Tectosilicate (framework silicate)
Varieties of quartz
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.
Faden quartz is a group of quartz crystals, usually tabular, with a white thread-like zone running through the interior.
Properties of quartz
Specific gravity: 2.65 to 2.66
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 polymorph tridymite may occur instead.
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 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 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 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 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).
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 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 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).
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 polymorphs of SiO2.
Alpha quartz is the low temperature, low pressure polymorph. 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 polymorph 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 polymorph 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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