Cristobalite
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The mineral cristobalite is a high-temperature polymorph of quartz and tridymite. It occurs as white octahedra in acidic volcanic rocks. Cristobalite is stable only above 1470 degrees Celsius, but can crystallize and persist metastably at lower temperatures. The crystal symmetry is cubic at very high temperature, but undergoes displacive phase transitions on cooling, the silica framework buckling to give tetragonal and then orthorhombic forms. The micrometre-scale spheres that make up precious opal are made of cristobalite, crystallized metastably at low temperature.
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References
- American Geological Institute Dictionary of Geological Terms.
- Reviews in Mineralogy and Geochemistry vol. 29. Silica: behavior, geochemistry and physical applications. Mineralogical Society of America, 1994.Template:Mineral-stub