Cristobalite
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Cristobalite 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... Wikipedia
Chemical formula: SiO2
Birefringence: 0.003
Color: Colorless, white
Crystal class: Trapezohedral (422)
Crystal habit: Octahedra or spherulites up to several cm in diameter
Crystal system: Tetragonal
IMA symbol: Crs