Cristobalite

Cristobalite structure
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High temperature polymorph of silicon dioxide (SiO2). Has the same chemical composition as coesite, stishovite, seifertite and tridymite but possesses a different crystal structure. This silica group mineral occurs in terrestrial volcanic rocks, martian and lunar meteorites, chondrites and impact glasses like Libyan Desert Glass. Cristobalite has a very open structure consisting of sheets of tetrahedra in 6-fold rings with tetrahedra pointing alternately up and down. The α to β inversion occurs at 268 °C. Cristobalite is stable only above 1470 °C, but can crystallize and persist metastably at lower temperatures.