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the stable form of silica (silicon dioxide, SiO2) between its melting point of 1,728 C (3,142 F) and 1,470 C (2,678 F), below which tridymite is the stable form. Cristobalite has two modifications: low-cristobalite, which occurs naturally up to 268 C (514 F) but is not stable; and high-cristobalite, which occurs above 268 C but is only stable above 1,470 C. Natural low-cristobalite usually occurs in sub-microcrystalline masses (see opal) or fibrous to columnar spherulites (see lussatite) in igneous rocks. Cristobalite has the same chemical composition as coesite, stishovite, quartz, and tridymite but has a different crystal structure. For detailed physical properties, see silica mineral (table)