Tridymite
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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... Wikipedia
Birefringence: δ < 0.004
Cleavage: indistinct,, imperfect
Crystal class: Disphenoidal (222); H–M symbol: (222)
Crystal habit: Platy – sheet forms
Crystal system: Orthorhombic; (α-tridymite)
Formula mass: 60.08 g/mol