMgSi2O6 + 3CaMg(SiO4)

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8) = Mg2Si2O6 + CaMgSi2O6 + MgAl2O4

forsterite, calcite and SiO2 to diopside and CO2
Mg2SiO4 + 2CaCO3 + 3SiO2 → 2CaMgSi2O6 + 2CO2 In high temperature environments with excess SiO2 diopside may form accoring to the above reaction.

forsterite, calcite and quartz to monticellite and CO2
Mg2SiO4 + 2CaCO3 + SiO2 → 2CaMg(SiO4) + 2CO2

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 ⇌ 3CaMg(SiO4) + 2 CO2
This reaction occurs during contact metamorphism of magnesian limestone.

forsterite, dolomite and H2O to calcite, hydroxylclinohumite and CO2
4Mg2SiO4 + CaMg(CO3)2 + H2O → Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite assemblage in the silica-rich dolomite in the aureole of the Alta granodiorite in Utah, USA, is probably due to the above reaction.

forsterite, enstatite and H2O to serpentine
2Mg2SiO4 + Mg2Si2O6 + 4H2O → 2Mg3Si2O5(OH)4
Serpentine is not stable in the presence of carbon dioxide, and may further react with it to form talc and magnesite.

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

forsterite and quartz to enstatite
Mg2SiO4 + SiO2 → 2MgSiO3
Forsterite is not stable in the presence of free SiO2 and will react with it to form enstatite according to the above reaction.

forsterite and talc to enstatite and H2O
Mg2SiO4 + Mg3Si4O10(OH)2 ⇌ 5MgSiO3 + H2O

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure.

monticellite and CO2 to åkermanite, forsterite and calcite
3CaMgSiO4 + CO2 ⇌ Ca2MgSi2O7 + Mg2O7 + CaCO3
The forward reaction occurs at temperature s above 890oC at and pressure above about 4.3 kbar.

monticellite and diopside to åkermanite and forsterite
3CaMgSiO4 + CaMgSi2O6 ⇌ 2Ca2MgSi2O7 + Mg2O7
Monticellite is stable below 890oC at pressure of about 4.3 kbar.

serpentine and brucite to forsterite and H2O
Mg3Si2O5(OH)4 + Mg(OH)2 ⇌ 2Mg2SiO4 + 3H2O
This reaction can proceed in either direction, depending on the external conditions. Early formation of forsterite.

serpentine and diopside to forsterite and talc
5Mg3Si2O5(OH)4 ⇌ 6Mg2SiO4 + Mg3Si4O10(OH)2 + 9H2O
In olivine-diopside- antigorite schist within the aureole of the tonalite in the southern Bergell Alps, Italy, at a higher grade of metamorphism than that which produces forsterite and tremolite:

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 ⇌ Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O + H2O
In lower grade assemblages associated with contact and regional metamorphism serpentine may form tremolite and forsterite according to the above reaction.

Fe and Cr-rich spinel , diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure.

spinel and tremolite to forsterite and magnesio-hornblende
MgAl2O4 + Ca2Mg5Si8O22(OH)2 ⇌ Mg2SiO4 + Ca2(Mg4Al)(Si7Al)O22(OH)2
This reaction occurs in some strongly metamorphosed serpentinite

talc and forsterite to anthophyllite and H2O
9Mg3Si4O10(OH)2 + 4Mg2SiO4 = 5Mg2Mg5Si8O22(OH)2 + 4H2O
This reaction occurs at a higher metamorphic grade than that forming forsterite and talc from serpentine

tremolite and dolomite to forsterite, calcite, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 11CaMg(CO3)2 → 8Mg2SiO4 + 13CaCO3 + 9CO2 + H2O

tremolite and forsterite to diopside, enstatite and H2O
Ca2Mg5Si8O22(OH)2 + Mg2SiO4 ⇌ 2CaMgSi2O6 + H2O
Equilibrium occurs at 840oC at a pressure of 4 kbar. Increasing temperature favours the forward reaction.

Common impurities: Fe

Gahnite

Formula: ZnAl2O4 multiple oxide, spinel group, gahnite-hercynite series, gahnite-spinel series.
Specific gravity: 4.57
Hardness: 7½ to 8
Streak: Grey
Colour: Dark blue-green, yellow, brown, black
Solubility:
Environments:

Pegmatites
Metamorphic environments

Gahnite occurs in crystalline schist, in granite pegmatites especially those rich in lithium, and in contact group metamorphosed limestone. It also occurs in high temperature replacement ore deposits in schist or marble.

Common impurities: Fe,Mg

Galena

Formula: PbS sulphide
Specific gravity: 7.2 - 7.6
Hardness: 2½
Streak: Lead-grey
Colour: Lead-grey
Solubility: Slightly soluble in hydrochloric and nitric acids
Environments:

Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Galena is a common primary mineral, found in hypothermal (high temperature) and mesothermal (moderate temperature) veins and in contact metamorphic deposits.
In hydrothermal veins it is often associated with anglesite, baryte, bornite, calcite, cerussite, chalcopyrite, dolomite, fluorite, marcasite, pyrite, quartz and sphalerite.

Alteration

In the oxidation zone of epithermal (low temperature) veins initially pyrite is oxidised to ferric sulphate, which is itself a strong oxidising agent. The ferric sulphate then reacts with galena to form anglesite.
Oxidation of pyrite:
pyrite + oxygen + H2O → ferrous sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidises to ferric (trivalent) sulphate and ferric hydroxide
ferrous sulphate + oxygen + H2O → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3

galena, ferric sulphate, water and oxygen to anglesite, ferrous sulphate and sulphuric acid
PbS + Fe2(SO4)3 + H2O + 3O → PbSO4 + 2FeSO4 + H2SO4
Galena is oxidised to anglesite and ferric iron is reduced to ferrous iron.

Galena may also dissolve in carbonic acid from percolating rainwater to form hydrogen sulphide, which is then oxidised to form anglesite.

galena and carbonic acid to Pb2+, hydrogen sulphide and HCO3-
PbS + 2H2CO3 → Pb2+ + H2S + 2HCO3-
then hydrogen sulphide, oxygen, Pb2+ and HCO3- to anglesite and carbonic acid
H2S + 2O2 + Pb2+ + 2HCO3- → PbSO4 + 2H2CO3

galena and oxygen to anglesite
In air, at outcrops of galena,
PbS + 2O2 → PbSO4
At ordinary temperatures the equilibrium is displaced far to the right, and the apparent stability of galena is a result of the slowness of the oxidation.

In the oxidation zone of epithermal veins galena alters to anglesite or cerussite depending on the acidity. Cerussite forms in more basic (alkaline) environments than anglesite.
During the progressive weathering of assemblages of supergene lead minerals anglesite disappears first, then cerussite, and finally only pyromorphite, mimetite and vanadinite persist.

Common impurities: Ag,Cu,Fe,Bi

Garnet

The garnet group is a group of nesosilicate (insular SiO4 groups) minerals including almandine, pyrope, spessartine, andradite, grossular and uvarovite.
Environments:

Plutonic igneous environments
Pegmatites
Clastic sedimentary environments
Placer deposits
Metamorphic environments (typical)
Hydrothermal replacement environments

Garnet is a common and widely distributed mineral occurring abundantly in some metamorphic rocks, and as an accessory constituent in some igneous rocks. It is also found in sedimentary environments including placers, and in hydrothermal replacement lodes.
Garnet may be found in granite, diorite, skarn, schist and gneiss.
Its most characteristic occurrence is in mica schist, hornblende schist and gneiss.
It is characteristic of the amphibolite facies, and it is also a mineral of the greenschist, granulite, blueschist and eclogite facies.
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid

Alteration

albite, diopside and magnetite to aegirine, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan.

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2 In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2 This reaction occurs at high temperature and pressure.

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

hypersthene, augite and a href="#spinel"target="_self">Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure.

Garronite

Garronite is a series with end members:
Garronite-Ca: Ca3(Al6Si10O32).14H2O
Garronite-Na: Na6(Al6Si10O32).8.5H2O
These minerals are tectosilicates (framework silicates), zeolite group
Specific gravity: 2.13 to 2.18
Hardness: 4½ to 5
Streak: White
Colour: Colourless, white
Garronite is most common in vesicles in basalt and other mafic rocks, often overgrown by or associated with phillipsite, and also as epitaxial overgrowths on phillipsite. At Mont St Hilaire, Canada, it occurs in some metamorphic rocks and in sodalite xenoliths.

Gaylussite

Formula: Na2Ca(CO3)2.5H2O hydrated normal carbonate
Specific gravity: 2.367
Hardness: 3 to 3½
Streak: White
Colour: Colourless, white
Solubility: Soluble with effervescence in acids. Slightly soluble in water
Environments

Chemical sedimentary environments

Gaylussite is usually found in soda lakes with natron, thermonatrite, trona, pirssonite, calcite and borax. At Borax Lake, California, USA, it is found with northupite and pirssonite.

Alteration

Gaylussite dehydrates with efflorescence in dry air. It alters readily to calcite. Calcite pseudomorphs after gaylussite have been described.

Gedrite

Formula: ☐Mg2(Mg3Al2)(Si6Al2)O22 (OH)2 inosilicate (chain silicate) amphibole
Specific gravity:
Hardness: 5½: to 6
Streak: White
Colour: Pale greenish-grey to brown
Environments:

Metamorphic environments

Gedrite occurs in contact and medium to high grade metamorphic rocks in association with garnet, cordierite, anthophyllite, cummingtonite, sapphirine, sillimanite, kyanite, quartz, staurolite and biotite. Ferrogedrite exists in low temperature, high pressure contact metamorphic geologic settings and remains stable up to 600°C-800°C due to its iron content.

Alteration

gedrite-ferro-gedrite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al4Si6O22(OH)2 + 2Si2 → 3(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 + H2O
In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction.

Gehlenite

Formula: Ca2Al(SiAl)O7 sorosilicate (Si2O7 groups) melilite group
Specific gravity: 2.9 to 3.06
Hardness: 5 to 6
Streak: white, greyish white
Colour: colourless, greenish grey, yellowish brown
Solubility: Insoluble in water; soluble in hydrochloric acid forming an insoluble silica gel
Environments:

Metamorphic environments

Gehlenite is a contact metamorphic mineral in limestone.

Common impurities: Ti,Fe,Mg,Mn,Na,K

Gibbsite

Formula:Al(OH)3 hydroxide
Specific gravity:
Hardness: 2½ to 3
Streak: White
Colour: White, light gray, light green, reddish white; reddish yellow (impure)
Solubility: Insoluble in water, hydrochloricand nitric acid; soluble in sulphuric acid

Plutonic igneous environments
Pegmatites
Clastic sedimentary environments
Chemical sedimentary environments

Gibbsite is a secondary mineral forming in the weathered surface zones in clay deposits and limestone, as well as low-silica igneous rocks and pegmatites. It is a constituent of the aluminium ore bauxite.

Alteration

kaolinite and H2O to gibbsite and quartz
Al2Si2O5(OH)4 + H2O ⇌ 2Al(OH)3 + 2SiO2

Common impurities: Fe,Ga

Gismondine

Formula: Ca2(Si4Al4)O16.8H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.12 to 2.28
Hardness: 4 to 5
Streak: White
Colour: Colourless, white, very pale pink
Environments:

Volcanic Igneous environments
Pegmatites
Sedimentary environments
Metamorphic environments

Gismondine occurs most commonly in olivine basalt, less commonly in nepheline basalt, and tuff, schist or pegmatites.

Glauberite

Formula: Na2Ca(SO4)2 sulphate
Specific gravity: 2.7 - 2.8
Hardness: 2½ - 3
Streak: White
Colour: Grey, colourless, yellowish, red
Solubility: Soluble in water
Environments:

Chemical sedimentary environments
Volcanic sublimates and hot spring deposits

Glauberite occurs in dry salt-lake beds or marine evaporite deposits, generally in desert climates, and in fumeroles. Camp Verde, Arizona, USA is known for pseudomorphs of calcite after glauberite.
At Lake Crosbie, Victoria, Australia, glauberite occurs associated with halite and gypsum in black mud under a salt crust which covers the lake. Glauberite dissolves in water, depositing gypsum, so pseudomorphs of gypsum after glauberite are not uncommon.

Glaucochroite

Formula: CaMn2+(SiO4) nesosilicate (insular SiO4 groups) olivine group
Specific gravity:
Hardness: 6
Streak: White
Colour: Bluish-grey, pink, brown or white
Environments:

Metamorphic environments

Glaucochroite is found in metamorphosed limestone containing manganese minerals

Alteration

bustamite, tephroite and calcite to glaucochroite and CO2
CaMn2+Si2O6 + Mn2+2(SiO4) + 2CaCO3 ⇌ 3CaMn2+(SiO4) + 2CO2

Glaucophane

Formula: ☐Na2(Mg3Al2)Si8O22 (OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.0 - 3.3
Hardness: 5½ - 6½
Streak: Light blue
Colour: Blue-grey, blue-lavender, blue-black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Metamorphic environments

Glaucophane is found only in metamorphic rocks such as schist, eclogite and marble. It is a low-temperature, relatively high pressure metamorphic mineral, occurring in association with jadeite, lawsonite and aragonite. It is a major constituent of glaucophane schist, and it is a characteristic mineral of the blueschist facies.

Alteration

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above.

albite and montmorillonite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz and H2O
8Na(AlSi3O8) + 2Ca0.5(Mg3.5Al0.5)Si8O20(OH)4.nH2O → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 15SiO2 +6H2O
This reaction occurs in low to intermediate metatmorphism.

Common impurities: Li,Ti,Cr,Mn,Ca,K,F,Cl

Gmelinite

Gmelinite is the name for three minerals:
Gmelinite-Ca: Ca2(Si8Al4)O24.11H2O
Gmelinite-K: K4(Si8Al4)O24.11H2O
Gmelinite-Na4(Si8Al4)O24.11H2O
These minerals are tectosilicates (framework silicates), zeolite group
Specific gravity: 2.04 to 2.17
Hardness: 4½
Streak: White
Colour: Colourless, white, yellow, greenish white, orange, pale green, pink, red, brown and grey
Environments:

Volcanic igneous environments
Pegmatites
Metamorphic environments
Hot spring deposits
Hydrothermal environments

Gmelinite generally occurs in silica-poor volcanic rocks, and also in sodium-rich pegmatites and 55 to 75oC geothermal wells. No sedimentary gmelinite has been reported. It is a product of diagenetic reactions, hydrothermal activity or alteration of basalt. Common associates are other zeolites, quartz, aragonite and calcite.

Goethite

Formula: FeO(OH) oxide containing hydroxyl
Specific gravity: 4.3
Hardness: 5 - 5½
Streak: Brown to brownish yellow
Colour: Yellow, brown to dark brown and reddish brown
Solubility: Slightly soluble in hydrochloric acid
Environments:

Pegmatites
Carbonatites
Clastic sedimentary environments
Hydrothermal vein environments

Goethite is a very common mineral, typically formed by the oxidation of iron-bearing minerals. It also forms as a direct inorganic or biogenic precipitate from water and it is widespread as a deposit in bogs and springs. Large quantities of goethite have resulted from the weathering of serpentine. It also occurs in vesicles in volcanic rocks, and in the oxidation zone of hypothermal (high temperature) hydrothermal veins.

Alteration

hematite and H2O to goethite
Fe2O3 + H2O ⇌ 2FeO(OH)
Both forward and reverse reactions are slow, but equilibrium in most natural environments is displaced to the left, favouring the formation of hematite.

Common impurities: Mn

Gold

Formula: Au native element
Specific gravity: 15.5 - 19.3
Hardness: 2½ to 3
Streak: Shining yellow
Colour: Shining yellow
Solubility: Insoluble in hydrochloric, sulphuric and nitric acids
Melting Point: 1062.4° ± 0.8°
Environments:

Plutonic igneous environments
Placer deposits (typical)
Hydrothermal replacement environments
Hydrothermal vein environments (typical)

Gold is a rare element that is widely distributed but in small amounts. Most gold occurs as the native metal. The chief sources of gold are hypothermal (high temperature) mesothermal (moderate temperatute) and epithermal (low temperature) hydrothermal gold-quartz veins, together with pyrite and other sulphides. When gold-bearing veins are weathered, the gold liberated either remains in place in the soil mantle, or is washed into neighbouring streams to form placer deposits.

Alteration

Gold is usually primary, but it can also occur as a secondary mineral derived from gold tellurides.
Pseudomorphs of gold after pyrite are extremely rare, but examples so well formed that they seem artificial are found at the Lena goldfields, Bodaibo Area, Eastern Siberia, Russia.
Common impurities: Ag,Cu,Pd,Hg

Gonnardite

Formula: (Na,Ca)2(Si,Al)5O10.3H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.21 to 2.36
Hardness: 4½ to 5
Streak: White
Colour: Colourless, white, yellow, pink to salmon-orange
Environments:

Volcanic Igneous environments
Pegmatites
Metamorphic environments (rarely)

Gonnardite occurs in silica-poor volcanic rocks and pegmatites, and rarely in skarn. Crystals are often found with cores of gonnardite and rims of thomsonite.

Alteration

Gonnardite forms overgrowths on natrolite.

Gordonite

Formula: MgAl2(PO4)2(OH)2.8H2O hydrated phosphate with hydroxyl
Specific gravity: 2.319
Hardness: 3½
Streak: White
Colour: Smoky-white, buff, colourless; crystals = pale pink or pale green on tips, colourless in transmitted light
Solubility: Soluble in acids

Environments:

Pegmatites
Hydrothermal environments

Gordonite is a rare secondary mineral formed from the alteration of variscite in nodules in limestone or as a late-stage hydrothermal mineral in complex granitic pegmatites.

Graphite

Formula: C native element
Specific gravity: 2.09 - 2.23
Hardness: 1 to 2
Streak: Black to steel-grey
Colour: Black to steel-grey
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Metamorphic environments (typical)
Hydrothermal vein environments

Graphite most commonly occurs in metamorphic where it may form a considerable proportion of the rock. In these cases, it has probably been derived from carbonaceous material of organic origin that has been converted into graphite during metamorphism. It can also be found in veins and in pegmatites.
It is a mineral of the greenschist, amphibolite and granulite facies.
Graphite is a common constituent of phyllite,
It also may be found in limestone, schist and gneiss.

Greenalite

Formula: (Fe2+, Fe3+2-3Si2O5(OH)4 phyllosilicate (sheet silicate) serpentine group
Specific gravity: 2.85 to 3.15
Hardness: 2½
Streak: Greenish grey
Colour: Green, light yellow-green
Environments:

Chemical sedimentary environments

Greenalite is a primary phase in some banded iron formations

Alteration

Mg-rich greenalite to olivine, Mg-rich grunerite and H2O
18(Fe2+, Mg))3Si2O5(OH)4 → 20(Fe,Mg)2SiO4 + 2(Fe2+,Mg)7Si8O22(OH)2 + 34H2O

Mg-rich greenalite to olivine, SiO2 and H2O
2(Fe2+, Mg))3Si2O5(OH)4 → 3(Fe,Mg)2SiO4 + SiO2 + 4H2O

Common impurities: Al,Mn,Mg

Grossular

Formula: Ca3Al2(SiO4)3 nesosilicate (insular SiO4 groups), garnet group
Specific gravity: 3.594
Hardness: 6½ to 7
Streak: White to pale brownish white
Colour: Brown, orange, red, yellow, green, white, colourless. colourless when pure (rare), commonly red orange to brown.
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Metamorphic environments

Grossular occurs in contact and regionally metamorphosed limestone; it is a mineral of the hornblende-hornfels, pyroxene-hornfels and amphibolite facies.

Alteration

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

epidote and quartz to anorthite, grossular and H2O
4Ca2Al3(SiO4)3(OH) + SiO2 → 5CaAl2Si2O8 + Ca3Al2(SiO4)3 + 2H2O
This reaction occurs as the degree of metamorphism increases

grossular, diopside,monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2
A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises.

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

Common impurities: Fe,Cr

Grunerite

Formula: ☐Fe2+2Fe2+5Si8O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.4 to 3.5
Hardness: 5 to 6
Streak: Colourless
Colour: Ashen, brown, brownish green, dark gray
Environments:

Metamorphic environments

Grunerite is common in iron formations due to medium to high grade contact metamorphism, and it also occurs in some blueschist facies quartzite.
Upon further metamorphism cummingtonite and grunerite give way to orthopyroxene or olivine.

Alteration

cummingtonite-grunerite and H2O to serpentine and SiO2
6(Fe,Mg)7Si8O22(OH)2 + 22H2O ⇌ 7(Fe,Mg)6Si4O10(OH)8 + 20SiO2

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
(Fe,Mg)7Si8O22(OH)2 + (Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)SiO3 + H2O

enstatite-ferrosilite, SiO2 and H2O to cummingtonite-grunerite
7(Mg,Fe2+)SiO3 + SiO2 + H2O ⇌ (Fe,Mg)7Si8O22(OH)2

fayalite, SiO2 and H2O to grunerite
7Fe2+2(SiO4) + 9SiO2 + 2H2O → 2Fe2+2Fe2+5Si8O22(OH)2 Fayalite may undergo partial regression to grunerite according to the above reaction.

ferro-actinolite to hedenbergite, grunerite, quartz and H2O
7Ca2Fe2+5Si8O22(OH)2 → 14CaFe2+Si2O6 + 3Fe2+2Fe2+5 Si8O22(OH)2 + 4SiO2 + 4H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, probably due to the instability of ferro-actinolite with rising temperature. The association of hedenbergite and grunerite has been widely described, and its formation may be due to the above reaction.

Mg-rich greenalite to olivine, Mg-rich grunerite and H2O
18(Fe2+, Mg))3Si2O5(OH)4 → 20(Fe,Mg)2SiO4 + 2(Fe2+,Mg)7Si8O22(OH)2 + 34H2O

Gummite

Gummite is a mixture of boltwoodite, clarkeite, curite, kasolite, soddyite, uraninite and uranophane.
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments

Gypsum

Formula: Ca(SO4).2H2O sulphate
Specific gravity: 2.3 - 2.4
Hardness: 1½ - 2
Streak: White
Colour: Colourless, white, yellowish, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:

Clastic sedimentary environments
Chemical sedimentary environments (typical)
Volcanic sublimates and hot spring deposits
Hydrothermal replacement environments

Gypsum is the commonest of the sulphate minerals, found in chemical sedimentary environments, where it frequently occurs interstratified with limestone and shale. It is usually found as a layer underlying beds of rock salt, having been deposited there as one of the first minerals to crystallise on the evaporation of salt waters. It may recrystallise in veins forming satin spar. It is also common as a gangue mineral in metallic veins, at fumaroles, and in the oxidation zones of sulphide deposits. Gypsum is also found in volcanic regions, especially where limestone has been acted on by sulphur vapours.
Gypsum is associated with many different minerals, the more common being halite, anhydrite, dolomite, calcite, sulphur, pyrite and quartz.

