Quartz

quartz

tridymite

cristobalite

coesite

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Formula: SiO2
Tectosilicate (framework silicate)

Varieties

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+.
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.
Citrine is a variety of quartz coloured yellow by submicroscopic distribution of colloidal ferric hydroxide and oxides, as well as Fe3+ substituting for Si.
Faden quartz is a group of quartz crystals, usually tabular, with a white thread-like zone running through the interior.
Ferruginous quartz is coloured red-brown by inclusions of hematite.
Green quartz owes its colour to inclusions of chlorite.
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.
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.

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 of quartz

In water, hydrochloric, sulphuric and nitric acid: quartz is insoluble at atmospheric pressure and temperature, but solubility increases rapidly with pressure and temperature.
In alkalis: SiO2 is attacked by alkaline substances (like potassium hydroxide, KOH). The speed of the reaction depends on the texture and crystal size: crystalline quartz will dissolve only very slowly in hot watery alkaline solutions, while amorphous SiO2, will be readily dissolved at room temperatures, according to the equation:
SiO2 + 2KOH → K2SiO3 + H2O
In hydrofluoric acid: Hydrofluoric acid, HF, will decompose quartz to form first silicon fluoride SiF4, then hydrofluorosilicic acid, H2SiF6:
SiO2 + 4HF → SiF4 + 2H2O
SiF4 + 2HF → H2SiF6
In natron or potash: All forms of silica dissolve in molten natron or potash (K2CO3)
SiO2 + Na2CO3.10H2O → Na2SiO3 + CO2 + 10H2O
SiO2 + K2CO3 → K2SiO3 + CO2
Weathering of silicate rocks: Carbonic acid H2CO3 present, for example, in meteoric waters, releases silicic acid and forms carbonates.
Formation of orthosilicic acid: Quartz dissolves in water at sufficiently high pressure and temperature forming orthosilicic acid, H4SiO4:
SiO2 + 2H2O → H4SiO4
Orthosilicic acid is a very weak acid, weaker than carbonic acid. It dissociates according to the reaction:
H4SiO4 + H2O ⇌ H3SiO4- + H3O+

Environments: Quartz occurs in every type of mineral environment.

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 paramorph tridymite may occur instead.

Localities

The Two Mile and Three Mile deposits, Paddy's River, Paddys River District, Australian Capital Territory, Australia, are skarn deposits at the contact between granodiorite and volcanic rocks. Quartz is a primary silicate that occurs in wide veins commonly intergrown with magnetite. Excellent crystals of milky quartz up to 4.5 cm long have been found, some coated with chlorite and magnetite (AJM 22.1.35).

At Bahia, Brazil, quartz pseudomorphs after talc have been found (KL p254).

At Irai, Brazil, quartz pseudomorphs after calcite have been found (KL p247).

At the Rock Candy mine, British Columbia, Canada, quartz pseudomorphs after fluorite have been found (KL p251).

The amethyst mines of the Thunder Bay area, Ontario, Canada, are the source of of some large groups of crystals of quartz variety amethyst. Some are of the typical purple amethyst colour, and some have a distinctly red colour, due to sub-surface inclusions of hematite (R&M 94.4.306-341).

At Tongbei, Fujian province, China, pseudomorphs of opal variety hyalite after quartz have been found with spessartine (KL p259).

At Cabiche, Quípama, Departamento de Boyacá, Colombia, quartz with bright yellow tips coloured by fibrous inclusions of an apparently amorphous mineral have been found. The fibres have not to date (May 2019) been identified unambiguously, but one possibility is halloysite (R&M 94.3.240-247).

At Göpfersgrün, Wunsiedel, Wunsiedel im Fichtelgebirge, Upper Franconia, Bavaria, Germany, talc pseudomorphs after quartz have been found (KL p234).

At Idar-Oberstein, Birkenfeld, Rhineland-Palatinate, Germany, quartz pseudomorphs after calcite have been found (R&M 95.3.275).

At Charcas, San Luis Potosi, Mexico, quartz pseudomorphs after danburite have been found (KL p250, R&M 95.3.275).

At the Berg Aukas mine, Grootfontein, Otjozondjupa Region, Namibia, quartz is a monor constituent in both the country rock and the orebodies. Drusy quartz is most commonly associated with descloizite and, together with calcite, dolomite, goethite and willemite, can form the matrix of some descloizite crystals (R&M 96.2.132).

At the Nikolaevskiy mine, Dalnegorsk, Russia, quartz pseudomorphs after beta-quartz have been found. Also at Dalnegorsk, quartz pseudomorphs after fluorite have been found (KL p246, 252).

At Mkobola district, Mpumalanga province, South Africa, a chalcedony pseudomorph after a quartz Japanese twin has been found (KL p258).

