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Tridymite is a high-temperature polymorph of quartz and usually occurs as minute tabular white or colorless pseudo-hexagonal triclinic crystals, or scales, in cavities in acidic volcanic rocks. Its chemical formula is SiO2. Tridymite is stable between 870 and 1470 degrees Celsius. It has a Mohs hardness of 6.5 to 7, a specific gravity of 2.28 to 2.33 and refractive indices of nα=1.471 - 1.482 nβ=1.472 - 1.483 nγ=1.474 - 1.488. Optically, tridymite has a birefringence of < .004 and has a 2V between 40 and 90 degrees, it may have wedge, complex or polysynthetic twinning.
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.
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