Alteration

anhydrite and water to gypsum
Ca(SO4) + 2H2O ⇌ Ca(SO4).2H2O
Gypsum is frequently formed by the hydration of anhydrite.

anorthite, H2SO4 and H2 to gypsum and kaolinite
CaAl2 Si2O8 + H2SO4 + 3H2O → CaSO4.2H2O + Al2Si2O5(OH)4

Gyrolite

Formula: NaCa16(Si23Al)O60(OH)8.14H2O phyllosilicate Formula:
Specific Gravity: 2.36 to 2.45
Hardness: 3 to 4
Streak: White
Colour: Brown, colourless, white, green
Solubility: Decomposed by acids
Environments:

Basaltic cavities

Gyrolite is a secondary mineral that occurs in amygdules in basalt. It is stable under saturated steam conditions from 120 to 200oC.
Common impurities: Fe, Mg

Halite

Formula: NaCl chloride
Specific gravity: 2.1 - 2.2
Hardness: 2
Streak: White
Colour: Colourless, white, redish, yellow, grey, blue
Solubility: Readily soluble in water
Melts at 804°.
Environments:

Chemical sedimentary environments
Volcanic sublimates and hot spring deposits

Halite is dissolved in the waters of salt springs, salt lakes and the ocean. It is a common mineral, occurring often in extensive beds and irregular masses, precipated by evaporation with gypsum, sylvite, anhydrite and calcite. It is a major salt in playa deposits of enclosed basins. It occurs as halite deposits in steppes and deserts, and in fumeroles.

Common impurities: I,Br,Fe,O

Halloysite

Formula: Al2Si2O5(OH)4.2H2O phyllosilicate (sheet silicate)
Specific gravity: 2.0 to 2.65
Hardness: 1 to 2
Streak: Paler than the color, or white.
Colour: White to tan, sometimes greenish or bluish, also chocolate brown to reddish. br Environments:

Plutonic igneous environments
Volcanic igneous environments
Pegmatites rarely
Hot spring deposits

Halloysite is a product of hydrothermal alteration or surface weathering of aluminosilicate minerals such as feldspar. Ongoing formation is observed at hot springs. It is frequently found as an alteration of basaltic rocks or in hydrothermally altered fissures in monzonite. It also may be derived from chlorite, mica, rhyolite, granite, volcanic ash, tuff and volcanic glass. It is found rarely in granite pegmatites and bauxite and clay. Commonly associated with kaolinite, illite, minerals of the smectite group (which includes montmorillonite and nontronite), allophane.

Alteration

Halloysite easily weathers to kaolinite, but can also be derived from it. It is sometimes associated with allophane and may be derived from it.

Common impurities: Ti,Ca,Na,K,Fe,Cr,Mg,Ni,Cu

Hanksite

Formula: KNa22(SO4)9(CO3)2Cl anhydrous compound sulphate containing halide
Specific gravity: 2.562
Hardness: 3 to 3½
Streak: White
Colour: Colourless to grey, yellow or almost black; colourless in transmitted light
Solubility: Readily soluble in water, effervesces weakly in dilute acids
Environments:

Chemical sedimentary environments

Hanksite occurs in lacustrine (lake) evaporite deposits. It is abundant at Searles Lake, California, USA, associated with halite, borax, trona and aphthitalite.

Harmotome

Formula: Ba2(Si12Al4)O32.12H2O tectosilicate (framework silicate), zeolite group

Hedenbergite

Formula: CaFe2+Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.5 - 3.6
Hardness: 5 - 6
Streak: Greenish, brownish, grey, also colourless
Colour: Dark green, greenish black, brownish black, black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Metamorphic environments

Hedenbergite is common in iron-rich metamorphic rocks, such as skarn, in alkaline granite and as xenoliths in kimberlite.

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2

diopside-hedenbergite and CO2 to enstatite-ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2 This reaction represents metamorphic reactions between the granulite and amphibolite facies.

ferro-actinolite to hedenbergite, grunerite, quartz and H2O
7Ca2Fe2+5Si8O22(OH)2 → 14CaFe2+Si2O6 + 3Fe2+2Fe2+5 Si8O22(OH)2 + 4SiO2 + 4H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, probably due to the instability of ferro-actinolite with rising temperature. The association of hedenbergite and grunerite has been widely described, and its formation may be due to the above reaction.

ferro-actinolite, calcite and quartz to hedenbergite, CO2 and H2O
Ca2Fe2+5Si8O22(OH)2 + 3CaCO3 + 2SiO2 ⇌ 5CaFe2+Si2O6 + 3CO2 + H2O In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, according to the above reaction, probably due to the instability of ferro-actinolite with rising temperature.

hematite, wüstite, quartz and calcite to andradite, hedenbergite and magnetite from
If wüstite, FeO, is also introduced hedenbergite and magnetite may form in addition to andradite:
hematite + wüstite + quartz + calciteandradite + hedenbergite + magnetite + CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

titanomagnetite (ilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2

Common impurities: Mn,Zn,Ti,Al,Mg,Na,K

Hematite

Formula: Fe2O3 oxide
Specific gravity: 5.2 - 5.3
Hardness: 6½
Streak: Reddish brown
Colour: Reddish brown, grey, black
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Clastic sedimentary environments
Metamorphic environments (typical)
Volcanic sublimates and hot spring deposits
Hydrothermal replacement environments
Hydrothermal vein environments

Hematite occurs as microscopic grains in almost all rocks, especially metamorphic rocks.
It is found in plutonic igneous environments as an accessory mineral in feldspar-rich igneous rocks such as granite, and in pegmatites and carbonatites.
Large ore bodies of hematite are usually of sedimentary origin. Hematite is also found in red sandstone as the cementing material that binds the quartz grains together.
Hematite occurs both in contact and regional metamorphic deposits, where it may have originated from the oxidation of limonite, siderite or magnetite.
It occurs in disseminated hydrothermal replacement deposits and in hydrothermal replacement lodes, as well as in the oxidation zone of epithermal (low temperature) and mesothermal (moderate temperature) hydrothermal veins.
It may also occur as a sublimation product in due to volcanic activity.

Hematite is a common constituent of marl.

Alteration

Hematite may form as an alteration product of ilmenite.

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O

calcite, hematite and quartz to andradite and CO2
3CaCO3 + Fe2O3 + 3SiO2 → Ca3Fe3+2Si3O12 + 3CO2

fayalite, oxygen and H2O to hematite and silicic acid
2Fe2SiO4 + O2 + 4H2O → 2Fe2O3 + 2H4SiO4
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above.

hematite and H2O to goethite
Fe2O3 + H2O ⇌ 2FeO(OH)
Both forward and reverse reactions are slow, but equilibrium in most natural environments is displaced to the left, favouring the formation of hematite.

hematite, wüstite, quartz and calcite to andradite, hedenbergite, magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2
If wüstite, FeO, is also introduced hedenbergite and magnetite may form in addition to andradite:

magnetite to hematite
2Fe3O4 + ½O2 ⇌ 3Fe2O3
Equilibrium is to one side or the other depending on temperature and pressure.

siderite, oxygen and H2O to hematite and silicic acid
2Fe2CO3 + O2 + 4H2O → 2Fe2O3 + 2H2CO3
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above.

Common impurities: Ti,Al,Mn,H2O

Hemimorphite

Formula: Zn4(Si2O7)(OH)2.H2O sorosilicate (Si2O7 groups)
Specific gravity: 3.475
Hardness: 4½ to 5
Streak: White
Colour: colourless, white, pale blue, pale green, gray, brown
Solubility: Slightly soluble in hydrochloric acid
Environments:

Hydrothermal replacement environments

Hemimorphite is a high temperature secondary mineral found in the oxidation portion of zinc deposits, associated with smithsonite, sphalerite, cerussite, anglesite and galena.
In the oxidation zone of epithermal veins sphalerite ZnS (primary) alters to secondary hemimorphite Zn4Si2O7(OH)2.H2O, smithsonite ZnCO3 and manganese-bearing willemite Zn2SiO4.

Common impurities: Cu,Fe

Hercynite

Formula: Fe2+Al2O4 multiple oxide
Specific gravity: 3.95
Hardness: 7½
Streak: Dark greyish green to dark green
Colour: Dark blue-green, yellow, brown, black
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Placer deposits
Metamorphic environments

Hercynite occurs in high grade metamorphosed ferruginous argillaceous sediments and in some mafic and ultramafic igneous rocks. Also in placers.

Alteration

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel-hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

spinel-hercynite, sillimanite and quartz to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10

staurolite, annite and O2 to hercynite, magnetite, muscovite,corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O

Heulandite

The heulandite group is a group of tectosilicates (framework silicates) and a sub-group of the zeolite group, comprising:

heulandite-Ba (Ba,Ca,K)5(Si27Al9)O72.22H2O,
heulandite-Ca (Ca,Na,K)5(Si27Al9)O72.26H2O,
heulandite-K (K,Ca,Na)5(Si27Al9)O72.26H2O,
heulandite-Na (Na,Ca,K)5(Si27Al9)O72.22H2O
heulandite-Sr (Sr,Ca,Na)5(Si27Al9)O72.24H2O
Specific gravity: 2.2
Hardness: 3½ - 4
Streak: White
Colour: Colourless, white, yellowish, red
Solubility: Moderately soluble in hydrochloric acid
Environments:

Pegmatites
Basaltic cavities

Heulandite is usually found in cavities of mafic igneous rocks associated with zeolites and calcite. It also occurs as druses in pegmatites.
It is found associated with stilbite and pyrite in water tunnels under Queens, New York City, USA, and with stilbite, datolite and chabazite at the Quarry Road Cut, also New York City.
It occurs on weathering scapolite in the Carlin trench, The Selleck Road Tremolite and Tourmaline Locality, West Pierrepont, St. Lawrence County, New York.
At Palabora, Limpopo Province, South Africa it has been reported in association with calcite, saponite, and fluorapophyllite.
Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that heulandite crystallised at temperatures from 65oC to 200oC at depths between 30m and 1200m.

Hornblende

Hornblende is a series between ferro-hornblende: ☐Ca2(Fe2+4Al) (Si7Al)O22(OH)2 and Magnesio-hornblende: ☐Ca2(Mg4Al) (Si7Al)O22(OH)2
Both are inosilicates (chain silicates) amphiboles
Specific gravity: 3.00 to 3.47
Hardness: 5 to 6
Streak: White
Colour: Green, black, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Metamorphic environments

Hornblende is an important and widely distributed primary, rock-forming mineral, occurring both in igneous and in metamorphic rocks; it is particularly characteristic of amphibolite in which hornblende and associated plagioclase feldspar are the major constituents.
In metamorphic environments hornblende is found both in contact and in regional metamorphic environments.
Hornblende is an essential constituent of ultramafic rocks
It is a common constituent of granite, syenite, diorite and gabbro.
It also may be found in trachyte, andesite, basalt and gneiss.
Hornblende is characteristic of the hornblende-hornfels and amphibolite facies, and it is also a mineral of the greenschist, granulite, blueschist and albite-epidote-hornfels facies.

Alteration

Hornblende characteristically alters from pyroxene both during the late stages of crystallisation of igneous rocks and during metamorphism.

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies.

epidote and chlorite to hornblende and anorthite
6Ca2Al3(SiO4)3(OH) + Mg5Al2Si3O18(OH)8 → Ca2Mg5Si8O22(OH)2 + 10CaAl2Si2O8
This reaction represents changes when the metamorphic grade increases from the greenschist facies to the amphibolite facies.

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

Al-rich hornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2

spinel and tremolite to forsterite and magnesio-hornblende
MgAl2O4 + Ca2Mg5Si8O22(OH)2 ⇌ Mg2SiO4 + Ca2(Mg4Al)(Si7Al)O22(OH)2
This reaction occurs in some strongly metamorphosed serpentinite.

Common impurities: Ti,Mn,Na,K

Hübnerite

Formula: Mn2+(WO4) anhydrous tungstate, but according to Mindat this species is misclassified in Dana 8th edition. There are no WO4 tetrahedra in it, so it should be classified as a simple oxide.
Hübnerite forms a complete solid solution series with ferberite.
Specific gravity: 7.12 to 7.18
Hardness: 4 to 4½
Streak: Greenish-grey, yellow to reddish-brown
Colour: Yellow-brown, reddish-brown, blackish brown, black, red (rare)
Environments:

Pegmatites
Clastic sedimentary environments
Metamorphic environments
Hydrothermal vein environments

Hübnerite occurs in high-temperature (hypothermal) hydrothermal veins and medium temperature metamorphic rocks. It also occurs in granite pegmatites and in alluvial and residual deposits.
Associated minerals include cassiterite, arsenopyrite, molybdenite, tourmaline, topaz, rhodochrosite and fluorite.

Hydrocerussite

Formula: Pb3(CO3)2(OH)2 anhydrous carbonate containing hydroxyl
Specific gravity: 6.8
Hardness: 3½
Streak: White
Colour: White or grey; colourless in transmitted light
Solubility: Soluble in acids with effervescence
Environments:

Metamorphic environments brHydrothermal replacement environments

At the type locality cerussite occurs in a metamorphosed Mn-Fe deposit. It is a secondary mineral usually developed in the oxidised portions of lead deposits

Hydroxylclinohumite

Formula: Mg9(SiO4)4(OH)2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.13
Hardness: 6½
Streak: White
Colour: Pale yellow to orange-yellow, almost colourless
Environments:

Metamorphic environments

Hydroxylclinohumite occurs in skarn rims around dolomitic marble xenoliths in gabbroic rocks.

Alteration

diopside, dolomite and H2O ⇌ hydroxylclinohumite, calcite and CO2
2CaMgSi2O6 + 7CaMg(CO3)2 + H2O ⇌ 4Mg2SiO4.Mg(OH)2 + 9CaCO3 + 5CO2
In the nodular dolomites, hydroxylclinohumite associated with calcite occurs in a narrow zone in the central parts of the nodules due to the above reaction.

forsterite, dolomite and H2O to calcite, hydroxylclinohumite and CO2
4Mg2SiO4 + CaMg(CO3)2 + H2O → Mg9(SiO4)4(OH)2 +CaCO3 + CO2
A forsterite-clinohumite assemblage in the silica-rich dolomite in the aureole of the Alta granodiorite in Utah, USA, is probably due to the above reaction.

tremolite, dolomite and H2O ⇆ hydroxylclinohumite, calcite and CO2
Ca2Mg5Si8O22(OH)2 + 13CaMg(CO3)2 + H2O ⇆ 2(4Mg2SiO4.Mg(OH)2) + 15CaCO3 + 11CO2

Hypersthene

Hypersthene is a term used for a mineral part way between enstatite and ferrosilite. It is not a valid mineral name.
Formulae:
Hypersthene (Mg,Fe)SiO3
Enstatite MgSiO3
Ferrosilite FeSiO3
Properties of hypersthene:
Specific gravity:
Hardness: 5½ to 6
Streak: Greyish white, greenish
Colour: Greyish white
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Environments:

Metamorphic environments

Hypersthene occurs in igneous rocks and schist and as pyroclasts (particles ejected during a volcanic eruption). It also may be found in basalt.

Alteration

augite and CO2 to hypersthene, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe)SiO3 + CaCO3 + SiO2

hypersthene, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

jadeite, diopside, magnetite and SiO2 to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction.

Ilmenite

Formula: Fe2+Ti4+O3 simple oxide
Specific gravity: 4.68 - 4.76
Hardness: 5 to 6
Streak: Black to reddish brown
Colour: Black
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites (typical)
Clastic sedimentary environments
Placer deposits

Ilmenite is a high-temperature primary mineral that is found in igneous environments and as a constituent of black sands.
It may be found in basalt, gabbro kimberlite and syenite. As a constituent of black sands it is associated with magnetite, rutile, zircon and monazite.

It is a mineral of the granulite facies.

Alteration

Ilmenite may alter to hematite or rutile.

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2 In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2 This reaction occurs at high temperature and pressure.

titanomagnetite (ilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2

Common impurities: Mn,Mg,V

Imogolite

Formula: Al2SiO3(OH)4 phyllosilicate (sheet silicate), allophane group
Specific gravity: 2.7
Hardness: 2 to 3
Streak: White
Colour: Yellowish white, light brownish yellow, blue, green, brown
Environments:

Clastic sedimentary environments

Imogolite occurs principally in soils derived from volcanic ash. It has been found associated with allophane, halloysite, vermiculite, goethite, gibbsite and quartz. Imogolite has been observed in cracks in weathered plagioclase.

Jadeite

Formula:NaAlSi2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.25 - 3.35
Hardness: 6
Streak: White
Colour: apple-green, greenish white, purplish blue, blue-green
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Metamorphic environments

Jadeite is found only in metamorphic rocks. It is a characteristic mineral of the blueschist facies.

Alteration

albite to jadeite and quartz
Na(AlSi3O8) → NaAlSi2O6 + SiO2
In quartz-jadeite rocks jadeite may be the product of the above reaction.
This reaction occurs at high pressure ranging from above 5 kbar at 0oC to above 20 kbar at 800oC.

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above.

albite and montmorillonite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz and H2O
8Na(AlSi3O8) + 2Ca0.5(Mg3.5Al0.5)Si8O20(OH)4.nH2O → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 15SiO2 +6H2O This reaction occurs in low to intermediate metatmorphism.

anorthite, albite and H2O to jadeite, lawsonite and quartz
CaAl2 Si2O8 + NaAlSi3O8 + 2H2O → NaAlSi2O6 + CaAl2(Si2O7)(OH)2.H2 + SiO2

jadeite, diopside, magnetite and SiO2 to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction.

jadeite and quartz"target="_self">quartz to albite
NaAlSi2O6 + SiO2 ⇌ NaAlSi3O8
High pressure favours the reverse reaction. Sometimes found in blueschist metamorphic rocks.

nepheline and albite to jadeite
NaAlSiO4 + NaAlSi3O8 ⇌ 2NaAlSi2O6 The forward reaction requires high pressure, 10 to 25 kbar, and temperature 600oC to 1,000oC.

Common impurities: Ti,Mn,Mg,Ca,K,H2O

Jarosite

Formula: KFe3+3(SO4)2(OH)6 sulphate, alunite group
Specific gravity: 2.9 - 3.3
Hardness: 3 - 4
Streak: Yellow
Colour: Ochre-yellow, brown to blackish brown
Solubility: Moderately soluble in hydrochloric acid
Environments:

Hydrothermal replacement environments

Jarosite is a secondary mineral found in the oxidation zone of hypothermal (high temperature) veins and sulphide deposits, formed by weathering in arid climates.

Common impurities: Na,Ag,Pb

Johannsenite

Formula: CaMnSi2O6 inosilicate pyroxene
Specific gravity: 3.56
Hardness: 6
Streak: White
Colour: Blue-green, grey-white, dark brown, colourless
Environments:

Metamorphic environments

Johannsenite is found in contact metamorphic environments

Alteration

johansennite and H2O to rhodonite and xonotlite
6CaMnSi2O6 + H2O → 6Mn2+SiO3 + Ca6Si6O17(OH)2
At Pueblo, Mexico, johansennite occurs in calcite veins in rhyolite and is altered to rhodonite and xonotlite according to the above equation.

Common impurities: Ti,Al,Fe,Mg,Na,K,C,P,H2O

K-feldspar

K-feldspars include microcline, orthoclase and sanidine, all of which have the formula KAlSi3O8 tectosilicates (framework silicates), and hyalophane, celsian, anorthoclase and buddingtonite
Specific gravity: 2.54 - 2.63
Hardness: 6 to 6 1/2
Streak: White
Colour: Colourless, white, grey, greyish yellow, yellowish, tan, pink, bluish green, greenish white, reddish white
Melting point: About 1,300oC at atmospheric pressure

Plutonic igneous environments
Volcanic igneous environments
Metamorphic environments

K-feldspars are essential constituents of rhyolite and common constituents of quartzolite.
They also may be found in diorite.
K-feldspars are minerals of the greenschist and amphibolite facies.

Alteration

dolomite, K-feldspar and H2O to phlogopite, calcite and CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O = KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of phlogopite according to the above equation.

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌ K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage.

K-feldspar and H+ to muscovite, quartz and K+
3KaAlSi3O8 + 2H+ ⇌ KAl2(AlSi3O100(OH)2 + 6SiO2 + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction.

montmorillonite and K-feldspar to illite (variety of muscovite), SiO2 and H2O
Al2Si4O10(OH)2.nH2 + KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O

muscovite to corundum, K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 + K(AlSi3O8) + H2O
This reaction takes place above temperatures ranging from 600oC at atmospheric pressure to about 720oC at pressure above 4 kbar.

muscovite and quartz to sillimanite, K-feldspar and H2O
KAl2(Si3Al)O10(OH)2 + SiO2 ⇌ Al2SiO5 + KAlSi3O8 + H2O
This equation represents changes that may occur in regional metamorphism when the metamorphic grade changes from the greenschist facies (left hand side) to the amphibolite facies (right hand side) or vice versa.
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat), hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down.

Common impurities: Fe,Ca,Na,Li,Cs,Rb,H2O,Pb

Kalsilite

Formula:KAlSiO4 tectosilicate (framework silicate)
Specific gravity: 2.6
Hardness: 6
Streak: White
Colour: colourless, grey, grey white, white
Environments:

Plutonic igneous environments
Volcanic igneous environments

Kalsilite is found as embedded grains in silica-undersaturated lavas and nepheline-bearing igneous rocks.

Common impurities: Fe,Mg,Ca,Na

Kaolinite

Formula: Al2Si2O5(OH)4 phyllosilicate (sheet silicate)
Specific gravity: 2.6
Hardness: 2 - 2½
Streak: white
Colour: white, brownish white, greyish white, yellowish white, greyish green
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with decomposition in hydrochloric acid
Environments:

Clastic sedimentary environments
Metamorphic environments

Kaolinite is always a secondary mineral derived from weathering or hydrothermal alteration of alumino-silicate minerals. It is a mineral of the zeolite facies, where clay minerals transform to illite, kaolinite and vermiculite.

