Tridymite
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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. 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.
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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