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.

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.

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.

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.

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.

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.

Types of metamorphism

Contact metamorphism

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

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.

Retrograde metamorphism

New minerals are often formed when the metamorphic grade increases (temperature and/or pressure increase). If the metamorphic grade decreases (temperature and/or pressure decrease) and new minerals are formed this is termed retrograde metamorphism.

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 boundaries of the different facies are not sharp, and there is appreciable overlap. Different sources give different values for the boundaries. The ones listed below are in fairly common usage. 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 5 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, wairakite, 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 450 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 (large crystals in a finer groundmass in a rock produced by metamorphic recrystallisation), 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 (large crystals in a finer groundmass in a rock produced by metamorphic recrystallisation), 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 6 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 1 to 10 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 800oC and pressure 2 to 14 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 above 700oC and pressure above 2 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 100 to 500oC and pressure greater than 4 kbar.
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 250oC 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.

Volcanic sublimates and hot spring deposits

Volcanic sublimates are minerals that form directly from volcanic gases during a discharge or eruption. Volcanic gases may also mix with steam and erupt through openings in the Earth's crust, called fumeroles. Some minerals form under the earth in geothermal wells, or in hot springs when the heated fluids rise to the surface. Minerals that crystallise directly from volcanic gases 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.