Alteration

anorthite, H2SO4 and H2 to gypsum and kaolinite
CaAl2 Si2O8 + H2SO4 + 3H2O → CaSO4.2H2O + Al2Si2O5(OH)4 anorthite to calcite and kaolinite in the early Earth's atmosphere
CO2 + H2O + anorthitecalcite + kaolinite
CO2 + 2H2O + CaAl2Si2O8 → CaCO3 + Al2Si2O5(OH)4

anorthite, H2O and CO2 ⇌ kaolinite and calcite
2CaAl2 Si2O8 + 4H2O + 2CO2 ⇌ Al4Si4O10(OH)8 + 2CaCO3
calcite is found as a low-temperature, late-stage alteraation product according to the above reaction.

kaolinite to diaspore, SiO2 and H2O
Al2Si2O5(OH)4 ⇌ 2AlO(OH) + 2SiO2 (aqueous) + H2O
At 1 kbar pressure kaolinite is stable at temperatures less than 300oC; it can be in equilibrium with quartz and water in solutions both saturated and undersaturated with quartz. Diaspore is stable at temperatures less than 400oC but only in solutions undersaturated with quartz. High temperature and low quartz saturation favours the forward reaction.

kaolinite and H2O to gibbsite and quartz
Al2Si2O5(OH)4 + H2O ⇌ 2Al(OH)3 + 2SiO2

kaolinite to pyrophyllite, diaspore and H2O
2Al2Si2O5(OH)4 → Al2Si4O10(OH)2 + 2AlO(OH) + 2H2O
In the absence of quartz, kaolinite breaks down on heating according to the above reaction.

kaolinite, dolomite, quartz and H2O to chlorite, calcite and CO2
Al2Si2O5(OH)4 + 5CaMg(CO3)2 + SiO2 + 2H2O ⇌ Mg5Al(AlSi3O10)(OH)8 + 5CaCO3 + 5CO2
Chlorite often forms in this way from reactions between clay minerals such as kaolinite and carbonates such as dolomite.

kaolinite and quartz to pyrophyllite and H2O
Al2Si2O5(OH)4 + 2SiO2 → Al2Si4O10(OH)2 + H2O
This reaction represents the breakdown of kaolinite in the presence of quartz. (If quartz is absent, diaspore is formed as well as pyrophyllite).

pyrophyllite and H2O to kaolinite and aqueous SiO2
Al2Si2O10(OH)2 + H2O → Al2Si2O5(OH)4 + 2SiO2
At 1 kbar pressure kaolinite is stable at temperatures less than 300oC; it can be in equilibrium with quartz and water in solutions both saturated and undersaturated with quartz. Pyrophyllite is stable at temperatures up to 450oC and above, but except for a very narrow band of temperature and composition, it is stable only with solutions supersaturated with quartz

kaolinite to pyrophyllite, diaspore and H2O
2Al2Si2O5(OH)4 → Al2Si4O10(OH)2 + 2AlO(OH) + 2H2O
In the absence of quartz, kaolinite breaks down on heating according to the above reaction.

muscovite, H+ and H2O to kaolinite and K+
2KAl2(AlSi3O10)(OH)2 + 2H+ + 3H2O ⇌ 3Al2Si2O5(OH)4 + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction.

Common impurities: Fe,Mg,Na,K,Ti,Ca,H2O

Kernite

Formula: Na2B4O6<(OH)2.3H2O borate
Specific gravity: 1.9
Hardness: 2½ - 3
Streak: White
Colour: When fresh colourless and transparent, but usually white and opaque
Solubility: Slightly soluble in water; readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Chemical sedimentary environments

Kernite occurs in chemical sedimentary environments associated with colemanite and ulexite. The kernite is believed to have formed from borax.
It alters to tincalconite on dehydration.

Kintoreite

Formula: PbFe3+3(PO4)(PO3OH)(OH)6

Kolbeckite

Formula: Sc(PO4).2H2O hydrated normal phosphate, variscite group
Specific gravity: 2.35
Hardness: 3 to 5
Streak: White
Colour: Colourless, light yellow; when impure: cyan-blue, blue-gray, apple-green
Solubility: Decomposed by acids
Environments:

Hydrothermal environments

Kolbeckite is a very rare secondary mineral in phosphate deposits and some hydrothermal veins. It is found in a quartz hübnerite-ferberite vein at the type locality, the Sadisdorf copper mine, Saxony, Germany.

Kyanite

Formula: Al2OSiO4 nesosilicate (insular SiO4 groups). Polymorph (same formula, different structure) of andalusite and sillimanite.
Specific gravity: 3.53 to 3.65
Hardness: 5½ to 7
Streak: White
Colour: Blue, green, orange, black
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Placer deposits
Metamorphic environments (typical)

Kyanite is a common mineral of high to very high pressure regionally metamorphosed rocks and typically it is found in gneiss and schist. It is often associated with garnet, staurolite and corundum.
It is a common constituent of eclogite, and it also may be found in hornfels and in garnet- omphacite-kyanite occurrences in kimberlite pipes.
Kyanite develops after staurolite and before sillimanite with increasing grade of metamorphism. It is a characteristic mineral of the amphibolite and granulite facies and it is also a mineral of the blueschist and eclogite facies.

Alteration

Andalusite, sillimanite and kyanite are polymorphs (same formula, different structure); they are in equilibrium at a pressure of 4.2 kbar and temperature 530oC. Under conditions of higher temperature and pressure andalusite may become unstable and invert to its polymorphs sillimanite or kyanite. Andalusite is stable at lower pressure and temperature up to about 900oC; kyanite is the high pressure polymorph, and sillimanite the high temperature polymorph.

talc and kyanite to cordierite, corundum and H2O
2Mg3Si4O10(OH)2 + 7Al2OSiO4 ⇌ 3Mg2Al4Si5O18 + Al2O3 + 2H2O

Langite

Formula: Cu4(SO4)(OH)6.2H2O hydrated sulphate containing hydroxyl
Specific gravity: 3.48 to 3.5
Hardness: 2½ to 3
Streak: Greenish blue
Colour: Blue, greenish-blue
Solubility: Insoluble in water. Readily soluble in acids or ammonia
Environments

Hydrothermal environments

Langite is a secondary copper mineral formed from the oxidation of copper sulphides, often as a post-mining deposit, typically found within oxidised copper-bearing veinstone within mine dumps and underground as flowstone on mine walls.

Laumontite

Formula: CaAl2Si4O12.4H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.23 - 2.41
Hardness: 3½ to 4
Streak: White
Colour: White, brown, yellow, pink
Solubility: Moderately soluble in hydrochloric acid
Environments:

Pegmatites
Metamorphic environments

Laumontite is a zeolite facies mineral, formed by the decomposition of analcime. When it is associated with prehnite it forms in the transition zone between the zeolite and greenschist facies
Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that laumontite crystallised at temperatures from 98oC to 230oC.

Alteration

laumontite to lawsonite, quartz and H2O
CaAl2Si4O12.4H2O→ CaAl2(Si2O7)(OH)2.H2O + 2SiO2 + 2H2O
In subduction zones, as the pressure rises to above about 1.5 kbar, laumontite alters to lawsonite according to the above reaction.

laumontite and calcite to prehnite, quartz, H2O and CO2
CaAl2Si4O12.4H2O + CaCO3 → Ca2Al(Si3Al)O10(OH)2 + SiO2 + 3H2O + CO2
Prehnite and pumpellyite form from calcium zeolites in the presence of calcite, as in the above equation.

Common impurities: Na,K,Fe

Lawsonite

Formula: CaAl2(Si2O7)(OH)2.H2O sorosilicate (Si2O7 groups)
Specific gravity: 3.1
Hardness: 7½
Streak: White
Colour: Blue, white
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Metamorphic environments

Lawsonite is a common constituent of gneiss and schist formed under low temperature and high pressure. It is a typical mineral of glaucophane schist, associated with chlorite, epidote, titanite, glaucophane, garnet and quartz.
Lawsonite also may be found in gneiss.

It is a mineral of the prehnite-pumpellyite and blueschist facies.

Alteration

anorthite, albite and H2O to jadeite, lawsonite and quartz
CaAl2 Si2O8 + NaAlSi3O8 + 2H2O → NaAlSi2O6 + CaAl2(Si2O7)(OH)2.H2O + SiO2

laumontite to lawsonite, quartz and H2O
CaAl2Si4O12.4H2O→ CaAl2(Si2O7)(OH)2.H2O + 2SiO2 + 2H2O
In subduction zones, as the pressure rises to above about 1.5 kbar, laumontite alters to lawsonite according to the above reaction.

lawsonite and jadeite to clinozoisite, paragonite, quartz and H2O
4CaAl2(Si2O7)(OH)2.H2 + NaAlSi2O6 ⇌ 2Ca2Al3[Si2o7][SiO4]O(OH) + NaAl2(Si3Al)O10(OH)2 + SiO2 +6H2
Clinozoisite and paragonite may have been derived from lawsonite by the above reaction.

Leadhillite

Formula: Pb4(SO4)(CO3)2(OH)2 compound carbonate
Polymorphs susannite and macphersonite
Hardness: 2½ to 3
Streak: White
Colour: Colourless to white, grey, yellowish, pale green to blue; colourless in transmitted light
Solubility: Soluble in nitric acid with effervescence, rendering a residue of lead sulphate. Exfoliates in hot water.
Environments:

Hydrothermal replacement environments

Leadhillite occurs with mimetite and melanotekite Pb2Fe3+2O2(Si2O7) at Tsumeb, Namibia.

At the Manila Mine in Arizona, USA, it occurs associated with anglesite, and in vugs with caledonite, diaboleite CuPb2Cl2(OH)4, linarite and lanarkite Pb2O(SO4).

It is found at several localities in Cumbria, England:
At Balliway Rigg it has been found in cavities in oxidised galena in quartz veins, associated with caledonite, mattheddleite Pb5(SiO4)1.5(SO4)1.5Cl and lanarkite Pb2O(SO4). It is sometimes associated with anglesite, cerussite or linarite.
At Red Gill Mine it occurs as crusts on quartz associated with numerous minerals including anglesite, linarite, caledonite, bindheimite Pb2Sb5+2O7, cerussite and susannite.
At Shortgrain leadhillite is common in the supergene post-mining assemblage, and in some specimens it replaces lanarkite Pb2O(SO4). Crystals are also produced by in situ oxidation in the vein. It is often associated with its polymorphs susannite and macphersonite, and sometimes occurs with scotlandite Pb(S4+O3), native silver or caledonite.
At Silver Gill it occurs associated with anglesite and caledonite.
At Driggith Mine dumps it occurs in oxidised galena-bearing fragments associated with caledonite, anglesite and cerussite.
At the Brae Fell Mine it occurs associated with anglesite, caledonite, cerussite and linarite.

Heating leadhillite causes it to reversibly transform into its polymorph susannite in the temperature range from 50 to 82°C

Lepidocrocite

Formula: Fe3+O(OH)

Lepidolite

Lepidolite is a series between polylithionite and trilithionite Formula:
Polylithionite: KLi2AlSi4O10F2
Trilithionite: KLi1.5Al1.5(Si3Al)O10F2
Both are phyllosilicates (sheet silicates) mica group
Specific gravity: 2.8 - 2.9
Hardness: 2 - 2½
Streak: White
Colour: Pink, lilac, reddish; The lavender pink colour is due to Mn3+. The colour of the mineral is not an indication of its lithium content.
Solubility: Slightly soluble in hydrochloric acid
Environments:

Pegmatites

Lepidolite is a comparatively rare mineral, found in lithium-rich pegmatites, usually associated with other lithium-bearing minerals such as pink and green tourmaline", amblygonite and spodumene, as well as microcline and quartz. It is often intergrown with muscovite in parallel layers. It appears that lepidolite forms late in the crystallisation of pegmatites, succeeding the more common muscovite and biotite of the outer pegmatite zones.

Common impurities: Polylithionite: Ti,Fe,Mn,Mg,Ca,Na,H2O

Leucite

Formula: K(AlSi2O6) tectosilicate (framework silicate)
Specific gravity: 2.45 - 2.5
Hardness: 5½ to 6
Streak: White
Colour: White, colourless
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments (rarely)
Volcanic igneous environments
Hydrothermal replacement environments

Leucite is very rare in plutonic masses. In volcanic environments leucite is found in potassium-rich mafic and ultramafic lavas, where it forms directly from cooling lava in low silica environments with high potassium content.
Leucite is a primary rock-forming mineral.
It may be found in andesite, basalt, diorite, gabbro, syenite and trachyte.
If sodium is abundant, nepheline occurs rather than leucite.
Leucite never occurs together with quartz; it reacts with free quartz to form K-feldspar.
In the oxidation zone it often transforms to pseudoleucite, which is a mixture of nepheline and orthoclase; further oxidation may break it down into kaolinite or clay.
In pre-tertiary rocks (older than 65 million years) leucite readily decomposes to zeolites, analcime and other secondary minerals.

Common impurities: Ti,Fe,Mg,Ca,Ba,Na,Rb,Cs,H2O

Lévyne

Formula: (Ca,Na2)3(Si12Al6)O36.18H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.09 to 2.16
Hardness: 4 to 4½
Streak: White
Colour: Colourless, white, grey, yellow, red
Environments:

Volcanic Igneous environments

Lévyne occurs most commonly in low silica volcanic rocks such as olivine basalt, and rarely in andesite, often associated with other zeolites, especially offretite and erionite.

Limonite

"Limonite" is a term for unidentified massive hydroxides and oxides of iron, with no visible crystals, and a yellow-brown streak. It is most commonly the mineral species goethite, but it can also consist of varying proportions of other minerals, including jarosite group minerals and hematite.

Linarite

Formula: CuPb(SO4)(OH)2 sulphate
Specific gravity: 5.3 - 5.5
Hardness: 2½
Streak: Light blue
Colour: Blue
Solubility: Moderately soluble in nitric acid
Environments:

Hydrothermal replacement environments

Linarite occurs as a secondary mineral in the oxidation zone of high temperature hydrothermal deposits, where it is formed as an alteration product of galena. In some Cumbrian localities it is found on limonitic quartz or partly decomposed galena with cerussite. It also occurs near the contact between galena and the surrounding matrix, if any copper sulphides are present. It may be formed by post-mining oxidation. The linarite, anglesite, brochantite assemblage is typical for some localities, and helps to distinguish linarite from azurite.
Linarite has been found pseudomorphed by chrysocolla, malachite and brochantite, and it is often intergrown with leadhillite.

Lizardite

Formula: Mg3Si2O5(OH)4 phyllosilicate (sheet silicate), serpentine group
Specific gravity: 2.55 to 2.60
Hardness: 2½
Streak: White
Colour: Green, green blue, yellow, white.

Metamorphic environments

Lizardite is formed by the alteration of magnesium silicates below 75oC

Alteration

See results for serpentine, which is a group of minerals including serpentine, chrysotile and lizardite, all of which share the same formula, although they have slightly different structures.

Magnesite

Formula: Mg(CO3) carbonate
Specific gravity: 2.98 to 3.02
Hardness: 3½ to 4½
Streak: White
Colour: Colourless, white, greyish-white, yellowish, brown, faintly pink, lilac-rose; colourless in transmitted light
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acids. Slightly affected by cold acids. Readily soluble in warm hydrochloric acid with effervescence. Slightly soluble in water with the solubility increasing with the presence of NaCl, Na2SO4, or CO2
Environments:

Chemical sedimentary environments

Magnesite is an evaporite mineral

Alteration

antigorite and magnesite to forsterite, CO2 and H2O
Mg3Si2O5(OH)4 + MgCO3 → 2Mg2SiO4 + CO2 + 2H2O

forsterite and CO2 to enstatite and magnesite
Mg2SiO4 + CO2 ⇌ MgSiO3 + MgCO3

magnesite and H2O to hydromagnesite Mg5(CO3)4(OH)2.4H2O and CO2
5MgCO3 + 5H2O → Mg5(CO3)4(OH)2.4H2O + CO2
Magnesite may react with water to form hydromagnesite or other low-temperature hydrated carbonates.

olivine and CO2 to enstatite- ferrosilite and magnesite- siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3

serpentine (chrysotile) and CO2 to talc, magnesite and H2O
2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + 3H2O
Serpentine (chrysotile) is not stable in the presence of carbon dioxide and reacts with it according to the above equation.

Common impurities: Fe,Mn,Ca,Co,Ni,ORG

Magnetite

Formula: Fe2+Fe3+2O4 oxide
Specific gravity: 5.175
Hardness: 5½ to 6½
Streak: Black
Colour: Black
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Volcanic igneous environments
Carbonatites
Clastic sedimentary environments
Chemical sedimentary environments
Placer deposits
Metamorphic environments (typical)
Hydrothermal replacement environments

Magnetite is found in igneous environments, carbonatites, sedimentary environments including placers, regional metamorphic environments, massive hydrothermal replacement deposits and hydrothermal replacement lodes. It is a common constituent of sedimentary and metamorphic banded iron formations, and in such occurrences it is of a chemical sedimentary origin. It is found in black sands often associated with corundum, forming emery. In some rocks magnetite may be one of the chief constituents and form large ore bodies.
Magnetite is a primary mineral.
It also may be found in andesite, basalt, gabbro, granite, kimberlite, rhyolite, syenite,

Alteration

aenigmatite, anorthite and O2 to hedenbergite, albite, ilmenite and magnetite
½Na4[Fe2+10Ti2]O4[Si12O36] + CaAl2Si2O8 + ½O2 = CaFe2+Si2O6 + 2NaAlSi3O8 + Fe2+Ti4+O3 + Fe2+Fe3+2O4

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan.

fayalite and H2O to magnetite, SiO2 and H2
3Fe2+2(SiO4) + 2H2O &38594; Fe2+Fe3+2O4 + 3SiO2 + 2H2
This reaction is highly exothermic

fayalite, H2O and O2 to cronstedtite and magnetite
6Fe2+2(SiO4) + 6H2O + ½O2 = 3Fe3Si2O5(OH)4 + Fe2+Fe3+2O4

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

hematite, wüstite, quartz and calcite to andradite, hedenbergite, magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2
If wüstite, FeO, is also introduced hedenbergite and magnetite may form in addition to andradite:

titanomagnetite (ilmenite combined with magnetite), quartz, and aegirine-hedenbergite to aenigmatite, hedenbergite, magnetite and O2
6(Fe2+Ti4+O3 + Fe2+Fe3+2O4) + 12SiO2 + 12(NaFe3+Si2O6 + CaFe2+Si2O6) ⇌ 3Na4[Fe2+10Ti2]O4[Si12O36] + 12CaFe2+Si2O6 + 2Fe2+Fe3+2O4 + 5O2

jadeite, diopside, magnetite and SiO2 to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction.

magnetite to hematite
Magnetite may convert to hematite, and vice versa, depending on the pressure and temperature, according to the equation:
magnetite + oxygen ⇌ hematite
2Fe3O4 + ½O2 ⇌ 3Fe2O3

olivine and H2O to serpentine, magnetite and H2
6(Mg1.5Fe0.5)SiO4 + 7H2O → 3Mg3Si2O5(OH)4 + Fe2+Fe3+2O4 + H2
The iron Fe in olivine does not enter into the serpentine, but recrystallises as magnetite.

staurolite, annite and O2 to hercynite, magnetite, muscovite,corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O

Common impurities: Mg,Zn,Mn,Ni,Cr,Ti,V,Al

Malachite

Formula: Cu2(CO3)(OH)2 anhydrous carbonate containing hydroxyl
Specific gravity: 3.6 - 4.05
Hardness: 3½ to 4
Streak: Green
Colour: Green
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Carbonatites
Hydrothermal replacement environments

Malachite is a very common secondary copper mineral found in the oxidation zones of high temperature hydrothermal copper deposits, often in limestone, associated with azurite, cuprite, native copper, and iron oxides.

Alteration

azurite and H2O to malachite and CO2
2Cu3(CO3)2(OH)2 + H2O → 3Cu2(CO3(OH)2 + CO2
Azurite is unstable under atmospheric conditions, and slowly converts to the more stable malachite according to the above reaction. This instability is evidenced by the existence of many pseudomorphs of malachite after azurite; pseudomorphs of azurite after malachite are extremely rare.

Common impurities: Zn,Co,Ni

Manganite

Formula: Mn3+O(OH) oxide containing hydroxyl
Specific gravity: 4.3 - 4.4
Hardness: 4
Streak:
Colour: Brownish black to black
Solubility: Slightly soluble in hydrochloric acid; moderately soluble in sulphuric acid
Environments:

Pegmatites
Hydrothermal vein environments

Manganite is found associated with other manganese oxides in deposits formed by meteoric waters. It is often found in epithermal (low temperature) hydrothermal veins associated with acanthite, baryte, cinnabar, pyrolusite, quartz, siderite and calcite.

It frequently alters to pyrolusite.

Marcasite

Formula: FeS2 sulphide
Specific gravity: 4.887
Hardness: 6 to 6½
Streak: Dark grey to black
Colour: Pale brass-yellow, tin-white on fresh exposures.
Solubility: Insoluble in hydrochloric acid and sulphuric acid
Environments:

Chemical sedimentary environments
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Marcasite most frequently occurs as replacement deposits also often in concretions and in limestone, and often in concretions and replacing organic matter and forming fossils in sedimentary beds, particularly coal beds. It is also found in chemical sedimentary environments, massive and disseminated hydrothermal replacement deposits and hydrothermal replacement lodes and in epithermal (low temperature) hydrothermal veins.
Marcasite may be found in clay, marl, shale and dolostone.
In hydrothermal veins it may be associated with pyrite.

Alteration

Marcasite is a mineral of low-temperature, near-surface, environments, forming from acid solutions. Pyrite, the more stable form of FeS2, forms in higher temperatures and lower acidity or alkaline environments.

Common impurities: Cu,As

Mawbyite

Formula: PbFe3+2(AsO4)2(OH)2

Mazzite

Mazzite is the name for two minerals
Mazzite-Mg: Mg5(Si26Al10)O72.30H2O
Mazzite-Na: Na8(Si28Al8)O72.30H2O
Tectosilicates (framework silicates), zeolite group
Specific gravity: 2.11
Streak: White
Colour: Colourless
Environments:

Volcanic igneous environments
Hydrothermal environments

Mazzite occurs in basalt associated with phillipsite, chabazite and offretite. It is of hydrothermal origin.

