Tridymite

Tridymite
Tabular tridymite crystals from Ochtendung, Eifel, Germany
General
Category	Oxide mineral (or tectosilicate), quartz group
Formula (repeating unit)	SiO2
IMA symbol	Trd[1]
Strunz classification	4.DA.10
Crystal system	Orthorhombic (α-tridymite)
Crystal class	Disphenoidal (222) H–M symbol: (222)
Space group	C2221
Identification
Formula mass	60.08 g/mol
Color	Colorless, white
Crystal habit	Platy – sheet forms
Cleavage	{0001} indistinct, {1010} imperfect
Fracture	Brittle – conchoidal
Mohs scale hardness	7
Luster	Vitreous
Streak	white
Specific gravity	2.25–2.28
Optical properties	Biaxial (+), 2V = 40–86°
Refractive index	'nα=1.468–1.482 nβ=1.470–1.484 nγ=1.474–1.486
Birefringence	δ < 0.004
Pleochroism	Colorless
Other characteristics	non-radioactive, non-magnetic; fluorescent, short UV=dark red
References	[2][3]

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^[2] (compound crystals comprising three twinned crystal components).