At the Witwatersrand Goldfield, South Africa, quartz is the most common mineral, constituting the bulk of the quartzite and the matrix of the conglomerate; it is also the most common mineral forming the conglomerate clasts. The latter are primarily white to grey vein quartz pebbles although less common quartzite and chert pebbles also occur. Quartz is found associated with baryte, calcite, chalcopyrite, clinochlore, dolomite, epidote, galena, gold, pyrite, pyrobitumen, pyrochlore and pyrrhotite. Several gold mines have produced aesthetic quartz crystals, typically transparent (R&M 96.4.341-342).

At the Welkom goldfield, Lejweleputswa District, Free State, South Africa, large specimens of quartz weighing several kilograms have been found (R&M 96.4.341-342).

At the President Brand Mine, Lejweleputswa District, Free State, South Africa, quartz crystals are partially to wholly coated by a thin film of light green clinochlore producing a glittering sheen. Clinochlore can also be included within some crystals imparting an attractive green colour to the quartz. Doubly terminated crystals have also been found here (R&M 96.4.341-342).

At the Mponeng Mine, West Wits, Far West Rand, West Rand District Municipality, Gauteng, South Africa, well formed quartz crystals to 30 cm, often as doubly terminated floaters, were common (R&M 96.4.341-342).

At the Kusasalethu Mine, Carletonville, Western Sector, Far West Rand, West Rand District Municipality, Gauteng, South Africa, noteworthy transparent crystals of quartz have been found (R&M 96.4.341-342).

At the Kopanang Mine, Klerksdorp, Dr Kenneth Kaunda District Municipality, North West, South Africa, a fault was intersected that yielded quartz specimens that are typically elongated prisms, transparent, and associated with brown, goethite-altered pyrrhotite and pyrite. Quartz specimens associated with grey rhombohedral calcite display distinct growth features via internal zoning, producing phantom forms (R&M 96.4.341-342).

At the Buffelsfontein gold mine, Stilfontein, Dr Kenneth Kaunda District, North West, South Africa, noteworthy transparent crystals of quartz have been found (R&M 96.4.341-342).

At Asar hill, Güğtı, Dursunbey district, Balikesir province, Marmara region, Turkey, quartz pseudomorphs after calcite have been found (KL p248).

At Wheal Mary Ann, Menheniot, Cornwall, England, UK, quartz pseudomorphs after fluorite have been found (R&M 95.3.275).

At the Fall Hill quarry, Ashover, Derbyshire, England, UK, quartz crystals occur on fluorite (RES p104).

At Calton Hill quarry, Buxton, Derbyshire, England, UK, quartz variety amethyst has been found lining a cavity in basalt (RES p116).

At Diamond Hill, Buxton, Derbyshire, England, UK, quartz variety eisenkiesel occurs in a baryte matrix (RES p134).

At Waterswallows quarry, Buxton, Derbyshire, England, UK, quartz variety amethyst has been found in a vug in dolerite (RES p135).

At Kenslow Knoll sandpit, Newhaven, Derbyshire, England, UK, quartz crystals occur on baryte (RES p114).

At the Harry Stoke mine, near Filton, South Gloucestershire, England, UK, quartz variety amethyst has been found with celestine and calcite (RES p168).

At Croft Quarry, Croft, Blaby, Leicestershire, England, UK, quartz is associated with analcime and is often well crystallised and showing left-hand faces. A cavity has been discovered with walls lined by a film of epidote with a little hematite, upon which was deposited a comb structure of thousands of small quartz crystals up to 12 mm in length and up to 5 mm in cross section. Of 52 crystals examined, 43 showed the development of the left trigonal pyramid {2111}, and rarely the left trigonal trapezohedron {6111} (JRS 20.24-25).

At the Cloud Hill quarry, Breedon on the Hill, Leicestershire, England, UK, quartz crystals have been found with hematite in a cavity in goethite, and also as variety eisenkiesel on a goethite matrix (RES p206).

At the Bardon Hill quarry, Coalville, Leicestershire, England, UK, quartz crystals have been found encrusted with dolomite (RES p194).

At Peldar Tor quarry, Spring Hill, Whitwick, Leicestershire, England, UK, quartz crystals have been found heavily included with chlorite (RES p201).

At the Snailbeach mine, near Minsterley, Shropshire, England, UK, quartz crystals coat both calcite rhombohedra and galena crystals. Quartz also occurs here with cerussite (RES p270, 272, 275, 276).

At the Edwin Richards quarry, Rowley Regis, Dudley, West Midlands, England, UK, quartz variety amethyst has been found with dolomite in a calcite vein (RES p330).

At the Finch mine, Hayden, Banner district, Gila county, Arizona, USA, quartz pseudomorphs after wulfenite have been found (KL p253).

At Black Canyon City, Maricopa county, Arizona, USA, large pseudomorphs of quartz after both anhydrite and aragonite have been found (R&M 94.2.166-167, KL p245).