Melanterite

Formula: Fe(SO4).7H2O sulphate
Specific gravity: 1.89
Hardness: 2
Streak: White
Colour: Colourless to white or green, also greenish-blue to blue with increased Cu content; colourless to pale green in transmitted light. Usually a yellowish-white after exposure to air and moisture.
Solubility: Readily soluble in water
Environments:

Hydrothermal replacement environments

Alteration

Melanterite is a secondary mineral formed by the oxidation of pyrite, marcasite and other iron sulphides in massive hydrothermal replacement deposits. It is often found in mines as a post-mining formation.
Melanterite may dehydrate to siderotil.

pyrite (primary) O and H2O to secondary melanterite and sulphuric acid.
FeS2 + 7O + 8H2O → FeSO4.7H2O + H2SO4
Melanterite indicates the presence of sulphuric acid, and it should be handled with care.

Common impurities: Cu,Mg

Melilite

Melilite is a group of tetragonal sorosilicates (Si2O7 groups). The most abundant members of the group are minerals in the åkermanite-gehlenite series
Formulae
Åkermanite: Ca2MgSi2O7
Gehlenite: Ca2Al(SiAl)O7
Melilite is a common component of magnesian skarn, and it also may be found in granite. It has been reported in a lamprophyre dike at Cripple Creek, Colorado, USA.
Quartz never occurs with melilite.

Merlinoite

Formula: K5Ca2(Si23Al9)O64.24H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.14 to 2.27
Hardness: 4½
Streak: White
Colour: Colourless to white
Environments:

Volcanic igneous environments
Pegmatites
Sedimentary environments
Basaltic cavities

Merlinoite is a secondary mineral occuring in cavities in basalt, in pegmatites, in pyroxene-rich volcanic ejecta, and as a diagenetic alteration in volcanic ash falls.
Common impurities: Fe

Merwinite

Formula: Ca3Mg(SiO4)2 nesosilicate (insular SiO4 groups)
Specific gravity: 3.15 to 3.32
Hardness: 6
Streak: White
Colour: White, colourless, light green, light gray green

Metamorphic environments

At the type locality merwinite is found in high temperature, low pressure contact metamorphic zones with marbles; in siliceous dolomitic limestone in contact metamorphic zones, it is formed at relatively elevated temperatures.

Alteration

monticellite and spurrite to merwinite and calcite
2CaMg(SiO4) + Ca5(SiO4)2(CO3) ⇌ 2Ca3Mg(SiO4)2 + CaCO3

monticellite, spurrite and quartz to merwinite and CO2
5CaMg(SiO4) + Ca5(SiO4)2(CO3) + SiO2 ⇌ 5Ca3Mg(SiO4)2 + 2CO2

Common impurities: Al, Fe

Mesolite

Formula: Na2Ca2(Si9Al6)O30.8H2O tectosilicate (framework silicate) zeolite group
Specific gravity: 2.26
Hardness: 5
Streak: White
Colour: Colourless, white, gray, yellowish
Solubility:
Environments: Occurs in amygdaloidal basalt and similar rocks

Basaltic cavities
Hydrothermal vein environments

Mesolite occurs in cavities in volcanic rocks, typically basalt, also in andesites, porphyries and hydrothermal veins

Common impurity: K

Localities

Iran

In the vicinity of Meshkinshahr, Ardabil Province, mesolite is the most common zeolite, almost always associated with thomsonite and analcime.

Metatorbernite

Formula:Cu(UO2)2(PO4)2.8H2O hydrated normal phosphate
The prefix "meta" denotes a dehydration product (of torbernite)
Specific gravity: 3.52 to 3.70
Hardness: 2½
Streak: Pale green
Colour: Pale green to dark green
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acids

Igeous environments
Hydrothermal environments

Metatorbernite is typically a secondary mineral, formed as a dehydration product of torbernite, which is unstable; it also may be formed directly above 75oC.

Common impurities: Ca,Ba,Mg

Metavariscite

Formula: Al(PO4).2H2O hydrated normal phosphate, variscite group, dimorph of variscite
Specific gravity: 2.54
Hardness: 3½
Streak: White
Colour: Light green; colourless in transmitted light

Environments:

Hydrothermal environments

Metavariscite occurs in cavities in variscite nodules at the type locality, Edison-Bird Mine, Utah, USA, and in phosphatised andesite at the Pacific Ocean island of Malpelo (Colombia).

Mica

The mica group is a group of sheet silicates, including muscovite and biotite.
Mica is a common constituent of most pegmatites, granite, diorite, gabbro, andesite and gneiss.
It also may be found in quartzolite, kimberlite, clay, skarn, amphibolite and eclogite.
It is never found in the granulite facies rocks.

Microcline

Formula: K(AlSi3O8) tectosilicate (framework silicate)
Microcline is a K-feldspar.
Amazonite is a variety of microcline
Adularia is a more ordered low-temperature variety of orthoclase or partially disordered microcline.
Properties of microcline:
Specific gravity: 2.54 - 2.57
Hardness: 6 to 6½
Streak: White
Colour: White, blue, green, pink, yellow
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments (microcline):

Pegmatites
Clastic sedimentary environments
Metamorphic environments

Microcline occurs mainly as a primary mineral in schist and gneiss. K-feldspars are essential constituents of granite, and microcline is the common K-feldspar of pegmatites. In sedimentary rocks microcline is present in feldspar-rich sandstone.
Microcline is characteristic of the granulite facies and it is also a mineral of the albite-epidote-hornfels, hornblende-hornfels and pyroxene-hornfels facies.

Alteration

phlogopite, calcite and quartz to diopside, microcline, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter according to the above reaction.

Common impurities in microcline: Fe,Ca,Na,Li,Cs,Rb,H2O,Pb

Microlite

Microlite is a group of minerals Formula: (Na,Ca)2Ta2O6(O,OH,F)
Specific gravity: 5.9 to 6.4
Hardness: 5 to 5½
Streak: light yellowish, brownish
Colour: pale yellow to reddish brown, sometimes emerald green
Solubility: Insoluble in water, hydrochloric and nitric acid; soluble with decomposition in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites

Microlite is found in granitic pegmatites, especially those rich in lepidolite or other lithium-bearing minerals, and in albite

Millerite

Formula: NiS sulphide
Specific gravity: 5.3
Hardness: 3½
Streak: Dark green
Colour: Brass-yellow
Solubility: Insoluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Environments:

Hydrothermal replacement environments

Millerite forms as a low-temperature mineral often in cavities and as an alteration of other nickel minerals, or as crystal inclusions in other minerals. Millerite occurs in nickel deposits, as an alteration product of other nickel ores.

Common impurities: Fe,Co,Cu

Millisite

Formula: NaCaAl6(PO4)4(OH)9.3H2O hydrated phosphate containing hydroxyl, wardite group
Specific gravity: 2.83
Hardness: 5½
Streak: White
Colour: White to light grey, colourless in transmitted light.
Environments:

Hydrothermal environments

Millisite is found in altered phosphate deposits. At the type locality, the Little Green Monster Mine, Utah, USA, it is associated with dehrnite, lewistonite and wardite.

Mimetite

Formula: Pb5(AsO4)3Cl arsenate
Specific gravity: 7.1
Hardness: 3½ - 4
Streak: White
Colour: Colourless, white, brown, orange, yellow, green, grey
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid
Environments:

Hydrothermal replacement environments

Mimetite is a secondary mineral occurring in the oxidation zone of high temperature hydrothermal lead deposits that also bear arsenic-containing minerals. While pyromorphite usually occurs only in the uppermost zones, mimetite can also appear in deeper deposit regions

Alteration

Solubility of mimetite
Pb5(AsO4)3Cl (solid) + 6H+ (aqueous) ⇌ 5Pb2+ (aqueous) + 3H2AsO-4 (aqueous) + Cl- (aqueous)

cerussite and aqueous H2AsO4-, Cl- and H+ to mimetite and aqueous H2CO3
5PbCO3 + 3H2AsO4- + Cl- + 7H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
or
5PbCO3 + 3HAsO42- + Cl- + 4H+ ⇌ Pb5(AsO4)3Cl + 5H2CO3
cerussite and mimetite can co-exist only under basic conditions at rather high PCO2.

Common impurities: Ca,F,Cr,V

Mirabilite

Formula: Na2(SO4).10H2O
Specific gravity: 1.490
Hardness: 1½ to 2
Streak: White
Colour: Colourless, white
Solubility: Soluble in water
Environments:

Chemical sedimentary environments

Mirabilite occurs in salt lakes, playas and springs, and as an efflorescence on alkali soils. At Great Salt Lake, Utah, USA, it crystallises during winter due to decreased solubility at low temperatures.

Alteration: Mirabilite loses its water in dry air and falls to a loose powder.

Molybdenite

Formula: MoS2 sulphide
Specific gravity: 4.7 - 4.8
Hardness: 1 - 1½
Streak: Dark grey
Colour: Lead-grey
Solubility: Moderately soluble in sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Molybdenite forms as an accessory mineral in some igneous rocks and in pegmatites. It is found in contact metamorphic deposits, and it is important in disseminated deposits of the porphyry (with coarse crystals or mineral grains phenocrysts in a finer groundmass) type. It is common as a primary mineral in hypothermal (high temperature) hydrothermal veins.
Molybdenite may be found in some granites, including aplite
In contact metamorphic deposits it is associated with lime silicates, scheelite and chalcopyrite.
In hypothermal (high temperature) hydrothermal veins it is associated with cassiterite, scheelite, hubnerite-ferberite and fluorite.

Monazite-(Ce)

Monazite-(Ce) is the overwhelmingly most common member of the monazite group. Formula: Ce(PO4) phosphate
Specific gravity: 5 - 5.5
Hardness: 5 to 5½
Streak: White
Colour: Commonly reddish brown to brown; shades of green to brown, yellow brown, rarely nearly white; yellow, colourless in transmitted light.
Solubility: Slightly soluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Clastic sedimentary environments
Metamorphic environments
Hydrothermal vein environments (infrequent)

Monazite is a comparatively rare mineral occurring as an accessory in some plutonic igneous rocks, in pegmatites and as rolled grains in sands because of its resistance to chemical attack and its high specific gravity.
It may be found in granite including aplite, syenite, schist, gneiss and granulite.
In clastic sedimentary deposits it is associated with other resistant and heavy minerals such as magnetite, ilmenite, rutile and zircon.

Montgomeryite

Formula: Ca4MgAl4(PO4)6(OH)4.12H2O hydrated phosphate with hydroxyl, montgomeryite group
Specific gravity: 2.530
Hardness: 4
Streak: White
Colour: Dark green to light green, colourless, red, yellow
Environments:

Pegmatites
Hydrothermal environments

Montgomeryite is a secondary mineral in sedimentary phosphate nodules and a late-stage mineral in highly oxidized phosphate nodules in granitic pegmatites. At the Little Green Monster Mine, Utah, USA, it occurs associated with wardite, englishite, gordonite, crandallite and apatite.

Monticellite

Formula: CaMg(SiO4) nesosilicate (insular SiO4 groups) olivine group
Specific gravity: 3.2
Hardness: 5½
Streak: White
Colour: Colourless, grey or greenish
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with gelatinous residue in hydrochloric acid
Environments:

Volcanic igneous environments <
Carbonatites
Metamorphic environments

Monticellite occurs as a relatively common mineral formed during metamorphism of siliceous dolostones, in contact metamorphic deposits between limestones and olivine gabbros and in skarns at granite-dolomitic limestone contacts. It occurs less frequently in kimberlites.

Alteration

During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite

åkermanite to monticellite and wollastonite
Ca2MgSi2O7 → CaMg(SiO4) + CaSiO3

diopside, forsterite and calcite to monticellite and CO2
CaMgSi2O6 + Mg2SiO4 + 2CaCO3 → 3CaMgSiO4 + 2CO2
This reaction requires a high temperature.

forsterite and åkermanite to diopside and monticellite
Mg2SiO4 + 2Ca2MgSi2O7 → CaMgSi2O6 + 3CaMg(SiO4)

forsterite, calcite and quartz to monticellite and CO2
Mg2SiO4 + 2 CaCO3 + SiO2 → 2CaMg(SiO4) + 2 CO2

forsterite, diopside and calcite to monticellite and CO2
Mg2SiO4 + CaMgSi2O6 + 2 CaCO3 ⇌ 3CaMg(SiO4) + 2 CO2
This reaction occurs during contact metamorphism of magnesian limestone.

grossular, diopside, monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2 A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises.

monticellite and CO2 to åkermanite, forsterite and calcite
3CaMgSiO4 + CO2 ⇌ Ca2MgSi27 + Mg2O7 + CaCO3
The forward reaction occurs at temperature s above 890oC at and pressure above about 4.3 kbar.

monticellite and diopside to åkermanite and forsterite
3CaMgSiO4 + CaMgSi2O6 ⇌ 2Ca2MgSi2O7 + Mg2O7
Monticellite is stable below 890oC at pressure of about 4.3 kbar.

monticellite and spurrite to merwinite and calcite
2CaMg(SiO4) + Ca5(SiO4)2(CO3) ⇌ 2Ca3Mg(SiO4)2 + CaCO3

monticellite, spurrite and quartz to merwinite and CO2
5CaMg(SiO4) + Ca5(SiO4)2(CO3) + SiO2 ⇌ 5Ca3Mg(SiO4)2 + 2CO2

Common impurities: Ti,Al,Fe,Mn,Zn,H2O

Montmorillonite

Formula: (Na,Ca)0.3(Al,Mg)2Si4O10(OH)2(OH)2. nH2O phyllosilicate (sheet silicate)
Specific gravity: 2.0 to 2.7
Hardness: 1 to 2
Streak: White
Colour: White, buff, yellow, green, rarely pale pink to red. Pink to red coloration is due to high valance Mn
Environments:

Clastic sedimentary environments
Chemical sedimentary environments

Montmorillonite is a common clay mineral. It is an alteration product of volcanic tuff and ash, or it may precipitate from water. It forms under alkaline conditions of poor drainage. It is a zeolite facies mineral.

Alteration

albite and montmorillonite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz and H2O
8Na(AlSi3O8) + 2Ca0.5(Mg3.5Al0.5)Si8O20(OH)4.nH2O → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 15SiO2 + 6H2O
This reaction occurs in low to intermediate metatmorphism.

montmorillonite and K-feldspar to illite, aqueous SiO2 and H2O
Al2Si4O10(OH)2.nH2 + KAlSi3O8 → KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O

Common impurities: Fe,K

Mordenite

Formula: (Na2,Ca,K2)4(Al8Si40)O96.28H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.10 to 2.15
Hardness: 4 to 5
Streak: White
Colour: Colourless, white, yellow, pink, orange, red
Environments:

Volcanic Igneous environments
Sedimentary environments
Hydrothermal environments

Mordenite occurs in silica rich (eg rhyolitic) volcanics, in volcanic ash beds, tuff, and rarely in olivine-basalt. It is of hydrothermal or sedimentary authigenic (formed in place) origin.
Common impurities: Mg

Mottramite

Formula: PbCu(VO4)(OH) anhydrous vanadate containing hydroxyl
Specific gravity: 5.9
Hardness: 3 to 3½
Streak: Yellowish
Colour: Grass-green, olive-green, yellow-green, siskin-green, blackish brown, nearly black
Solubility: Readily soluble in acids
Environments:

Hydrothermal environments

Mottramite is a secondary mineral associated with descloizite in oxidised zones, especially in sandstone.
Common impurities: Zn

Muscovite

Formula: KAl2(AlSi3O10)(OH)2 phyllosilicate (sheet silicate), mica group
Muscovite forms a continuous series with celadonite KMgFe3+Si4O10(OH)2

Illite series minerals are K-deficient varieties of muscovite
Sericite is a variety of muscovite
Specific gravity: 2.77 - 2.88
Hardness: 2½
Streak: White
Colour: White, yellow
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments (muscovite):

Plutonic igneous environments
Pegmatites
Metamorphic environments
Environments (sericite):

Pegmatites
Metamorphic environments

In the Bowen reaction series muscovite is intermediate between orthoclase (higher temperature) and quartz (lower temperature).
Muscovite is a widespread and common rock-forming mineral. It is a primary mineral typically found in granite and granite pegmatites, where it is associated with quartz and feldspar. It is also very common in metamorphic rocks, being the chief constituent of some mica schists. In the chlorite zone of metamorphism muscovite is a characteristic constituent of albite-chlorite-muscovite schist. Muscovite is a mineral of the albite-epidote-hornfels, hornblende-hornfels, prehnite-pumpellyite, greenschist, amphibolite, blueschist and eclogite facies.

Sericite is a variety of muscovite that occurs as fibrous aggregates with a silky lustre. The development of sericite from feldspar and other minerals, such as topaz kyanite, spodumene, and andalusite, is a common feature of retrograde metamorphism (changes that occur during uplift and cooling of a rock). Sericite also forms as an alteration of the wall rock of hydrothermal ore veins.

The illite series minerals frequently contain randomly sequenced montmorillonite/beidellite layers. They occur as constituents of some shale, soil and recent sediments. Illite is a mineral of the zeolite and prehnite-pumpellyite facies. In the zeolite facies clay minerals transform to illite, kaolinite and vermiculite.

Muscovite is an essential constituent of phyllite, and a common constituent of granite. It also may be found in gneiss, schist and amphibolite

At the Chickering mine, New Hampshire, USA, muscovite is a component of the outer pegmatite zones, having formed before the melt became enriched in lithium in the inner zones, forming the lithium-rich mica lepidolite. It also occurs much later as a druse lining moulds where elbaite crystals have dissolved.
At Yaogangxian, Hunan, China, it sometimes occurs coating earlier formed arsenopyrite and ferberite, and at the Santo Nino mine, Arizona, USA, it encloses rutile.

Alteration

The fine-grained "pinite", which is mainly composed of muscovite and clay minerals, occurs as an alteration product of cordierite.

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases

K-feldspar and H+ to muscovite, silica and K+
3KaAlSi3O8 + 2H+ ⇌ KAl2(AlSi3O100(OH)2 + 6SiO2 (aqueous) + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction.

montmorillonite and K-feldspar to illite (variety of muscovite), aqueous SiO2 and H2O
Al2Si4O10(OH)2.nH2 + KAlSi3O8 ⇌ KAl2(AlSi3)O10(OH)2 + 4SiO2 + nH2O

muscovite to corundum, K-feldspar and H2O
KAl2(AlSi3O10)(OH)2 ⇌ Al2O3 + K(AlSi3O8) + H2O
This reaction takes place above temperatures ranging from 600oC at atmospheric pressure to about 720oC at pressure above 4 kbar.

muscovite, H+ and H2O to kaolinite and K+
2KAl2(AlSi3O10)(OH)2 + 2H+ + 3H2O ⇌ 3Al2Si2O5(OH)4 + 2K+
Low temperature and a low K+/H+ ratio favour the forward reaction.

muscovite and quartz to sillimanite, K-feldspar and H2O
KAl2(Si3Al)O10(OH)2 + SiO2 ⇌ Al2SiO5 + KAlSi3O8 + H2O
This equation represents changes that may occur in regional metamorphism when the metamorphic grade changes from the greenschist facies (left hand side) to the amphibolite facies (right hand side) or vice versa.
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat), hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down.

sillimanite, annite and H2O to staurolite, muscovite, SiO2 and O2
31Al2SiO5 + 4KFe2+3(AlSi3O10)(OH)2 + 6H2O → 34Fe2+2Al9si4O23(OH) + KAl2 (AlSi3O10)(OH)2 + 7 SiO2 + 1.5O2
Staurolite may occur as a product of retrograde metamorphism according to the above reaction.

spodumene, K+ and H+ to muscovite, quartz and Li+
3LiAlSi2O6 + K+ + 2H+ → KAl3Si3O10(OH)2 + 3SiO2 + 3Li+
The direct conversion of spodumene to muscovite liberates silica, but quartz is not usually present in pseudomorphs of muscovite after spodumene, and this requires an explanation.

staurolite, annite and O2 to hercynite, magnetite, muscovite,corundum, SiO2 and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O

Other reactions:
The fine-grained variety sericite replaces beryl, topaz and tourmaline in pegmatites.
Muscovite itself alters to illite.

Common impurities: Cr,Li,Fe,V,Mn,Na,Cs,Rb,Ca,Mg,H2O

Nahcolite

Formula: NaH(CO3) acid carbonate
Specific gravity: 2.21
Hardness: 2½
Streak: White
Colour: Colourless, white, grey, buff
Solubility: Readily soluble in water forming an alkaline solution that gives off carbon dioxide when heated. Also soluble in glycerine.
Environments

Chemical sedimentary environments

Nahcolite is a lacustrine (lake) evaporite mineral.

Localities

Italy

In old Roman conduit at Naples, Italy, nahcolite occurs admixed with trona and thermonatrite as an efflorescence.

Kenya

In Little Lake Mogadi, Kenya, it occurs as fibrous pseudomorphs after gaylussite, and as alteration rims around thermonatrite.

USA

At Searles Lake, California, nahcolite occurs in thin beds associated with gaylussite, thénardite, burkeite, northupite, borax and halite.

Natrite

Formula: Na2CO3 anhydrous carbonate
Specific gravity: 2.54
Hardness: 3½
Streak: White
Colour: Grey-white to colourless
Solubility: Soluble in water
Environments:

Chemical sedimentary environments

Natrite occurs in deep workings and drill cores in mines in evaporite rocks in the Kola Peninsula, Russia. In air it rapidly hydrates to thermonatrite.

Natrolite

Formula: Na2(Si3Al2)O10.2H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.2 - 2.4
Hardness: 5 - 5½
Streak: White
Colour: Colourless, white, yellow
Solubility: Moderately soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Pegmatites
Basaltic cavities

Natrolite is characteristically found lining cavities in basalt associated with other zeolites and calcite. It is also found in syenite and nepheline syenite.
Geothermal wells have been drilled through a thick series of basalt flows in western Iceland, where it was found that natrolite crystallised at temperatures from 70oC to 100oC at depths between 450m and 1200m.

Common impurities: Ca,K

Natron

Formula: Na2CO3.10H2O hydrated normal carbonate
Specific gravity: 1.478
Hardness: 1
Streak: White
Colour: Colourless, white, grey, yellow
Solubility: Soluble in water forming alkali solution, effervesces in acid.
Environments:

Chemical sedimentary environments

On slight heating natron decomposes thermonatrite and H2O. It forms from water solution below 32oC but when other salts are present the temperature limit may be lower. It occurs in soda lakes. In the Eastern Gobi desert it is associated with thermonatrite, trona, gaylussite and calcite.

