Cristobalite
| Cristobalite | |
|---|---|
Cristobalite spheres formed via devitrification from the obsidian matrix (California, USA) 5.9×3.8×3.8 cm.
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| General | |
| Category | Oxide mineral |
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Formula (repeating unit) |
SiO2 |
| 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) |
| Identification | |
| Color | Colorless, white |
| Crystal habit | Octahedra or spherulites up to several cm large |
| 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 | 1713 °C (β) |
| References | |
The mineral cristobalite is a high-temperature polymorph of silica, meaning that 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. Cristobalite occurs as white octahedra or spherulites in acidic volcanic rocks and in converted diatomaceous deposits in the Monterey Formation of the US state of California and similar areas. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures. It is named after Cerro San Cristóbal in Pachuca Municipality, Hidalgo, Mexico.
The persistence of cristobalite outside of its thermodynamic stability range occurs because the transition from cristobalite to quartz or tridymite is "reconstructive", requiring the breaking up and reforming of the silica framework. These frameworks are composed of SiO4tetrahedra in which every oxygen atom is shared with a neighbouring tetrahedron, so that the chemical formula of silica is SiO2. The breaking of these bonds required to convert cristobalite to tridymite and quartz requires considerable activation energy and may not happen on a human time frame. Framework silicates are also known as tectosilicates.
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