Silica polymorphs

Quartz is a hard mineral composed of silica (silicon dioxide). Its atoms are linked in a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen atom being shared between two tetrahedra, giving an overall chemical formula of SiO2. Therefore, quartz is classified structurally as a framework silicate mineral and compositionally as an oxide mineral. Quartz is the second most common mineral or mineral group in Earth's lithosphere, comprising about 12% by mass.

thumb|alt=A map of the world displaying the countries where opal is most commonly found; Australia, Ethiopia, Brazil, Mexico and the United States of America are highlighted.|Main opal producing countries

Chalcedony ( or ) is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. These are both silica minerals, but they differ in that quartz has a trigonal crystal structure, while moganite is monoclinic. Chalcedony's standard chemical structure (based on the chemical composition of quartz) is (silicon dioxide).

Tridymite is a high-temperature polymorph of silica and usually occurs as minute tabular white or colorless pseudo-hexagonal crystals, or scales, in cavities in felsic volcanic rocks. Its chemical formula is SiO2. Tridymite was first described in 1868 and the type location is in Hidalgo, Mexico. The name is from the Greek tridymos for triplet as tridymite commonly occurs as twinned crystal trillings (compound crystals comprising three twinned crystal components).

Cristobalite ( ) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group, which also include coesite, tridymite and stishovite. It is named after Cerro San Cristóbal in Pachuca Municipality, Hidalgo, Mexico.

Coesite () is a form (polymorph) of silicon dioxide (SiO2) that is formed when very high pressure (2–3 gigapascals), and moderately high temperature (), are applied to quartz. Coesite was first synthesized by Loring Coes, Jr., a chemist at the Norton Company, in 1953.

thumb|Pressure-temperature diagram for various forms of silicon dioxide, including stishovite (at top left)

Moganite is a tectosilicate mineral with the chemical formula SiO2 (silicon dioxide) that was discovered in 1976. The mineral was initially described as a new form of silica, on the basis of specimens found in the Barranco de Medio Almud ravine. This ravine is in the municipality of Mogán, on the island of Gran Canaria, in the Canary Islands region of Spain. In later work, the mineral derived its name from this locality. In 1994, the International Mineralogical Association (IMA) disapproved moganite as a valid mineral, since it was considered to be indistinguishable from quartz. Subsequent stu

Keatite is a silicate mineral with the chemical formula SiO2 (silicon dioxide) that was discovered in nature in 2013. It is a tetragonal polymorph of silica first known as a synthetic phase. It was reported as minute inclusions within clinopyroxene (diopside) crystals in an ultra high pressure garnet pyroxenite body. The host rock is part of the Kokchetav Massif in Kazakhstan.

Melanophlogite (MEP) is a rare silicate mineral and a polymorph of silica (SiO2). It has a zeolite-like porous structure which results in relatively low and not well-defined values of its density and refractive index. Melanophlogite often overgrows crystals of sulfur or calcite and typically contains a few percent of organic and sulfur compounds. Darkening of organics in melanophlogite upon heating is a possible origin of its name, which comes from the Greek for "black" and "to be burned".

Seifertite is a silicate mineral with the formula SiO2 and is one of the densest polymorphs of silica. It has only been found in Martian and lunar meteorites, where it is presumably formed from either tridymite or cristobalite – other polymorphs of quartz – as a result of heating during the atmospheric entry and impact to the Earth, at an estimated minimal pressure of 35 GPa. It can also be produced in the laboratory by compressing cristobalite in a diamond anvil cell to pressures above 40 GPa. The mineral is named after Friedrich Seifert (born 1941), the founder of the Bayerisches G