Nepheline

Formula: NaAlSiO4 tectosilicate (framework silicate). It forms partial solid solutions with both albite and anorthite.
Specific gravity: 2.6 - 2.7
Hardness: 5 - 6
Streak: White
Colour: Colourless, white, grey, yellow to brownish, reddish and greenish
Solubility: Readily soluble in hydrochloric acid
Environments:

Plutonic igneous rocks
Volcanic igneous environments
Pegmatites
Carbonatites

Nepheline is a primary rock-forming mineral that requires a high soda and low silica environment, and it never occurs together with quartz.
Nepheline may be found in andesite, basalt, diorite, gabbro, mafic igneous rocks (characteristic), syenite and trachyte.

Alteration

albite to nepheline and quartz
Na(AlSi3O8) ⇌ NaAlSiO4 + 2SiO2

nepheline and NaCl from the fluid to sodalite
6NaAlSiO4 + NaCl ⇌ 2Na4(Si3Al3)O12Cl

nepheline and H4SiO4 (silicic acid) to analcime and H2O
NaAlSiO4 + H4SiO4 ⇌ Na(AlSi2O6).H2O + H2O

nepheline and albite to jadeite
NaAlSiO4 + NaAlSi3O8 ⇌ 2NaAlSi2O6
The forward reaction requires high pressure, 10 to 25 kbar, and temperature 600oC to 1,000oC.

Common impurities: Mg,Ca,H2O

Nontronite

Formula: Na0.3Fe3+2(Si,Al)4O10(OH)2.nH2O phyllosilicate
Specific Gravity: 2.06 to 2.32
Hardness: 1 to 2
Streak: White
Colour: Green, olive-green, yellow-green, yellow, orange, brown
Environments:

Volcanic igneous environments
Clastic sedimentary environments
Metamorphic environments
Hydrothermal environments

Nontronite is a weathering product of basalt and other mafic and ultra-mafic volcanic rocks. It also occurs in poorly drained volcanic ash soils, in some hydrothermally altered mineral deposits, and contact metamorphosed limestone. It is authigenic (formed in place) in recent marine sediments.
Common impurities: Ti,Mg,Ca

Northupite

Formula: Na3Mg(CO3)2Cl anhydrous carbonate containing halogen
Specific gravity: 2.38
Hardness: 3½
Streak: White
Colour: Colourless, pale yellow, grey, brown
Solubility: Readily soluble in dilute acids with effervescence. Decomposed by hot water with the separation of magnesium carbonate.
Environments:

Chemical sedimentary environments
Clastic sedimentary environments

Northupite occurs in continental evaporite deposits and in oil shales. At Searles Lake, California, USA it is associated with tychite and pirssonite. In drill cores in the oil shales at Green River, Wyoming, USA, it occurs with shortite, bradleyite, trona, pirssonite and gaylussite.

Offretite

Formula: KCaMg(Si13Al5)O36.15H2O tectosilicate (framework silicate) zeolite group
Specific gravity: 2.10 to 2.13
Hardness: 4
Streak: White
Colour: Colourless, white, yellow, golden
Environments:

Volcanic igneous environments
Hydrothermal environments

Offretite occurs in veins and fractures in basaltic rocks, usually intergrown with erionite or as epitaxial overgrowths on it. Offretite is not found in Si-rich volcanics, pegmatites or altered volcanic ash tuff (where erionite is abundant). Offretite is of hydrothermal origin. At Passo Forcal Rosso, Italy, chabazite-offretite epitaxial overgrowths occur. In Hunter Valley, NSW, Australia offretite occurs on plates of lévyne.
Common impurities: Mg

Okenite

Formula: Ca10Si18O46.18H2O phyllosilicate
Specific Gravity: 2.28 to 2.33
Hardness: 4½ to 5
Streak: White
Colour: Colourless, white, pale yellow, blue
Environments:

Carbonatites
Basaltic cavities

Okenite occurs in cavities in basalt or related eruptive rocks. In limestone at Crestmore, California, USA, and in carbonatites. It is often associated with zeolites, apophyllite, calcite, prehnite and quartz.
Common impurities: Al,Fe,Sr,Na,K

Olivine

Olivine is a series between forsterite Mg2SiO4 and fayalite Fe2SiO4. These are both nesosilicates (insular SiO4 groups).
Solubility: Insoluble in water, nitric and sulphuric acid; soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Volcanic igneous environments

Olivine is a common primary, rock-forming mineral, varying in amount from an accessory to a major constituent. It is the first major mineral to crystallise in the discontinuous branch of the Bowen reaction series.
Olivine is an essentail constituent of kimberlite. It is a common constituent of gabbro, dunite, peridotite and basalt.
It also may be found in andesite, diorite, gabbro, granite,
It is associated with plagioclase feldspar, pyroxene, magnetite, corundum, chromite and serpentine.
quartz never occurs with olivine.
Olivine is a mineral of the granulite facies.

Alteration

cummingtonite-grunerite and olivine to enstatite-ferrosilite and H2O
(Fe,Mg)7Si8O22(OH)2 + (Mg,Fe)2SiO4 ⇌ 9(Mg,Fe2+)SiO3 + H2O

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

Mg-rich greenalite to olivine, Mg-rich grunerite and H2O
18(Fe2+, Mg))3Si2O5(OH)4 → 20(Fe,Mg)2SiO4 + 2(Fe2+,Mg)7Si8O22(OH)2 + 34H2O

Mg-rich greenalite to olivine, SiO2 and H2O
2(Fe2+, Mg))3Si2O5(OH)4 → 3(Fe,Mg)2SiO4 + SiO2 + 4H2O

hypersthene, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

olivine and CO2 to enstatite- ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3

olivine and H2O to serpentine, magnetite and H2
6(Mg1.5Fe0.5)SiO4 + 7H2O → 3Mg3Si2O5(OH)4 + Fe2+Fe3+2O4 + H2
The iron Fe in olivine does not enter into the serpentine, but recrystallises as magnetite.

olivine and SiO2 to enstatite-ferrosilite
(Mg,Fe)2SiO4 + SiO2 → 2(Mg,Fe2+)SiO3

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction

Mg-rich siderite and SiO2 to olivine, orthopyroxene and CO2
3(Fe,Mg)(CO3)→ (Fe,Mg)2SiO4 + 2SiO2 → (Fe,Mg)2SiO4 + 3CO2

Omphacite

Formula: (Ca,Na)(Mg,Fe,Al)Si2O6 inosilicate (chain silicate) pyroxene group
Specific gravity: 3.29 - 3.39
Hardness: 5 - 6
Streak: Greenish white
Colour: Dark green
Environments:

Metamorphic environments

Omphacite is an essential constituent of eclogite as a result of high pressure and high temperature metamorphism.
It is also found in kimberlites.
It is a characteristic mineral of the eclogite facies.

Alteration

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2 In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2 This reaction occurs at high temperature and pressure.

diopside and albite to omphacite and quartz
CaMgSi2O6 + xNaAlSi3O8 ⇌ CaMgSi2O6.xNaAlSi2O6 + SiO2

labradorite, albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2

Common impurities:Ti,Cr,Mn,K,H2O

Opal

Formula: SiO2.nH2O tectosilicate (framework silicate)
Specific gravity: 1.9 -2.2
Hardness: 5 - 6½
Streak: White
Colour: Colourless, translaprent (hyalite), whitish, bluish with a play of rainbow colours (precious opal), red to orange, translucent (fire opal), green, red, brown, yellow, opaque (common opal)
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Pegmatites
Clastic sedimentary environments
Basaltic cavities
Hot spring deposits

Opal is a low temperature secondary mineral that develops in a wide variety of rocks as cavity and fracture fillings; it may be deposited by hot springs at shallow depths, and it may replace the cells in wood and the shells of clams. The largest accumulations of opal are formed from silica-secreting organisms.

Orpiment

Formula: As2S3 sulphide
Specific gravity: 3.48
Hardness: 1½ - 2
Streak: Light yellow
Colour: Lemon yellow to orange yellow
Solubility: Slightly soluble in nitric acid
Environments:

Volcanic sublimates and hot spring deposits
Hydrothermal vein environments

Orpiment is a rare mineral, usually associated with realgar and formed under similar conditions. It occurs in epithermal (low temperature) hydrothermal silver and lead ore veins, together with cinnabar, realgar and calcite. It is also found in hot springs and fumaroles. It is an alteration product of arsenic minerals, especially realgar.

Common impurities: Hg,Ge,Sb

Orthoclase

Formula: K(AlSi3O8) tectosilicate (framework silicate), K-feldspar.
Adularia is a more ordered low-temperature variety of orthoclase or partially disordered microcline.
Properties of orthoclase
Specific gravity: 2.55 - 2.63
Hardness: 6
Streak: White
Colour: White, green, yellow, pink
Solubility: Insoluble in hydrochloric acid, sulphuric and nitric acids
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments

In the Bowen reaction series orthoclase is the first major mineral to crystallise after the two branches, continuous and discontinuous, combine.
K-feldspars are essential constituents of granite and syenite, and major constituents of granodiorite. When these rocks have cooled at moderate depth and at reasonably fast rates orthoclase is the characteristic K-feldspar. In more slowly cooled granite and syenite microcline is the characteristic K-feldspar.

Adularia is a low temperature form of either orthoclase or partially disordered microcline. It occurs mainly in low temperature veins in gneiss and schist. It is a mineral of the zeolite facies.

Alteration

biotite and SiO2 to enstatite-ferrosilite, orthoclase and H2O
K(Mg,Fe)3(AlSi3O10)(OH)2 + 3SiO2 → 3(Mg,Fe2+)SiO3 + KAlSi3O8 + H2O
Enstatite-ferrosilite may develop from the breakdown of biotite according to the above reaction.

Common impurities: Na,Fe,Ba,Rb,Ca

Overite

Formula: CaMgAl(PO4)2(OH).4H2O hydrated phosphate with hydroxyl, overite group
Specific gravity: 2.53
Hardness: 3½ to 4
Streak: White
Colour: Light apple-green to colourless; colourless in transmitted light
Solubility: Readily soluble in hot nitric acid
Environments:

Pegmatites
Hydrothermal environments

Overite is a secondary phosphate mineral found in altered phosphate nodules in sediments and in granitic pegmatites. At the type locality, Clay Canyon, Utah, USA, it is associated with crandallite and apatite.

Paragonite

Formula: NaAl2(Si3Al)O10(OH)2 phyllosilicate (sheet silicate), mica group, forms a series with muscovite.
Specific gravity: 2.78
Hardness: 2½
Streak: White
Colour: Colourless, pale yellow
Solubility: Insoluble in water and hydrochloric acid
Environments:

Metamorphic environments

Paragonite is a metamorphic mineral formed under a broad range of pressure-temperature conditions.
Paragonite is a common constituent of eclogite.
It is also found in schist and gneiss.
It is a mineral of the greenschist and blueschist facies.

Alteration

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

lawsonite and jadeite to clinozoisite, paragonite, quartz and H2O
4CaAl2(Si2O7)(OH)2.H2O + NaAlSi2O6 ⇌ 2Ca2Al3[Si2o7][SiO4]O(OH) + NaAl2(Si3Al)O10(OH)2 + SiO2 +6H2
Clinozoisite and paragonite may have been derived from lawsonite by the above reaction.

Pargasite

Formula: NaCa2(Mg4Al)(Si6Al2)O22(OH)2 inosilicate (chain silicate)
Specific gravity: 3.069 - 3.181
Hardness: 5 to 6
Streak: White
Colour: Green, brown
Environments:

Metamorphic environments

Pargasite is a common component of skarn metamorphosed from siliceous limestone. It also may occur in schist and amphibolite.

Common impurities: Ti,Cr,Mn,K,F,H2O,P

Parisite

Formula:
Parisite-(Ce): CaCe2(CO3)3F2
Parisite-(La): CaLa2(CO3)3F2
Both are anhydrous carbonates containing halogen
Specific gravity: 4.33 to 4.39
Hardness:
Streak: Light yellow
Colour: Brown to yellow; colourless to yellow in transmitted light
Solubility: Soluble in hot strong acids
Environments:

Hydrothermal vein environments

Parisite is found in calcite veins in hydrothermal deposits

Paulingite

Paulingite is the name for two minerals
Paulingite-Ca: (Ca,K,Na,Ba,☐)10(Si,Al)42O84.34H2O
Paulingite-K: (K,Ca,Na,Ba,☐)10(Si,Al)42O84.34H2O
tectosilicates (framework silicates), zeolite group
Specific gravity: 2.10 to 2.22
Hardness: 5
Streak: White
Colour: Colourless, light yellow, orange, red
Environments:

Volcanic igneous environments

Paulingite occurs in basalt associated with erionite and pyrite.

Pectolite

Formula: NaCa2Si3O8(OH) inosilicate (chain silicate) wollastonite group
Specific gravity: 2.8 - 2.9
Hardness: 4½ - 5
Streak: White
Colour: White, grey, sometimes greenish, yellowish, colourless
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Basaltic cavities

Pectolite is a secondary mineral similar in occurrence to zeolites. It is found lining cavities in basalt, associated with zeolites, prehnite and calcite.

Periclase

Formula: MgO simple oxide
Specific gravity: 3.55 to 3.57
Hardness: 5½
Streak: White
Colour: Colourless, grayish white, yellow, brownish yellow, green, black
Solubility: Insoluble in water, soluble in hydrochloric, nitric and sulphuric acid
Environments:

Metamorphic environments

Periclase is found as a product of contact metamorphism of dolomites and magnesites

Alteration

During the progressive metamorphism of silica-rich dolostones the following approximate sequence of mineral formation is often found, beginning with the lowest temperature product: talc, tremolite, diopside, forsterite, wollastonite, periclase, monticellite

brucite to periclase and H2O
Mg(OH)2 ⇌ MgO + H2O

Common impurity: Fe

Perovskite

Formula: CaTiO3 simple oxide
Specific gravity: 3.98 to 4.26
Hardness: 5½
Streak: Colourless, greyish white
Colour: Dark brown, black, red-brown, yellow shades
Solubility: Insoluble in hydrochloric and nitric acids; slightly soluble in sulphuric acid
Environments:

Plutonic igneous environments
Carbonatites
Metamorphic environments

Perovskite is an accessory mineral in silica-poor rocks, such as nepheline syenite, kimberlite (common) and carbonatite. It also occurs in calcareous skarn.

Common impurities: Fe,Nb,Ce,La,TR

Petalite

Formula: LiAlSi4O10 phyllosilicate (sheet silicate)
Specific gravity: 2.3 to 2.5
Hardness: 6 to 6½
Streak: White
Colour: Colourless, white, grey, pink
Solubility: Insoluble in acids
Environments:

Pegmatites

Petalite is found in granite pegmatites associated with cleavelandite, quartz and lepidolite.

Alteration

Petalite alters to montmorillonite.

Common impurities: Mg,Fe,Na,Ca,K,H2O

Pharmacosiderite

Formula: KFe3+4(AsO4)3(OH)4.6-7H2O

Phenakite

Formula: Be2(SiO4) nesosilicate, phenakite group
Specific gravity: 2.98
Hardness: 7½ to 8
Streak: White
Colour: Colourless, white, yellow, pale rose
Solubility: Insoluble in acids
Environments:

Pegmatites
Clastic sedimentary environments
Chemical sedimentary environments
Hydrothermal vein environments

Phenakite occurs in granite pegmatites with microcline, topaz and quartz, and also in schist as a product of beryl alteration. At Takovaya in the Russian Urals, phenakite occurs in schist with emerald and chrysoberyl.

Phillipsite

The phillipsite group is a group of tectosilicates (framework silicates) and a sub-group of the zeolite group, comprising:
Phillipsite-Ca: Ca3(Si10Al6)O32.12H2O
Phillipsite-K: K6(Si10Al6)O32.12H2O
Phillipsite-Na: Na6(Si10Al6)O32.12H2O
Phillipsite forms a series with harmotome
Specific gravity: 2.2
Hardness: 4 to 5
Streak: White
Colour: White
Solubility: Moderately soluble in hydrochloric acid
Environments:

Volcanic igneous environments
Clastic sedimentary environments
Chemical sedimentary environments
Hydrothermal vein environments

Phillipsite is a common zeolite in volcanic rocks, ore veins, lithified rhyolitic vitric tuff (consolidated pyroclastic rock), saline lake deposits, and ocean floor sediments. Forms in Iceland in geothermal wells at 60 to 85oC
Phillipsite is a mineral of the zeolite facies

Phlogopite

Formula: KMg3(AlSi3O10)(OH)2 phyllosilicate (sheet silicate) mica group
Specific gravity: 2.78 - 2.85
Hardness: 2 to 3
Streak: White
Colour: Brown, gray, green, yellow, or reddish brown
Solubility: Slightly soluble in sulphuric acid
Environments:

Carbonatites
Metamorphic environments

Phlogopite is found in metamorphosed Mg-rich limestone, dolostone and ultramafic rocks.

It is an essential constituent of kimberlite.
It also may be found in dolostone and skarn.

Alteration

almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks.

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies.

dolomite, K-feldspar and H2O to phlogopite, calcite and CO2
3CaMg(CO3)2 + KAlSi3O8 + H2O = KMg3AlSi3O10(OH)2 + 3CaCO3 + 3CO2
In the presence of Al and K the metamorphism of dolomite leads to the formation of phlogopite according to the above equation.

Al-rich hornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2

phlogopite, calcite and quartz to diopside, microcline, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter according to the above reaction.

Common impurities: Mn,Ba,Cr,Na,Ti,Ni,Zn,Ca,Li,Rb,H2O

Phosgenite

Formula: Pb2(CO3)Cl2 anhydrous carbonate containing halogen
Specific gravity:
Hardness: 2 to 3
Streak: White
Colour: Colourless, white, yellow, brown, greenish or pink; colourless in transmitted light
Solubility: Moderately soluble in nitric acid with effervescence. Decomposed slowly in cold water, which extracts lead chloride
Environments:

Hydrothermal replacement environments

Phosgenite is a secondary mineral found in the weathered zone of lead ore deposits. It readily alters to, and is replaced by, cerussite

Pirssonite

Formula: Na2Ca(CO3)2.2H2O hydrated normal carbonate
Specific gravity: 2.352
Hardness: 3 to 3½
Streak: White
Colour: Colourless, white, greyish
Solubility: Soluble in cold dilute acids with effervescence
Environments

Chemical sedimentary environments

Pirssonite is an evaporite mineral.

Localities

Namibia

At the Otjiwalundo Salt Pan, Kunene Region, pirssonite occurs with trona and thénardite.

USA

At Borax Lake, California, pirssonite occurs with northupite, tychite and gaylussite.

Near Green River, Wyoming, it has been recovered from drill cores with shortite, bradleyite, northupite, gaylussite and trona.

Plagioclase

Plagioclase feldspars form a continuous series of tectosilicates (framework silicates) from
anorthite CaAl2Si2O8 to
albite NaAlSi3O8. They include oligoclase (10 to 30% anorthite), andesine (30 to 50% anorthite), labradorite (50 to 70% anorthite) and bytownite (70 to 90% anorthite), as well as the end members.
Environments:

Plutonic igneous environments
Volcanic igneous environments
Metamorphic environments

The plagioclase feldspars are widely distributed as rock-forming minerals, and more abundant than the K-feldspars. They are minerals of the granulite facies. Plagioclase is an essential constituent of diorite, rhyolite, andesite and basalt.
It is a common constituent of quartzolite and also may be found in granite.
It is a mineral of the hornblende-hornfels, pyroxene-hornfels, amphibolite and granulite facies.

Platinum

Formula: Pt native element
Specific gravity: 21.4
Hardness: 4 - 4½;
Streak: Grey
Colour: Silver grey
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Clastic sedimentary environments
Placer deposits
Hydrothermal environments

Most platinum occurs as the native metal in ultramafic rocks, especially dunite, associated with olivine, chromite, pyroxene and magnetite. It has been mined extensively in placers which are usually close to the platinum-bearing parent rock. It also occurs in quartz veins.

Pollucite

Formula: Cs(Si2Al)O6.nH2O
Specific gravity: 2.67 to 3.03
Hardness: 6Æ to 7
Streak: White
Colour: Colourless, white, grey, pink, blue, violet
Solubility: Slightly soluble in hot hydrochloric acid; readily soluble in HF
Environments:

Pegmatites

Pollucite occurs only in rare-element, complex lithium-cesium-bearing granitic pegmatites.

Alteration

Quartz pseudomorphs after pollucite have been found. Pollucite also alters to analcime, adularia, lepidolite, spodumene and clay.
Common impurities: Fe,Ca,K,Rb

Prehnite

Formula: Ca2Al(Si3Al)O10(OH)2 phyllosilicate (sheet silicate)
Specific gravity: 2.8 - 3.0
Hardness: 5½ - 6
Streak: White
Colour: Colourless, white, grey, greenish, yellowish, reddish
Solubility: Slightly soluble in hydrochloric acid
Environments:

Plutonic igneous environments
Basaltic cavities

Prehnite occurs as a secondary mineral lining cavities in basalt and related rocks, and in alpine crevices. It is associated with zeolites, datolite, pectolite and calcite. Prehnite is characteristic of the prehnite-pumpellyite facies, and it is also a mineral of the zeolite facies.

Alteration

laumontite and calcite to prehnite, quartz, H2O and CO2
CaAl2Si4O12.4H2O + CaCO3 → Ca2Al(Si3Al)O10(OH)2 + SiO2 + 3H2O + CO2
Prehnite and pumpellyite form from the Ca zeolites in the presence of calcite, as in the above equation.

Proustite

Formula: Ag3AsS3 sulphosalt
Specific gravity: 5.5 - 5.7
Hardness: 2½;
Streak: Scarlet red
Colour: Red
Solubility: Slightly soluble in nitric acid
Environments:

Hydrothermal vein environments

Proustite is a late forming mineral in the oxidation and enriched zone in epithermal (low temperature) hydrothermal veins, associated with other silver minerals and sulphides.

Pseudobrookite

Formula: (Fe3+2Ti)O5 multiple oxide, member of the armacolite-pseudobrookite series
Specific gravity: 4.39
Hardness: 6
Streak: Reddish brown to ochre yellow
Colour: Black, dark reddish brown
Solubility: Soluble in hot hydrochloric and sulphuric acid
Environments:

Volcanic igneous environments
Volcanic sublimates and hot spring deposits
Basaltic cavities

Pseudobrookite is usually formed from hot vapours in volcanic rocks, associated with tridymite, hematite, magnetite, sanidine, apatite and rutile.

Localities

France

At Riveau Grande, Puy de Dome, pseudobrookite occurs in cavities in andesite with tridymite, hypersthene and sanidine.

On the Island of Reunion in the Indian Ocean it is a product of hot volcanic gases acting on basalt.

Germany

At Hessen-Brucker, Hessen, it is a product of hot volcanic gases acting on basalt.

