Cristobalite
Cristobalite (/krɪˈstoʊbəˌlaɪt/) is a mineral polymorph of silica that is formed at very high temperatures. It has the same chemical formula as quartz, SiO2, but a distinct crystal structure. Both quartz and cristobalite are polymorphs with all the members of the quartz group, which also include coesite, tridymite and stishovite. It is named after Cerro San Cristóbal in Pachuca Municipality, Hidalgo, Mexico.
| Cristobalite | |
|---|---|
Cristobalite spherulites formed by devitrification from the obsidian matrix. Specimen from California, US; size: 5.9 cm × 3.8 cm × 3.8 cm (2.3 in × 1.5 in × 1.5 in). | |
| General | |
| Category | Oxide mineral, quartz group |
| Formula (repeating unit) | SiO2 |
| IMA symbol | Crs |
| Strunz classification | 4.DA.15 |
| Dana classification | 75.1.1.1 |
| Crystal system | Tetragonal |
| Crystal class | Trapezohedral (422) |
| Space group | P41212, P43212 |
| Unit cell | a = 4.9709(1) Å, c = 6.9278(2) Å; Z = 4 (α polytype) |
| Structure | |
| Jmol (3D) | Interactive image |
| Identification | |
| Color | Colorless, white |
| Crystal habit | Octahedra or spherulites up to several cm in diameter |
| Twinning | on {111} |
| Fracture | Conchoidal |
| Tenacity | Brittle |
| Mohs scale hardness | 6–7 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent |
| Specific gravity | 2.32–2.36 |
| Optical properties | Uniaxial (−) |
| Refractive index | nω = 1.487 nε = 1.484 |
| Birefringence | 0.003 |
| Pleochroism | None |
| Melting point | 1,713 °C (3,115 °F) (β) |
| References | |
It is used in dentistry as a component of alginate impression materials as well as for making models of teeth.
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