Tectosilicate (framework silicate)
Cristobalite is a paramorph of quartz that exists both as α and as β phases. β-cristobalite is the stable form of SiO2 from 1,460oC to the melting point, 1,728oC. It exists as a metastable phase below 1,470oC because the transition to tridymite proceeds very slowly. The transformation from β to α-cristobalite occurs at 268oC for pure cristobalite, but may be as low as 175oC if a high level of impurities exists.
Specific gravity: 2.2 to 2.33
Colour: Colourless, white, also blue grey, brown, grey, yellow
Common impurities: Fe,Ca,Al,K,Na,Ti,Mn,Mg,P
Volcanic igneous environments
Cristobalite occurs both as α and as β forms as a component of
opal. Crystalline material occurs in silica-rich volcanic
rocks. It is characteristic in vesicles in rhyolite,
andesite and trachyte,
is associated with tridymite,
pyroxene, fayalite and
magnetite, and also in
It is also found in thermally metamorphosed sandstone
At atmospheric pressure, with increasing temperature tridymite alters to cristobalite at 1,470oC, and cristobalite melts at 1,705oC. tridymite, cristobalite and beta quartz can co-exist in equilibrium at a point with temperature about 1,400 oC and pressure 30 kbar (QP).
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