Italy

Pseudobrookite occurs on lava from the 1872 eruption of Vesuvius with hematite, magnetite and sellaite.

Romania

In Uroiu, Transylvania, pseudobrookite occurs in cavities in andesite with tridymite, hypersthene and garnet.

USA

At Red Cone, Crater Lake, Oregon, pseudobrookite occurs in cavities in basalt with hypersthene and apatite.

In the Thomas Range, Utah, pseudobrookite is occurs in cavities in rhyolite associated with topaz, bixbyite, hematite, red beryl and ilmenite.

Psilomelane

Psilomelane is an obsolete group name for hard black manganese oxides

Pyrargyrite

Formula: Ag3SbS3 sulphosalt
Specific gravity: 5.85
Hardness: 2½; - 3
Streak: Cherry red
Colour: Red to black
Solubility: Slightly soluble in nitric acid
Environments:

Hydrothermal replacement environments
Hydrothermal vein environments

Pyrargyrite is a late-stage, low temperature mineral in the enrichment zone of epithermal (low temperature) hydrothermal silver ore veins.

Pumpellyite

The pumpellyite group of minerals includes:
Pumpellyite-(Al): Ca2Al3(Si2O7)(SiO4)(OH,O)2.H2O
Pumpellyite-(Fe2+): Ca2Fe2+Al2(Si2O7)(SiO4) (OH,O)2.H2O
Pumpellyite-(Fe3+): Ca2(Fe3+,Mg)Al2(Si2O7)(SiO4) (OH,O)2.H2O
Pumpellyite-(Mg): Ca2MgAl2(Si2O7)(SiO4)(OH,O)2.H2O
Pumpellyite-(Mn2+): Ca2Mn2+Al2(Si2O7)(SiO4) (OH,O)2.H2O
These are all sorosilicates (Si2O7 groups)
Hardness: Mg 5½
Streak: Mg white
Colour: Greenish black due to divalent iron Fe2+, green due to manganese Mg, blue, blackish green, brown, pale grey due to divalent manganese Mn2+, pink-brown
Environments:

Metamorphic environments

Pumpellyite is characteristic of the prehnite-pumpellyite facies, and it is also a mineral of the blueschist facies.
Prehnite and pumpellyite form from calcium zeolites in the presence of calcite.

Pyrite

Formula: FeS2 sulphide
Specific gravity: 4.8 - 5
Hardness: 6 to 6½
Streak: Greenish black
Colour: Pale brass-yellow
Solubility: Insoluble in hydrochloric acid and sulphuric acid; slightly soluble in nitric acid
Environments:

Plutonic igneous environments
Carbonatites
Clastic sedimentary environments
Chemical sedimentary environments
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments
Basaltic cavities

Pyrite is the most common and widespread of the sulphide minerals. It forms at both high and low temperature, but the largest masses probably at high temperature. It is found in plutonic igneous environments including pegmatites and carbonatites; it is a common mineral in clastic and also chemical sedimentary rocks, being both primary and secondary. It occurs in contact metamorphic deposits, disseminated hydrothermal replacement deposits, hydrothermal replacement lodes and as a primary mineral in hypothermal (high temperature) and mesothermal (moderate temperature) hydrothermal veins.
Pyrite may be found in dolostone and limestone.
It is associated with many minerals but found most frequently with chalcopyrite and sphalerite.

Alteration

Marcasite and pyrite are polymorphs (same formula, different structure). Marcasite is a mineral of low-temperature, near-surface, environments, forming from acid solutions. Pyrite is the more stable form of FeS2, and forms in higher temperatures and lower acidity or alkaline environments.

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + H2O → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4
The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + H2O → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulphate is a strong oxidizing agent, and it attacks sulphide minerals in hydrothermal veins, to form soluble salts which trickle downwards through the deposit to be redeposited in the enrichment zone.

pyrite (primary) O and H2O to secondary melanterite and sulphuric acid.
FeS2 + 7O + 8H2O → FeSO4.7H2O + H2SO4
Melanterite indicates the presence of sulphuric acid, and it should be handled with care.

Common impurities: Ni,Co,As,Cu,Zn,Ag,Au,Tl,Se,V

Pyrochlore

Pyrochlore is a group of minerals Formula: (Na,Ca)2Nb2(O,OH,F)2
Specific gravity: 4.5
Hardness: 5 to 5½
Streak: light brown, yellowish brown
Colour: orange, brownish red, brown, black
Solubility: Insoluble in water, soluble with decomposition in sulphuric acid

Plutonic igneous environments
Carbonatites
Clastic sedimentary environments

Pyrochlore is a primary mineral, forming in pegmatites and in igneous rocks dominated by carbonate minerals. It is an accessory mineral in silica-poor rocks, often occurring with magnetite, apatite and zircon. It also accumulates in some detrital deposits.

Common impurities: U,Th

Pyrolusite

Formula: MnO2 simple oxide, rutile group
Specific gravity: 4.9 - 5.1
Hardness: 6
Streak: Black
Colour: Silver-grey to flat black
Solubility: Moderately soluble in hydrochloric acid; insoluble in nitric acid
Environments:

Clastic sedimentary environments
Hydrothermal replacement environments
Hydrothermal vein environments

Manganese is present in small amounts in most crystalline rocks. When dissolved from these rocks, it may be redeposited as various minerals, but chiefly as pyrolusite. Nodular deposits of pyrolusite are found in bogs, on lake bottoms, and on the floors of seas and oceans. Beds of manganese ores are found enclosed in residual clay, derived from the decay of manganiferous limestone. Pyrolusite is also found in the oxidation zone of epithermal (low temperature) hydrothermal veins with quartz and various metallic minerals.

Pyromorphite

Formula: Pb5(PO4)3Cl phosphate
Isistructural with mimetite and vanadinite
Specific gravity: 7.04
Hardness: 3½ to 4
Streak: White
Colour: Green to dark green, yellow, greenish-yellow or yellowish-green, orangish-yellow, shades of brown, white and colourless; colourless or faintly tinted in transmitted light. colourless when pure
Solubility: Slightly soluble in hydrochloric acid and sulphuric acid; moderately soluble in nitric acid and KOH; slightly soluble in carbonated water
Environments:

Hydrothermal replacement environments

Pyromorphite is usually a secondary lead mineral found in the oxidation zones of high temperature hydrothermal lead deposits associated with other oxidised lead and zinc minerals. Lead will generally precipitate as primary galena from ore fluids rich in sulphur and lead. Removal of sulphur by precipitation of sulphides, however, may lead ultimately to an ore fluid from which galena cannot be precipitated, even with a high concentration of lead in solution. In these circumstances, pyromorphite, as well as cerussite and anglesite, could be precipitated as a primary mineral.

Pyromorphite forms a complete series with mimetite (lead chloride arsenate), and many specimens are intermediates between the two end-members.
Pyromorphite forms pseudomorphs after galena and cerussite (common), and galena forms pseudomorphs after pyromorphite. Other pseudomorphs include apatite after pyromorphite and plumbogummite encrusted on and replacing pyromorphite.

Galena is sometimes epitaxial on pyromorphite.

At Ingray Gill, Cumbria, England, pyromorphite crystals encrust mottramite on a number of specimens. These crystals, which are late in the supergene paragenesis, are ideal endmember pyromorphite.

Alteration

Although phosphorus is more abundant (0.099%) in the Earth's crust than chlorine (0.017%), chlorine is widely distributed in the surface environment but phosphorus occurs only as a trace element in most environments. In addition, only small amounts of chlorine are required to form pyromorphite but phosphorous is a major constituent. Hence the availability of phosphorus is likely to be an important factor in the formation of pyromorphite, especially in an oxidised lead deposit where lead is abundant. The commonest source of phosphorus is phosphate rock, which is mostly apatite.
5Pb2+ (from galena) + 3(PO4)3- (from apatite) + Cl- (abundant) ⇌ Pb2+5(PO4)3-3Cl- (pyromorphite)
If phosphorous is not available, galena will weather to anglesite or cerussite depending on the acidity.

Solubility of pyromorphite
Pb5(PO4)3Cl (solid) + 6H+ (aqueous) ⇌ 5Pb2+ (aqueous) + 3H2PO-4 (aqueous) + Cl- (aqueous)

Common impurities: F,Ra,Ca,Cr,V,As

Pyrope

Formula: Mg3Al2(SiO4)3 nesosilicate (insular SiO4 groups), garnet group
Specific gravity: 3.58
Hardness: 7 - 7½
Streak: White
Colour: Dark red, blood red
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Placer deposits
Metamorphic environments

Pyrope occurs in some ultramafic rocks such as mica-peridotite or kimberlite and associated serpentinite. Garnet is one of the commonest minor minerals in kimberlite.
Pyrope is an essential constituent of eclogite, and a common constituent of kimberlite.
It is a characteristic mineral of the eclogite facies.

Alteration

almandine and phlogopite to pyrope and annite
Fe2+3Al2(SiO4)3 + KMg3AlSi3O12(OH)2 ⇌ Mg3Al2Si3O12 + KFe3AlSi3O10(OH)2
This assemblage is commonly formed during amphibolite facies metamorphism of pelitic rocks.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction

Common impurities: Fe,Mn,Ca

Pyrophyllite

Formula: Al2Si4O10(OH)2 phyllosilicate (sheet silicate)
Specific gravity: 2.65 to 2.90
Hardness: 1 to 2
Streak: White
Colour: White, gray, pale blue, pale green, pale yellow, brownish green
Solubility: Slightly soluble in sulphuric acid
Environments:

Metamorphic environments
Hydrothermal vein environments

Pyrophyllite is found both in hydrothermal veins and in bedded deposits in schistose metamorphic rocks, where is is often associated with kyanite.
It is a mineral of the greenschist facies

Alteration

kaolinite to pyrophyllite, diaspore and H2O
2Al2Si2O5(OH)4 → Al2Si4O10(OH)2 + 2AlO(OH) + 2H2O
In the absence of quartz, kaolinite breaks down on heating according to the above reaction.

kaolinite and quartz to pyrophyllite and H2O
Al2Si2O5(OH)4 + 2SiO2 → Al2Si4O10(OH)2 + H2O
This reaction represents the breakdown of kaolinite in the presence of quartz. (If quartz is absent, diaspore is formed as well as pyrophyllite).

pyrophyllite and H2O to kaolinite and aqueous SiO2
Al2Si2O10(OH)2 + H2O → Al2Si2O5(OH)4 + 2SiO2
At 1 kbar pressure kaolinite is stable at temperatures less than 300oC; it can be in equilibrium with quartz and water in solutions both saturated and undersaturated with quartz. Pyrophyllite is stable at temperatures up to 450oC and above, but except for a very narrow band of temperature and composition, it is stable only with solutions supersaturated with quartz

pyrophyllite to andalusite or kyanite, quartz and H2O
Al2Si4O10(OH)2 ⇌ Al2SiO5 + 3SiO2 + H2O The equilibrium temperature for this reaction at 1.8 kbar pressure is 414oC, with the reaction going to the right at higher temperatures, and to the left at lower temperatures. The pressure determines whether kyanite or andalusite is formed. Above about 2.5 kbar it is kyanite, and andalusite at lower pressure.

pyrophyllite and diaspore to andalusite and H2O
Al2Si4O10(OH)2 + 6AlO(OH) ⇌ 4Al2SiO5 + 4H2O

Pyroxene

Pyroxenes are single chain inosilicates with the general formula M2M1(Si2O6).
There are two groups of pyroxenes, orthopyroxenes such as enstatite Mg2(Si2O6) and clinopyroxenes such as aegirine NaFe3+Si2O6, augite (Ca,Mg,Fe)2Si2O6 and diopside CaMg(Si2O6).
In the discontinuous branch of the Bowen reaction series pyroxene is intermediate between olivine (higher temperature) and amphibole (lower temperature).
Solubility: Insoluble in water, hydrochloric, nitric and sulphuric acid
Environments:

Plutonic igneous environments
Volcanic igneous environments

Pyroxenes are characteristic of the pyroxene-hornfels facies, and are also minerals of the granulite facies.
Pyroxenes are primary minerals.
They are essential constituents of kimberlite, ultramafic rocks and basalt.
Pyroxenes are common but not essential constituents of rhyolite, andesite and eclogite.
They also may be found in granite, syenite, diorite, gabbro, trachyte and hornfels.

Alteration

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2 This reaction occurs at high temperature and pressure.

jadeite, diopside, magnetite and SiO2 to aegirine, kushiroite (pyroxene) and hypersthene
2NaAlSi2O6 + CaMgSi2O6 + Fe2+Fe3+2O4 + SiO2 ⇌ 2NaFe3+Si2O6 + CaAlAlSiO6 + MgFeSi2O6
Aegirine in blueschist facies rocks may be formed by the above reaction.

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction

Mg-rich siderite and SiO2 to olivine, orthopyroxene and CO2
3(Fe,Mg)(CO3)→ (Fe,Mg)2SiO4 + 2SiO2 → (Fe,Mg)2SiO4 + 3CO2

Pyrrhotite

Formula: Fe7S8 sulphide
Specific gravity: 4.6
Hardness: 4
Streak: Dark grey
Colour: Bronze
Solubility: Insoluble in water, nitric and sulphuric acid; soluble with decomposition in hydrochloric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments (typical)
Hydrothermal vein environments

Pyrrhotite is a primary mineral that may be found as a minor constituent of some igneous rocks. It is also found in pegmatites, contact metamorphic deposits, and in the enriched zone of hypothermal (high temperature) hydrothermal veins.
Pyrrhotite may be found in gabbro and norite (a gabbro where the main mafic mineral is orthopyroxene). It occurs in them as disseminated grains or as large masses associated with pentlandite, chalcopyrite and other sulphides.
At Desolation Prospect, Mount Isa, Australia, trace amounts of pyrrhotite occur as small inclusions in pyrite. Grains of galena are present in the pyrrhotite, which also contains minor cobalt and nickel.
At the Cobar Deposit, Australia, pyrrhotite is associated with early stage gold and with later stage chalcopyrite-cubanite. Where chalcopyrite-cubanite-pyrrhotite overprints early stage gold-bismuthinite, bismuth, gold, scheelite and pyrite apparently are recrystallised.
At Yaogangxian, China, pyrrhotite is often coated with small pyrite crystals.

Common impurities: Ni,Co,Cu

Quartz

Formula: SiO2 tectosilicate (framework silicate) Citrine is a variety of quartz coloured yellow by submicroscopic distribution of colloidal ferric hydroxide and oxides, as well as Fe3+ substituting for Si.
Rose quartz is a variety of quartz which, when massive, is coloured by scattering of tiny orientated rutile needles and/or the presence of Ti3+ in channels and voids. Al3+ is usually also present. Transparent single crystals of rose quartz are coloured by substitutional phosphorus.
Amethyst is a variety of quartz coloured by Fe3+ substituting for Si in tetrahedral co-ordination, and then the action of natural irradiation producing Fe4+.
Smoky quartz is a variety of quartz coloured dark brown to black due to the presence of Al3+ in the tetrahedral site. If Fe3+ is present in greater concentration than Al3+, irradiation of clear quartz produces smoky quartz at first, but further irradiation further ionises the iron and causes charge transfer between Fe4+ and a trapped hole on an oxygen atom of the Al3+ tetrahedron producing amethyst.
Blue quartz is a blue variety of quartz. Some examples of blue quartz are coloured by submicron-size inclusions of ilmenite which produce scattering. Other examples owe their colour to submicron size inclusions of rutile, tourmaline or amphibole, and in rare cases to incorporation of cobalt.
Green quartz owes its colour to inclusions of chlorite.
Red-brown quartz is coloured by inclusions of hematite.

Properties of quartz: Specific gravity: 2.65 to 2.66
Hardness: 7
Streak: White
Colour: White or colourless, also grey, yellow, purple, pink, brown, black. Also may be coloured by blue, green or red-brown by inclusions of other minerals. Solubility: Insoluble in hydrochloric, sulphuric and nitric acid. At temperatures above 100°C and high pressures the solubility of quartz increases quickly. At 300°C it is between 700 and 1200 mg/l, depending on the pressure.
Environments:

Plutonic igneous environments
Volcanic igneous environments
Pegmatites
Carbonatites
Clastic sedimentary environments
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments
Basaltic cavities

In the Bowen reaction series quartz is the last major mineral to crystallise out.
It is the most common mineral found on the surface of the Earth; it occurs in plutonic igneous environments including pegmatites and carbonatites, in sedimentary environments, in contact and regional metamorphic environments, in hydrothermal deposits and it is the principal constituent of hydrothermal veins.
Smoky quartz occurs in alpine fissures and veins; rose quartz occurs in pegmatites.

Quartz is generally a primary, rock-forming mineral, but it may also be of secondary origin.

It is an essential constituent of quartzolite, granite, pegmatites, rhyolite and sandstone.
It is a common constituent of diorite, basalt, phyllite, gneiss and eclogite.
Quartz also may be found in syenite, gabbro, trachyte, andesite, clay, limestone and dolostone.

Quartz occurs in all metamorphic facies with the possible exception of the sanidite facies, where the high temperature polymorph tridymite may occur instead.

Alteration

Alpha quartz is stable at low pressures and up to about 573oC, and beta quartz, also stable at low pressures, is stable from about 573 to 870oC. It can exist metastably above 870oC.

aegirine and CaO to andradite, quartz and Na2O
2NaFe3+Si2O6 + 3CaO → Ca3Fe3+2 (SiO4)3 + SiO2 + Na2O This is a high temperature process.

aegirine, epidote and CO2 to albite, hematite, quartz, calcite and H2O
4NaFe3+Si2O6 + 2Ca2(Al2Fe3+ [Si2O7](SiO4)O(OH) + 4CO2 → 4Na(AlSi3O8) + 3Fe2O3 + 2SiO2 + 4CaCO3 + H2O

albite to jadeite and quartz
Na(AlSi3O8) → NaAlSi2O6 + SiO2
In quartz-jadeite rocks jadeite may be the product of the above reaction.
This reaction occurs at high pressure ranging from above 5 kbar at 0oC to above 20 kbar at 800oC.

albite to nepheline and quartz
Na(AlSi3O8) ⇌ NaAlSiO4 + 2SiO2

albite, chlorite and calcite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz, CO2 and H2O
8Na(AlSi3O8) + (Mg4.0Fe2.0)(AlSi3O10)(OH)8 + CaCO3 → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 2SiO2 + CO2 + 2H2O
In low to intermediate metamorphism jadeite-glaucophane assemblages may arise from reactions such as the one above.

albite, diopside and magnetite to aegirine, Si2O6, garnet and quartz
2Na(AlSi3O8) + CaMgSi2O6 + Fe2+Fe3+2O4 ⇌ 2NaFe3+Si2O6 + Si2O6 + CaMgFe2+Al2(SiO4)3 + SiO2
This reaction may occur in blueschist facies rocks in Japan.

albite and montmorillonite to Ca, Mg-rich jadeite, Al-rich glaucophane, quartz and H2O
8Na(AlSi3O8) + 2Ca0.5(Mg3.5Al0.5)Si8O20(OH)4.nH2O → 5(Na0.8Ca0.2)(Mg0.2Al0.8Si2)6 + 2Na2(Mg3Al2)(Al0.5Si7.5)O22(OH)2 + 15SiO2 +6H2O
This reaction occurs in low to intermediate metatmorphism.

amphibole and anorthite to chlorite, epidote and quartz
CaFe5Al2Si7O22(OH)2 + 3CaAl2Si2O8 + 4H2O → Fe5Al2Si3O10(OH)8 + 2Ca2Al3Si3O12(OH) + 4SiO2

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

calcium amphibole, calcite and quartz to diopside-hedenbergite, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Fe,Mg)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

calcium amphibole, grossular and quartz to diopside- hedenbergite, anorthite, pyrope-almandine and H2O
2Ca2(Mg,Fe2+)3Al4Si6O22(OH)2 + Ca3Al2(SiO4)3 + SiO2 = 3Ca(Fe,Mg)Si2O6 + 4Ca(Al2Si2O8) + (Mg,Fe2+)3Al2(SiO4)3 + 2H2O
Diopside-hedenbergite occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies, where it may form according to the above reaction.

analcime and quartz to albite and H2O
Na(AlSi2O6).H2O + SiO2 ⇌ Na(AlSi3O8) + H2O

ankerite, dolomite and quartz to augite and CO2
Ca(Mg,Fe)(CO3)2 + 2SiO2 → Ca(Mg,Fe)Si2O6 + 2CO2

ankerite-dolomite and quartz to diopside-hedenbergite and CO2
Ca(Fe,Mg)(CO3)2 + 2SiO2 = Ca(Fe,Mg)Si2O6 + 2CO2

anorthite, albite and H2O to jadeite, lawsonite and quartz
CaAl2 Si2O8 + NaAlSi3O8 + 2H2O → NaAlSi2O6 + CaAl2(Si2O7)(OH)2.H2 + SiO2

augite and CO2 to hypersthene, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe)SiO3 + CaCO3 + SiO2

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2 This reaction occurs at high temperature and pressure.

calcite, hematite and quartz to andradite
3CaCO3 + Fe2O3 + 3SiO2 → Ca3Fe3+2Si3O12 + 3CO2

chlorite, muscovite and quartz to biotite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al(AlSi3O10)(OH)8 + KAl2(AlSi3O10)(OH)2 + 2SiO2 → K(Mg,Fe2+)3(AlSi3O10)(OH)2 + (Mg,Fe2+)2Al4Si5O18 + 4H2O
This reaction ocurs when the metamorphic grade increases

chlorite and quartz to enstatite- ferrosilite, Fe rich cordierite and H2O
(Mg,Fe2+)4Al4Si2O10(OH)8 + 5SiO2 → 2(Mg,Fe2+)SiO3 + (Mg,Fe2+2)2Al4Si5O18 + 4H2O
Enstatite-ferrosilite also occurs in medium grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content according to the above equation.

chloritoid and quartz to staurolite, almandine and H2O
23Fe2+Al2O(SiO4)(OH)2 + 8SiO2 ⇌ 4Fe2+2Al9Si4O23(OH) + 5Fe2+3Al2(SiO4)3 + 21H2O