At the Potter-Cramer mine, Vulture Mining District, Maricopa county, Arizona, USA, secondary colourless quartz crystals fill vugs and fractures within the host rock. Surprisingly the quartz fluoresces bright bluish white under SWUV, but not MW or LW, and it does not exhibit phosphorescence. It is not known what activates the fluorescence (R&M 96.1.34).

At the South Belmont Mine, Santa Cruz county, Arizona, USA, fine epimorphs of quartz after calcite have been found (R&M 94.2.160).

In the South Comobabi Mountains, Pima county, Arizona, USA, epimorphs of quartz after calcite have been found, sometimes with hematite inclusions (R&M 94.2.161).

At the Emmons pegmatite, Greenwood, Oxford county, Maine, USA, quartz can occur as crystals up to 25 cm long. Very dark smoky quartz crystals tend to occur in late-stage secondary phosphate assemblages with rhodochrosite. The Emmons pegmatite is an example of a highly evolved boron-lithium-cesium-tantalum enriched pegmatite (R&M 94.6.514).

At the Little Gem amethyst mine, Jefferson county, Montana, USA, quartz occurs in a pegmatite outcrop hosted in biotite-hornblende granite and containing pockets of significant size, mostly enclosed partially or completely within microcline feldspar. Pockets in the core of the pegmatite contain only quartz. Elsewhere they contain quartz and microcline, but seldom albite, with occasional minor quantities of epidote, schorl, anatase, pyrite (typically altered to limonite) and goethite.
Quartz occurs in a number of varieties, including milky quartz, rock crystal, smoky quartz and amethyst. Amethyst is a late stage generation of quartz, often the last, exhibiting habits including sceptres and orientated overgrowths. The purple colour is caused by trace quantities of iron as Fe3+, so where there is insufficient iron rock crystal or smoky quartz form in preference to amethyst.
Sceptres are very common, generally with milky or smoky stems and amethyst heads.
Jacaré or crocodile quartz has overgrowths that crystallised from the hydrothermal fluids that created secondary minerals. The substrate is milky to smoky quartz, and the jacaré is usually amethyst.
Many of the quartz crystals have fluid inclusions (R&M 93.6.498-516).

At the PC Mine, Cataract Mining District, Jefferson county, Montana, USA, quartz occurred as single crystals, groups, and as Japan-law twins. Specimens are mostly colourless, rarely smoky. Individual crystals are up to 30 cm long and groups up to 76 cm. Inclusions include anhydrite, hematite, sericite, schorl and pyrite. It is estimated that more than four thousand specimens of Japan-law twins were found here.
A twin is the symmetrical intergrowth of two crystals. A Japan-law twin is a contact twin with the c-axes inclined to each other at an angle of 84.55 degrees. A Reichenstein-Grieserntal twin is a rare contact twin with the inclination of the c-axes at 76.43 degrees. Only five are known to have been found at the PC mine (R&M 96.490-501)

In Nebraska, USA, quartz pseudomorphs after gypsum have been found (KL p249).

At Cookes Peak mining district, Luna county, New Mexico, USA, quartz occurs as massive replacement of limestone bodies, and as crystals lining cavities in these bodies. It is occasionally associated with fluorite, and is sometimes found as quartz epimorphs after fluorite. Excellent epimorphs of quartz after calcite have also been found, some associated with fluorite (R&M 94.3.234-235).

At the Devil's Den locality, Mount Tabor, Rutland county, Vermont, USA, collecting is no longer permitted, but in the past it has been a prolific source of various varieties of quartz, including faden quartz, smoky quartz and Mount Tabour "twins". The "twins" are pairs of parallel crystals that share a common prism face and a pair of terminations with a church-and-steeple appearance (R&M 94.3.266-272).

Alteration

Quartz is ubiquitous and a component of many reactions which are detailed elsewhere, according to the other components.
The only alterations considered here are transformations between the different paramorphs of SiO2. Silica minerals stability diagram

Alpha quartz is the low temperature, low pressure paramorph. At atmospheric pressure it is stable up to 573oC, when it alters to beta quartz, the beta quartz alters to tridymite at 870oC. Alpha quartz, beta quartz and coesite can co-exist at a point where the temperature is about 1,360oC and the pressure 34 kbar.
At atmospheric pressure and 1,470oC tridymite alters to cristobalite, and cristobalite melts at 1705oC.
Tridymite, cristobalite and beta quartz can co-exist in equilibrium at a point with temperature about 1,400 oC and pressure 30 kbar.
Coesite is a high pressure paramorph of quartz. With increasing pressure, at 800oC alpha quartz alters to coesite at about 30 kbar pressure, then coesite alters to the ultra-high pressure paramorph stishovite at about 90 kbar.
beta quartz and coesite can co-exist in equilibrium with the silica melt at a point where the temperature is about 2,410oC and the pressure 45 kbar.
With further increase in pressure and temerature, coesite can continue to exist up to about 2,770oC and 110 kbar pressure, at which point coesite, stishovite and the silica melt are in equilibrium (QP).

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