Tridymite
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).
| Tridymite | |
|---|---|
Tabular tridymite crystals from Ochtendung, Eifel, Germany | |
| General | |
| Category | Oxide mineral (or tectosilicate), quartz group |
| Formula (repeating unit) | SiO2 |
| IMA symbol | Trd |
| 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 | |
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