Fe-rich cordierite and diopside- hedenbergite to enstatite- ferrosilite, anorthite and quartz
(Mg,Fe)2 Al4Si5O18 + 2Ca(Mg,Fe)Si2O6 = 4(Mg,Fe2+)SiO3 + 2Ca(Al2Si2O8) + SiO2

diopside and albite to omphacite and quartz
CaMgSi2O6 + xNaAlSi3O8 ⇌ CaMgSi2O6.xNaAlSi2O6 + SiO2

diopside-hedenbergite and CO2 to enstatite- ferrosilite, calcite and quartz
Ca(Mg,Fe)Si2O6 + CO2 → (Mg,Fe2+)SiO3 + CaCO3 + SiO2

dolomite and quartz to diopside and CO2
CaMg(CO3)2 + 2SiO2 → CaMgSi2O6 + 2CO2
In siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.

dolomite and quartz to forsterite, calcite and CO2
2CaMg(CO3)2 + SiO2 → Mg2SiO4 + 2CaCO3 + 2CO2
In siliceous dolostone dolomite and quartz may react to form either diopside or forsterite, with diopside forming at a lower temperature than forsterite.


dolomite, quartz and H2O to tremolite, calcite and CO2
5CaMg(CO3)2 + 8SiO2 + H2O → Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 7CO2
This is a metamorphic reaction in dolomitic limestone

dolomite and chert (a variety of quartz) to talc and calcite
3CaMg(CO3)2 + 4SiO2 + H2O → Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2
This is a very low-grade metamorphic reaction occurring at temperature between about 150oC and 250oC.

enstatite, calcite and quartz to diopside and CO2
3Mg2Si2O6 + 2CaCO3 + 2SiO2 ⇌ + 2CaMgSi2O6 + 2CO2
Enstatite is uncommon in the more calcareous hornfels due to reactions such as the above.

enstatite-ferrosilite and H2O to serpentine and quartz
6(Mg,Fe2+)SiO3 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2

enstatite-ferrosilite, diopside-hedenbergite, albite, anorthite and H2O to amphibole and quartz
3(Mg,Fe2+)SiO3 + Ca(Mg,Fe2+)Si2O6 + NaAlSi3O8 + Ca(Al2Si2O8) + H2O ⇌ NaCa2(Mg,Fe)4Al(Al2O6)O22(OH)2 + 4SiO2
This reaction represents metamorphic reactions between the granulite and amphibolite facies.

enstatite-ferrosilite, K-feldspar and H2O to biotite and quartz
3(Mg,Fe2+)SiO3 + K(AlSi3O8) + H2O ⇌ K(Mg,Fe)3(AlSi3O10)(OH)2+ 3SiO2
The forward reaction leads to an amphibolite facies assemblage.

enstatite-ferrosilite, quartz and H2O to cummingtonite- grunerite
7(Mg,Fe2+)SiO3 + SiO2 + H2O ⇌ (Fe,Mg)7Si8O22(OH)2

epidote and quartz to anorthite, grossular and H2O
4Ca2Al3(SiO4)3(OH) + SiO2 → 5CaAl2Si2O8 + Ca3Al2(SiO4)3 + 2H2O
This reaction occurs as the degree of metamorphism increases

ferro-actinolite to hedenbergite, grunerite, quartz and H2O
7Ca2Fe2+5Si8O22(OH)2 → 14CaFe2+Si2O6 + 3Fe2+2Fe2+5 Si8O22(OH)2 + 4SiO2 + 4H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, probably due to the instability of ferro-actinolite with rising temperature. The association of hedenbergite and grunerite has been widely described, and its formation may be due to the above reaction.

ferro-actinolite, calcite and quartz to hedenbergite, CO2 and H2O
Ca2Fe2+5Si8O22(OH)2 + 3CaCO3 + 2SiO2 ⇌ 5CaFe2+Si2O6 + 3CO2 + H2O
In some calc-silicate rocks hedenbergite is the product of metamorphism of iron-rich sediments, according to the above reaction, probably due to the instability of ferro-actinolite with rising temperature.

forsterite, calcite and quartz to monticellite and CO2
Mg2SiO4 + 2CaCO3 + SiO2 → 2CaMg(SiO4) + 2CO2

forsterite and quartz to enstatite
Mg2SiO4 + SiO2 → 2MgSiO3
Forsterite is not stable in the presence of free SiO2 and will react with it to form enstatite according to the above reaction.

gedrite-ferro-gedrite and quartz to enstatite-ferrosilite, Fe-rich cordierite and H2O
(Mg,Fe2+)5Al4Si6O22(OH)2 + 2Si2 → 3(Mg,Fe2+)SiO3 + (Mg,Fe2+)2Al4Si5O18 + H2O
In the pyroxene-hornfels facies enstatite-ferrosilite may develop from gedrite-ferro-gedrite according to the above reaction.

grossular, diopside, monticellite, calcite and H2O to vesuvianite, quartz and CO2
10Ca3Al2(SiO4)3 + 3CaMgSi2O6 + 3CaMg(SiO4) + 2CaCO3 + 8H2O ⇌ 2Ca19Al10Mg3(SiO4)10 (Si2O2)4O2(OH)8 + 3SiO2 + 2CO2
A common association in calc-silicate metamorphism can be represented by the above equation. Vesuvianite stability will tend to increase with increasing water and decrease as the activity of CO2 rises.

hematite, wüstite, quartz and calcite to andradite, hedenbergite, magnetite and CO2
2Fe2O3 + 2FeO + 5SiO2 + 4CaCO3 → Ca3Fe3+2(SiO4)3 + CaFe2+Si2O6 +Fe2+Fe3+2O4 +4CO2

hornblende, calcite and quartz to Fe-rich diopside, anorthite, CO2 and H2O
Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + 3CaCO3 + 4SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 2Ca(Al2Si2O8) + 3CO2 + H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

Al-rich hornblende, grossular and quartz to Fe-rich diopside, anorthite, almandine and H2O
2Ca2(Mg,Fe2+)3(Al4Si6)O22(OH)2 + Ca3Al2Si3O12 + 2SiO2 = 3Ca(Mg,Fe2+)Si2O6 + 4CaAl2Si2O8 + (Mg,Fe2+)Al2Si3O12 + 2H2O
Fe-rich diopside occurs commonly in regionally metamorphosed calcium-rich sediments and basic igneous rocks belonging to the higher grades of the amphibolite facies. The above reaction is typical.

Al-rich hornblende, spinel, quartz and K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2

jadeite and quartz to albite
NaAlSi2O6 + SiO2 ⇌ NaAlSi3O8
High pressure favours the reverse reaction.

kaolinite and H2O to gibbsite and quartz
Al2Si2O5(OH)4 + H2O ⇌ 2Al(OH)3 + 2SiO2

kaolinite and quartz to pyrophyllite and H2O
Al2Si2O5(OH)4 + 2SiO2 → Al2Si4O10(OH)2 + H2O
This reaction represents the breakdown of kaolinite in the presence of quartz. (If quartz is absent, diaspore is formed as well as pyrophyllite).

labradorite (variety of anorthite), albite, forsterite and diopside to omphacite, garnet and quartz
3CaAl2Si2O8 + 2Na(AlSi3O8) + 3Mg2SiO4 + nCaMgSi2O6 → (2NaAlSi2O6 + nCaMgSi2O6) + 3(CaMg2)Al2(SiO4)3 + 2SiO2
This reaction occurs at high temperature and pressure.

laumontite to lawsonite, quartz and H2O
CaAl2Si4O12.4H2O→ CaAl2(Si2O7)(OH)2.H2O + 2SiO2 + 2H2O
In subduction zones, as the pressure rises to above about 1.5 kbar, laumontite alters to lawsonite according to the above reaction.

laumontite and calcite to prehnite, quartz, H2O and CO2
CaAl2Si4O12.4H2O + CaCO3 → Ca2Al(Si3Al)O10(OH)2 + SiO2 + 3H2O + CO2
Prehnite and pumpellyite form from the Ca zeolites in the presence of calcite, as in the above equation.

monticellite, spurrite and quartz to merwinite and CO2
5CaMg(SiO4) + Ca5(SiO4)2(CO3) + SiO2 ⇌ 5Ca3Mg(SiO4)2 + 2CO2

muscovite and quartz to sillimanite, K-feldspar and H2O
KAl2(Si3Al)O10(OH)2 + SiO2 ⇌ Al2SiO5 + KAlSi3O8 + H2O
This equation represents changes that may occur in regional metamorphism when the metamorphic grade changes from the greenschist facies (left hand side) to the amphibolite facies (right hand side) or vice versa.
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat), hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down.

phlogopite, calcite and quartz to diopside, microcline, H2O and CO2
KMg3(AlSi3O10)(OH)2 + 3CaCO3 + 6SiO2 = 3CaMgSi2O6 + K(AlSi3O8) + H2O + 3CO2
In reaction zones between interbedded carbonate and pelitic beds of the calc-mica schists, phlogopite may alter according to the above reaction.

pyrophyllite to andalusite, quartz and H2O
Al2Si4O10(OH)2 ⇌ Al2SiO5 + 3SiO2 + H2O
The equilibrium temperature for this reaction at 1.8 kbar pressure is 414oC, with the reaction going to the right at higher temperatures, and to the left at lower temperatures.

pyrophyllite and H2O to kaolinite and quartz
Al2Si2O10(OH)2 + H2O → Al2Si2O5(OH)4 + 2SiO2

quartz and calcite to wollastonite and CO2
3SiO2 + 3CaCO3 ⇌ Ca3Si3O9 + 3CO2 (gaseous)
This is a contact metamorphic change occurring at temperatures from about 600°C such as in the immediate border zone of an igneous intrusion into limestone. High pressure inhibits the forward reaction by suppressing the formation of gaseous CO2 rhodochrosite and quartz to rhodonite and CO2
MnCO3 + SiO2 → MnSiO3 + CO2
This is a metamorphic reaction occurring in manganese deposits and manganese-rich iron formations.

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2 (Mg3Fe3+2)Si8O22(OH)2 + H2O

spodumene, quartz and Na+ to albite and Li+
LiAlSi2O6 + SiO2 + Na+ → NaAlSi3O8 + Li+
Whether spodumene breaks down into albite or into eucryptite and albite depends largely on the presence or absence of quartz.

staurolite, annite and O2 to chercynite, magnetite, muscovite,corundum, quartz and H2O
2Fe2+2Al9Si4O23(OH) + KFe2+3 (AlSi3O10)(OH)2 +2O2 → 4Fe2+Al2O4 + Fe2+Fe3+2O4 + KAl2 (AlSi3O10)OH)2 + 4Al2O3 + 8SiO2 + 2H2O

talc to anthophyllite, quartz and H2O
7Mg3Si4O10(OH)2 → 3☐Mg2Mg5Si8O22(OH)2 + 4SiO2 + 4H2O
This reaction occurs when the degree of metamorphism increases

talc and calcite to dolomite, quartz and H2O
Mg3Si4O10(OH)2 + 3CaCO3 + 3CO2 ⇌ 3CaMg(CO3)2 + 4SiO2 + H2O

talc, calcite and quartz to tremolite, CO2 and H2O
5Mg3Si4O10(OH)2 + 6CaCO3 +4SiO2 → 3Ca2Mg5Si8O22(OH)2 + 6CO2 +2H2O
Metamorphism of siliceous carbonate rock causes the formation of hydrous phases such as talc and tremolite.

tremolite, calcite and quartz to diopside, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2 → 5CaMgSi2O6 + 3CO2 + H2O
This is a medium-grade metamorphic change occurring at temperatute between about 450oC and 600oC.

tremolite to diopside, enstatite, quartz and H2O
Ca2Mg5Si8O22(OH)2 ⇌ 2CaMgSi2O6 + 3MgSiO3 + SiO2 + H2O
At 8 kbar pressure the equilibrium temperature is about 930oC, with the forward reaction favoured by higher temperatures.

tremolite, calcite and quartz to diopside, CO2 and H2O
Ca2Mg5Si8O22(OH)2 + 3CaCO3 + 2SiO2 → 5CaMgSi2O6 + 3CO2 + H2O
This is a medium-grade metamorphic change occurring at temperatute between about 450oC and 600oC.

Common impurities: H,Al,Li,Fe,Ti,Na,Mg,Ge,etc

Ranciéite

Formula: (Ca,Mn2+)0.2(Mn4+,Mn3+)O2.0.6H2O multiple oxide
Specific gravity: 3.2
Hardness: 2½ to 3
Streak: Dark brown
Colour: Black, brownish, violet
Environments:

Clastic sedimentary environments

Localities

France

At the type locality, Rancié, Occitanie, ranciéite occurs in cavities in limonite.

Italy

At the Filhols Mine, Romano, it occurs in lake-deposited tuffs, associated with halloysite.

Realgar

Formula: AsS sulphide
Specific gravity: 3.5 - 3.6
Hardness: 1Æ
Streak: Orange yellow
Colour: Deep red to orange
Solubility: Slightly soluble in nitric acid
Environments:

Chemical sedimentary environments
Volcanic sublimates and hot spring deposits
Hydrothermal vein environments (typical)

Realgar is a common epithermal (low temperature) hydrothermal vein mineral in veins of lead, silver and gold ores associated with orpiment, other arsenic minerals and stibnite. It also occurs as a volcanic sublimation product and as a deposit in hot springs, and in carbonate sedimentary rocks.

Realgar decomposes to pararealgar with exposure to orange light, and decays to form orpiment.

Rhodochrosite

Formula: Mn(CO3) carbonate
Specific gravity: 3.3 - 3.6
Hardness: 3½ - 4
Streak: White
Colour: Rose-pink, light red, yellowish grey, brownish
Solubility: Slightly soluble in water with the solubility rate increasing with the presence of CO2. Begins to dissociate at about 300˚ with the formation of CO2 and MnO. Moderately soluble in hydrochloric, sulphuric and nitric acid.
Environments:

Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Rhodochrosite is a comparatively rare mineral, occurring as a primary mineral in epithermal (low temperature), mesothermal (moderate temperature) and hypothermal (high temperature) hydrothermal veins, with ores of silver, lead, copper, and other manganese minerals. It also occurs as large deposits in metamorphic rocks

Alteration

rhodochrosite and quartz to rhodonite and CO2
MnCO3 + SiO2 → MnSiO3 + CO2
This is a metamorphic reaction occurring in manganese deposits and manganese-rich iron formations.

rhodonite and rhodochrosite to tephroite and CO2
Mn2+SiO3 + Mn(CO3) → Mn2+2(SiO4) + CO2

Common impurities: Fe,Ca,Mg,Zn,Co,Cd

Rhodonite

Formula: Mn2+SiO3 inosilicate (chain silicate)
Specific gravity: 3.57 to 3.76
Hardness: 5½ to 6½
Streak: White
Colour: Red, pink, brownish-red, gray
Solubility: Slightly soluble in hydrochloric acid
Environments:

Clastic sedimentary environments
Metamorphic environments
Hydrothermal environments

Rhodonite is found in manganese bearing deposits formed by hydrothermal, contact and regional metamorphic and sedimentary processes

Alteration

johansennite and H2O to rhodonite and xonotlite
6CaMnSi2O6 + H2O → 6Mn2+SiO3 + Ca6Si6O17(OH)2 At Pueblo, Mexico, johansennite occurs in calcite veins in rhyolite and is altered to rhodonite and xonotlite according to the above equation.

rhodochrosite and quartz to rhodonite and CO2
MnCO3 + SiO2 → MnSiO3 + CO2
This is a metamorphic reaction occurring in manganese deposits and manganese-rich iron formations.

rhodonite and rhodochrosite to tephroite and CO2
Mn2+SiO3 + Mn(CO3) → Mn2+2(SiO4) + CO2

Common impurities: Al,Ca,Fe,Zn

Riebeckite

Formula: ☐Na2(Fe2+3Fe3+2) Si8O22(OH)2 inosilicate (chain silicate) amphibole
Specific gravity: 3.4
Hardness: 5 to 5½
Streak: Greenish brown
Colour: Black, blue, grey, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Metamorphic environments

Riebeckite occurs most commonly in igneous rocks such as granite, syenite, nepheline syenite and related pegmatites, and in carbonatites. It is present in some schist of regional metamorphic origin.
Riebeckite also may be found in granite,

Alteration

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2 (Mg3Fe3+2)Si8O22(OH)2 + H2O

Common impurities: Fe,Ti,Mg,Al,Mn

Rutile

Formula: TiO2 simple oxide, rutile group
Specific gravity: 4.23(2)
Hardness: 6 to 6½
Streak: Greyish black
Colour: Red, blue, brown, yellow, violet
Solubility: Insoluble in hydrochloric and nitric acid; slightly soluble in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Carbonatites
Clastic sedimentary environments
Placer deposits
Metamorphic environments

Rutile is found in high pressure, high temperature igneous rocks. It is a common but not essential constituent of eclogite, and it also may be found in granite, kimberlite, limestone, dolostone, mica schist and gneiss.
It often occurs as slender crystals inside quartz and mica. Rutile is found in considerable quantities in black sands associated with ilmenite, magnetite, zircon and monazite.
It is a mineral of the greenschist, amphibolite, granulite and eclogite facies.

Alteration

Rutile is the principle alteration mineral of ilmenite Fe2+Ti4+O3. Rutile can replace ilmenite, and may in turn be associated with later growth crystals of ilmenite.

amphibole, chlorite, paragonite, ilmenite, quartz and calcite to garnet, omphacite, rutile, H2O and CO2
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + Mg5Al(AlSi3O10)(OH)8 + 3NaAl2(Si3Al)O10(OH)2 + 4Fe2+Ti4+O3 + 9SiO2 + 4CaCO3 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 8H2O + 4CO2
In low-grade rocks relatively rich in calcite the garnet-omphacite association may be due to reactions such as the above.

amphibole, clinozoisite, chlorite, albite, ilmenite and quartz to garnet, omphacite, rutile and H2O
NaCa2(Fe2Mg3)(AlSi7)O22(OH)2 + 2Ca2Al3[Si2o7][SiO4]O(OH) + Mg5Al(AlSi3O10)(OH)8 + 3NaAlSi3O8 + 4Fe2+Ti4+O3 + 3SiO2 → 2(CaMg2Fe3)Al4(SiO4)6 + 4NaCaMgAl(Si2O6)2 + 4TiO2 + 6H2O
In low-grade rocks relatively poor in calcite the garnet-omphacite association may be developed by the above reaction.

augite, albite, pyroxene, anorthite and ilmenite to omphacite, garnet, quartz and rutile
2MgFe2+Si2O6 + Na(AlSi3O8) + Ca2Mg2Fe2+Fe3+AlSi5O18 + 2Ca(Al2Si2O8) + 2Fe2+Ti4+O3 → NaCa2MgFe2+Al(Si2O6)3 + (Ca2Mg3Fe2+4)(Fe3+Al5)(SiO4)9 + SiO2 + 2TiO2
This reaction occurs at high temperature and pressure.

Common impurities: Fe,Ta,Nb,Cr,V,Sn

Sanidine

Formula: K(AlSi3O8) tectosilicate (framework silicate)
Sanidine is a high-temperature K-feldspar.
Specific gravity: 2.53 - 2.56
Hardness: 6
Streak: White
Colour: Colourless, transparent to opaque grey, grey brownish
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Volcanic igneous environments
Pegmatites

Sanidine occurs as phenocrysts (larger crystals embedded in a finer-grained matrix in an igneous rock) in volcanic igneous rocks such as rhyolite and trachyte. It is characteristic of rocks that cooled quickly from an initial high temperature of eruption.

Sanidine may occur in hornfels.

It is characteristic of the sanidinite facies.
Common impurities: Fe,Ca,Na,H2O

Sapphirine

Formula: Mg4(Mg3Al9O4)[Si3Al9O36] inosilicate (chain silicate), non-pyroxene
Specific gravity: 3.4 to 3.5
Hardness: 7½
Streak: Colourless
Colour: Light blue, blue-gray, green, greenish gray, rarely yellow-brown or pink

Environments:

Plutonic igneous environments
Metamorphic environments

Sapphirine is found in high temperature metamorphic rocks or xenoliths with abundant aluminium and magnesium and low silicon. It may occur as a primary magmatic mineral
It is a mineral of the granulite facies.

Alteration

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies.

Al-richhornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2

spinel-hercynite, sillimanite and SiO2 to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10

Scapolite

A series between Marialite and Meionite
Marialite Na4Al3Si9O24Cl
Meionite Ca4Al6Si6O24(CO3)
These are tectosilicates (framework silicates)
Specific gravity: 2.5 - 2.8
Hardness: 5 - 6½
Streak: White
Colour: Colourless, white, grey, yellow, green, blue, red to violet
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid

Environments:

Pegmatites
Metamorphic environments

Scapolite is characteristic in limestone as a contact metamorphic mineral.
Scapolite may occur in schist and gneiss.
It is associated with diopside, amphibole, garnet, apatite, titanite and zircon.
Scapolite is a mineral of the amphibolite and granulite facies.

Alteration

In many cases scapolite is derived by alteration from plagioclase feldspar.

Schairerite

Formula: Na21(SO4)7ClF6 anhydrous sulphate containing halogen
Specific gravity: 2.616
Hardness: 3½
Streak: White
Colour: Colourless
Solubility: Slowly soluble in water
Environments:

Chemical sedimentary environments

Schairerite was found in a drill core in a lower salt bed at Searles Lake, California, USA, with gaylussite, tychite, pirssonite and hanksite.

Scheelite

Formula: Ca(WO4) tungstate
Specific gravity: 5.9 - 6.1
Hardness: 4½ to 5
Streak: White
Colour: Tan, golden-yellow, colourless, white, greenish, dark brown, etc.; colourless in transmitted light May be compositionally colour zoned.
Solubility: Slightly soluble in hydrochloric acid

Environments:

Plutonic igneous environments
Pegmatites
Placer deposits
Metamorphic environments (common)
Hydrothermal vein environments

Scheelite is a primary mineral found in granite pegmatites and contact metamorphic deposits. It occurs in the oxidation zone of hypothermal (high temperature) hydrothermal veins in granitic rocks and in alluvial deposits. In hydrothermal veins it is associated with apatite, arsenopyrite, cassiterite, fluorite, molybdenite, topaz and hubnerite-ferberite.

Alteration

Scheelite may form as an alteration product of ferberite.

Common impurities: Mo,Nb,Ta

Schorl

Schorl is the commonest member of the tourmaline group.
Formula: NaFe2+3Al6(Si6O18) (BOSub>3)3(OH)3(OH) cyclosilicate (ring silicate)
Specific gravity: 3.18 - 3.22
Hardness: 7
Streak: Brown Greyish-white to bluish-white.
Colour: Bluish-black to black, sometimes brownish-black, rarely greenish-black.

Environments (tourmaline):

Plutonic igneous environments
Pegmatites
Metamorphic environments (common)
Hydrothermal vein environments


Tourmaline is found in granite and granite pegmatites, in metamorphic rocks and as a primary mineral in hypothermal (high temperature) hydrothermal veins.

Common impurities: Mn,Mg,Ca,Li,Cr,Ti,F,K

Scolecite

Formula: Ca(Si3Al2)O10.3H2O tectosilicate (framework silicate), zeolite group
Specific gravity: 2.24 to 2.31
Hardness: 5 to 5½
Streak: White
Colour: Colourless, white, pink, salmon, red, green
Solubility: Soluble in common acids
Environments:

Volcanic Igneous environments
Metamorphic environments
Hydrothermal environments

Scolecite occurs in basalt, andesite, gneiss, and amphibolite, in contact metamorphic and in hydrothermal environments. It is associated with other zeolites, calcite, quartz and prehnite.

Common impurities: Na,K

Scorodite

Formula: Fe3+(AsO4).2H2O

Searlesite

Formula: NaBSi2O5(OH)2 phyllosilicate (sheet silicate)
Specific gravity: 2.46 to 2.49
Hardness: 3½
Streak: White
Colour: White, colourless
Solubility: Readily soluble in hydrochloric acid
Environments:

Pegmatites
Clastic sedimentary environments
Chemical sedimentary environments

Searlesite occurs in boron-bearing evaporite deposits, in sedimentary rocks rich in sodium and potassium, and in pegmatites, also rich in sodium and potassium.

Common impurities: Al,Fe,Mg,H2O

Segnitite

Formula: PbFe3+3(AsO4)(AsO3OH)(OH)6 hydrated arsenate containing hydroxyl
Segnitite forms a series with beudantite
Specific gravity: >4.2
Hardness: 4
Streak: Pale yellow
Colour: Greenish to yellowish-brown

Localities

Australia

At the Kintore pit, New South Wales, segnitite is found in the oxidised zone of lead-zinc sulphide ore bodies where it overgrows beudantite on a matrix of goethite encrusting quartz and small spessartine crystals. It is also associated with mimetite, carminite, bayldonite, agardite-Y and mawbyite.

United Kingdom

At Roughton Gill, Cumbria, segnitite occurs with carminite and beudantite in fractures in quartz-rich rock.

USA

At the San Rafael Mine, Nevada, segnitite occurs as a crystalline aggregate in pods of oxidised galena and anglesite with kaolinite and covellite. In the altered rock bordering the pods segnitite occurs with scorodite, carminite and mimetite, and with adamite in pods of limonite gossan.

Sellaite

Formula: MgF2 normal halide
Specific gravity: 3.15
Hardness: 5
Streak: White
Colour: Colourless, white; colourless in transmitted light.
Solubility: Slightly soluble in water. Decomposed by concentrated sulphuric acid.
Environments

Chemical sedimentary environments

Localities

Brazil

At a magnesite mine in Brumado, Bahia, sellaite occurs in vugs associated with magnesite and quartz.

France

At Moutiers, Savoie, sellaite occurs in bituminous dolomite-anhydrite rock in a glacial moraine.

Italy

At Carrara, Tuscany, it occurs in cavities in marble.

Serpentine

The serpentine subgroup is a group of phyllosilicates (sheet silicates) including lizardite, chrysotile and antigorite. These three minerals all have the same formula, Mg3Si2O5(OH)4, but different structures, ie they are polymorphs of each other.
Environments

Metamorphic environments

Serpentine is a common mineral and widely distributed, frequently in disseminated particles, in places in such quantity as to make up practically the entire rock mass.
It may be found in kimberlite and skarn.
It is usually formed by alteration of magnesium silicates, especially olivine, pyroxene and amphibole.
It is frequently associated with magnesite, chromite, and magnetite.

Alteration

cummingtonite-grunerite and H2O to serpentine and SiO2
6(Fe,Mg)7Si8O22(OH)2 + 22H2O ⇌ 7(Fe,Mg)6Si4O10(OH)8 + 20SiO2

enstatite-ferrosilite and H2O to serpentine and quartz
6(Mg,Fe2+)SiO3 + 4H2O ⇌ (Fe,Mg)6Si4O10(OH)8 +2SiO2

forsterite and H2O to serpentine and brucite
2Mg2SiO4 + 3H2O ⇌ Mg3Si2O5(OH)4 + Mg(OH)2
The forward reaction is highly exothermic. Stable equilibrium occurs at 350°C for pressure 0.5 kbar, 380°C for pressure 2.0 kbar, 400°C for 3.5 kbar, 420°C for 5.0 kbar and 430°C for 6.5 kbar.

forsterite, SiO3 and H2O to serpentine
3Mg2SiO4 + SiO2 + 4H2O → 2Mg3Si2O5 (OH)4
This reaction is highly exothermic

forsterite, enstatite and H2O to serpentine
2Mg2SiO4 + Mg2Si2O6 + 4H2O → 2Mg3Si2O5(OH)4
Serpentine (chrysotile) is not stable in the presence of carbon dioxide, and may further react with it to form talc and magnesite.

forsterite, fayalite, H2O and CO2 to serpentine, magnetite and methane
18 Mg2SiO4 + 6Fe2SiO4 + 26H2O + CO2 → 12Mg3Si2O5(OH)4 + 4Fe3O4 + CH4

olivine and H2O to serpentine, magnetite and H2
6(Mg1.5Fe0.5)SiO4 + 7H2O → 3Mg3Si2O5(OH)4 + Fe2+Fe3+2O4 + H2
The iron Fe in olivine does not enter into the serpentine, but recrystallises as magnetite.

serpentine (chrysotile) and CO2 to talc, magnesite and H2O
2Mg3Si2O5(OH)4 + 3CO2 → Mg3Si4O10(OH)2 + 3MgCO3 + 3H2O
Serpentine (chrysotile) is not stable in the presence of carbon dioxide and reacts with it according to the above equation.

serpentine, aegirine and quartz to magnesio-riebeckite and H2O
Mg3Si2O5(OH)4 + 2NaFe3+Si2O6 + 2SiO2 → Na2(Mg3Fe3+2)Si8O22(OH)2 + H2O

serpentine and brucite to forsterite and H2O
Mg3Si2O5(OH)4 + Mg(OH)2 → 2Mg2SiO4 + 3H2O
This reaction can proceed in either direction, depending on the external conditions. Early formation of forsterite.

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 ⇌ Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O + H2O
In lower grade assemblages associated with contact and regional metamorphism serpentine may form tremolite and forsterite according to the above reaction.

serpentine and diopside to tremolite, forsterite and H2O
5Mg3Si2O5(OH)4 + 2CaMgSi2O6 = Ca2Mg5Si8O22(OH)2 + 6Mg2SiO4 + 9H2O

Shortite

Formula: Na2Ca2(CO3)3 anhydrous normal carbonate
Specific gravity: 2.6
Hardness: 3
Streak: White
Colour: Colourless, light yellow, light green
Solubility: Decomposes in water giving a calcium carbonate precipitate.
Environments:

Carbonatites
Chemical sedimentary environments

Shortite occurs with calcite and pyrite as single crystals in bands in the montmorillonite clay associated with trona at the Green River formation, Wyoming, USA. It also occurs as minute inclusions in apatite from Tororo carbonatite complex, Eastern Region, Uganda.

Siderite

Formula: Fe(CO3) carbonate
Specific gravity: 3.7 - 3.9
Hardness: 4 - 4½
Streak: White
Colour: Yellowish white, yellowish brown to dark brown
Solubility: Moderately soluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites
Carbonatites (common)
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Siderite is frequently found mixed with clay minerals, in concretions with concentric layers. As black-band ore it is found, contaminated by carbonaceous material, in extensive stratified formations in shale and commonly associated with coal measures. It is also formed by the action of iron-rich solutions on limestone. Siderite is a common vein mineral associated with silver minerals, pyrite, chalcopyrite, tetrahedrite and galena in the oxidation zone of hypothermal (high temperature) hydrothermal veins.
Siderite may be found in limestone.

Alteration

olivine and CO2 to enstatite- ferrosilite and magnesite-siderite
(Mg,Fe)2SiO4 + CO2 → (Mg,Fe2+)SiO3 + (Mg,Fe)CO3

siderite, oxygen and H2O to hematite and silicic acid
2Fe2CO3 + O2 + 4H2O → 2Fe2O3 + 2H2CO3
On prolonged exposure to the air Fe2+ compounds are oxidised to Fe3+ compounds according to reactions such as the one above.

siderite and SiO2 to fayalite and CO2
2Fe(CO3) + SiO2 = Fe2+2(SiO4) + 2CO2

Mg-rich siderite and SiO2 to olivine, orthopyroxene and CO2
3(Fe,Mg)(CO3)→ (Fe,Mg)2SiO4 + 2SiO2 → (Fe,Mg)2SiO4 + 3CO2

Common impurities: Mn,Mg,Ca,Zn,Co

Siderotil

Formula: (Fe,Cu)(SO4).5H2O hydrated sulphate
Specific gravity: 2.1 to 2.2
Hardness: 2½
Streak: White
Colour: Yellowish, white, light green; colourless in transmitted light
Environments:

Hydrothermal environments

Siderotil is a rare supergene mineral formed by the oxidation of iron sulphides or the dehydration of cuprian melanterite. It tends to occur in association with other supergene sulphates such as jarosite. It can be a product of post-mining oxidation.

Sillimanite

Formula: Al2SiO5 nesosilicate (insular SiO4 groups). Polymorph (same formula, different structure) of andalusite and kyanite.
Specific gravity: 3.23 to 3.27
Hardness: 6½ to 7½
Streak: White
Colour: Colourless, white, yellow, brown, green, grey
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments (occasional)
Metamorphic environments (common)

The occasional occurrence of sillimanite in granitic rocks is normally the result of contamination by clay-rich xenoliths, but hydrothermal activity may also be responsible. Sillimanite usually occurs in high pressure and high temperature thermal aureoles around intrusive rocks, and in high pressure and temperature regionally metamorphosed rocks.
In contact metamorphosed rocks it may occur in sillimanite-cordierite gneiss or sillimanite-biotite hornfels.
In regionally metamorphosed rocks it is found in quartz-muscovite- biotite-oligoclase- muscovite-sillimanite schist. In silica-poor rocks it may be associated with corundum.
Sillimanite also may be found in gneiss and hornfels.

It is a characteristic mineral of the amphibolite and granulite facies, and it is also a mineral of the pyroxene-hornfels facies. Alteration

Andalusite, sillimanite and kyanite are polymorphs (same formula, different structure); they are in equilibrium at a pressure of 4.2 kbar and temperature 530oC. Under conditions of higher temperature and pressure andalusite may become unstable and invert to its polymorphs sillimanite or kyanite. Andalusite is stable at lower pressure and temperature up to about 900oC; kyanite is the high pressure polymorph, and sillimanite the high temperature polymorph.

muscovite and quartz to sillimanite, K-feldspar and H2O
KAl2(Si3Al)O10(OH)2 + SiO2 ⇌ Al2SiO5 + KAlSi3O8 + H2O
This equation represents changes that may occur in regional metamorphism when the metamorphic grade changes from the greenschist facies (left hand side) to the amphibolite facies (right hand side) or vice versa.
The forward reaction is strongly endothermic (absorbs heat) and the reverse reaction in exothermic (gives out heat), hence the forward reaction is favoured by high temperatures, as the system adjusts to bring the temperature back down.

sillimanite, annite and H2O to staurolite, muscovite, SiO2 and O2
31Al2SiO5 + 4KFe2+3(AlSi3O10)(OH)2 + 6H2O → 34Fe2+2Al9si4O23(OH) + KAl2 (AlSi3O10)(OH)2 + 7 SiO2 + 1.5O2
Staurolite may occur as a product of retrograde metamorphism according to the above reaction.

spinel-hercynite, sillimanite and SiO2 to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10

Common impurities: Fe

Silver

Formula: Ag native element
Specific gravity: 9.6 - 12
Hardness: 2½ - 3
Streak: Silver white
Colour: Silver white
Solubility: Insoluble in hydrochloric acid; slightly soluble in sulphuric acid; moderately soluble in nitric acid
Environments:

Plutonic igneous environments
Placers
Hydrothermal vein environments

Native silver is found in the enriched zone of hypothermal (high temperature) hydrothermal veins, and sometimes also as a primary mineral, either in epithermal (low temperature) veins associated with sulphides, zeolites, calcite, baryte, fluorite and quartz, or in hypothermal (high temperature) veins associated with uraninite, arsenides and sulphides of cobalt, nickel and silver and native bismuth.

Alteration

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + H2O → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4

The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + H2O → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3

Ferric sulphate is a strong oxidizing agent; it attacks silver according to the reaction:

silver + ferric sulphate → silver sulphate + ferrous sulphate
2Ag + Fe2(SO4)3 → Ag2SO4 + 2FeSO4

Common impurities: Au,Hg,Cu,Sb,Bi

Smithsonite

Formula: Zn(CO3) carbonate
Specific gravity: 4.3 - 4.5
Hardness: 5
Streak: White
Colour: Colourless, white, yellow, brown, red, green, blue, grey
Solubility: Readily soluble in hydrochloric, sulphuric and nitric acid
Environments:

Clastic sedimentary environments
Hydrothermal replacement environments

Smithsonite is often found as a secondary mineral in the oxidation zone of zinc ore deposits in limestone. It has also been observed in sedimentary deposits and as a direct oxidation product of sphalerite.
It is associated with sphalerite, galena, hematite, cerussite, calcite and limonite. It is often found as pseudomorphs after calcite. In the oxidation zone of epithermal veins sphalerite ZnS (primary) alters to secondary hemimorphite, smithsonite and manganese-bearing willemite.

Common impurities: Fe,Co,Cu,Mn,Ca,Cd,Mg,In

Sodalite

Formula: Na4(Si3Al3)O12Cl tectosilicate (framework silicate)
Specific gravity: 2.1 - 2.3
Hardness: 5 - 6
Streak: White
Colour: Blue, grey, white, colourless
Solubility: Moderately soluble in hydrochloric acid; slightly soluble in sulphuric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments

Sodalite is a relatively rare primary rock-forming mineral that never occurs together with quartz. It is also a product of volcanic eruptions and it is sometimes found in contact metamorphosed rocks.
It may be found in andesite, basalt, diorite, limestone, dolostone, gabbro, syenite and trachyte.
Sodalite is associated with nepheline, cancrinite and other feldspathoids.

Alteration

albite and NaCl (aqueous) to sodalite and silica
6Na(AlSi3O8) + 2NaCl → 2Na4(Si3Al3)O12Cl + 12SiO2

nepheline and NaCl from the fluid to sodalite
6NaAlSiO4 + NaCl ⇌ 2Na4(Si3Al3)O12Cl

Common impurities: Fe,Mn,K,Ca,H2O,S

Spessartine

Formula: Mn2+3Al2(SiO4)3 nesosilicate (insular SiO4 groups), garnet group
Specific gravity: 4.19
Hardness: 7
Streak: White
Colour: Pink, orange, light brown to dark brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Plutonic igneous environments
Pegmatites
Metamorphic environments

Spessartine occurs in plutonic igneous rocks, pegmatites and metamorphic manganese deposits. It may be found in granite.

Common impurities: Ti,Fe,Mg,Ca,H2O,Y

Sphalerite

Formula: ZnS sulphide
Specific gravity: 3.9 - 4.2
Hardness: 3½ - 4
Streak: White, yellow to brown if iron is present
Colour: Yellow, brown, red, green, black, rarely colourless
Solubility: Moderately soluble in hydrochloric acid; slightly soluble in nitric acid; insooluble in water
Environments:

Igneous environments
Sedimentary environments
Metamorphic environments
Hydrothermal replacement environments
Hydrothermal vein environments

Sphalerite is extremely common. Its occurrence and mode of origin are similar to those of galena, with which it is commonly found. Sphalerite occurs in granite, gabbro, sedimentary deposits and contact metamorphic deposits. It is found as a primary mineral in hypothermal (high temperature) hydrothermal veins associated with arsenopyrite, galena and quartz, and in replacement deposits associated with chalcopyrite, galena and pyrite.
Sphalerite with only minor galena occurs associated with pyrrhotite, pyrite and magnetite.
It also may be found in gabbro and granite.
In the oxidised zone of epithermal veins, sphalerite (primary) alters to secondary hemimorphite, smithsonite and manganese-bearing willemite.

Alteration

Sphalerite is a dimorph of wurtzite. The transition from sphalerite to wurtzite occurs around 1020oC.

Oxidation of pyrite forms ferrous (divalent) sulphate and sulphuric acid:
pyrite + oxygen + H2O → ferric sulphate + sulphuric acid
FeS2 + 7O + H2O → FeSO4 + H2SO4

The ferrous (divalent) sulphate readily oxidizes to ferric (trivalent) sulphate and ferric hydroxide:
ferrous sulphate + oxygen + H2O → ferric sulphate + ferric hydroxide
6FeSO4 + 3O + 3H2O → 2Fe2(SO4)3 + 2Fe(OH)3
Ferric sulfate is a strong oxidizing agent; it attacks sphalerite as below:

sphalerite, ferric sulphate and water to zinc sulphate, ferrous sulphate and sulphuric acid
ZnS + 4Fe2(SO4)3 + 4H2O → ZnSO4 + 8FeSO4 + 4H2SO4

Common impurities: Mn,Cd,Hg,In,Tl,Ga,Ge,Sb,Sn,Pb,Ag

Spinel

Formula: MgAl2O4 multiple oxide
Specific gravity: 3.6
Hardness: 8
Streak: White
Colour: Red, violet, blue, yellow, colourless
Solubility: Insoluble in hydrochloric and nitric acid; slightly soluble in sulphuric acid
Environments:

Placer deposits
Metamorphic environments

Spinel is a common high-temperature mineral occurring in contact metamorphosed limestone and metamorphic argillaceous (clay-rich) rocks poor in SiO2. It also occurs as an accessory mineral in many dark igneous rocks and it is frequently found frequently as rolled pebbles in stream sands.
In contact metamorphic rocks such as marble spinel is associated with phlogopite, pyrrhotite, chondrodite and graphite.
It is a mineral of the granulite facies.

Alteration

anorthite, enstatite, spinel, K2O and H2O to Al-rich hornblende, Mg-rich sapphirine and phlogopitephlogopite
2.5Ca(Al2Si2O8) + 10MgSiO3 + 6MgAl2O4 + K2O + 3H2O → Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 3Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2
This reaction occurs in the granulite to amphibolite facies.

enstatite-ferrosilite and andalusite to Fe-rich cordierite and spinel-hercynite
5(Mg,Fe2+)SiO3 + 5 Al2SiO5 → 2(Mg,Fe2+)2Al4Si5O18 + (Mg,Fe2+)Al2O4
In medium-grade thermally metamorphosed argillaceous rocks originally rich in chlorite and with a low calcium content, the association of andalusite with enstatite-ferrosilite is excluded by the above reaction.

enstatite-ferrosilite, Fe-rich diopside and Fe, Cr-rich spinel to garnet and olivine
2(Mg,Fe2+)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

forsterite and anorthite to clinoenstatite, diopside and spinel
2Mg2SiO4 + CaAl2Si2O8 ⇌ 2MgSiO3 + CaMgSi2O6 + MgAl2O4
The reaction can proceed in either direction, depending on the ambient conditions.

forsterite and anorthite to enstatite, diopside and spinel
2Mg2SiO4 + Ca(Al2Si2O8) = Mg2Si2O6 + CaMgSi2O6 + MgAl2O4

Al-rich hornblende, spinel, quartz, K2O and H2O to anorthite, Mg-rich sapphirine and phlogopite
Ca2.5Mg4Al(Al2Si6)O22(OH)2 + 4 MgAl2O4 + 6SiO2 + K2O + H2O → 2.5Ca(Al2Si2O8) + Mg2Al4SiO10 + 2KMg3(AlSi3O10)(OH)2

hypersthene, augite and Fe and Cr-rich spinel to garnet and olivine
2(Mg,Fe)SiO3 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ Ca(Mg,Fe)2(Al,Cr)2(SiO4)3 + (Mg,Fe)2SiO4

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine
(Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4
The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction

Fe and Cr-rich spinel, diopside and enstatite to forsterite, anorthite and chromite
MgFeAl2Cr2O8 + CaMgSi2O6 + Mg2Si2O6 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
This reaction occurs at fairly low temperature and pressure.

spinel-hercynite, sillimanite and quartz to sapphirine
7(Mg,Fe2+)Al2O4 + 2Al2SiO5 + SiO2 → 4(Mg,Fe)1.75Al4.5Si0.75O10

spinel and tremolite to forsterite and magnesio-hornblende MgAl2O4 + Ca2Mg5Si8O22(OH)2 ⇌ Mg2SiO4 + Ca2(Mg4Al)(Si7Al)O22(OH)2
This reaction occurs in some strongly metamorphosed serpentinite

orthopyroxene, Fe-rich diopside and Fe and Cr-rich spinel to Fe, Ca and Cr-rich pyrope and olivine (Mg,Fe)2Si2O6 + Ca(Mg,Fe)Si2O6 + (Mg,Fe)(Al,Cr)2O4 ⇌ (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Mg,Fe)2Ca(Al,Cr)2Si3O12 + (Fe,Mg)2SiO4

The garnet-bearing peridotites are considered to have originated in a high-pressure environment according to the reaction

Fe and Cr-rich spinel, diopside and enstatite to olivine, anorthite and chromite
MgFe2+Al2Cr2O8 + CaMgSi2O6 + 2MgSiO3 ⇌ 2Mg2SiO4 + Ca(Al2Si2O8) + Fe2+Cr2O4
In high temperature and high pressure environments olivine is produced according to the above reaction.

Common impurities: Ti,Fe,Zn,Mn,Ca

Spodumene

Formula: LiAlSi2O6 inosilicate (chain silicate) pyroxene group
Kunzite is a variety of spodumene
Specific gravity: 3.1 - 3.2
Hardness: 6½ - 7
Streak: White
Colour: Colourless, white, pink and lilac (kunzite), green (hiddenite), yellow, brown
Solubility: Insoluble in hydrochloric, sulphuric and nitric acid
Environments:

Pegmatites

Spodumene is found almost exclusively as a primary mineral in lithium-rich pegmatites.

Alteration

At the White Picacho District, Arizona, USA, the alteration sequence for spodumene is
spodumene → eucryptite + albitemuscovite + albitemuscovite

spodumene, quartz and Na+ to albite and Li+
LiAlSi2O6 + SiO2 + Na+ → NaAlSi3O8 + Li+
Whether spodumene breaks down into albite or into eucryptite and albite depends largely on the presenc