Cristobalite

Formula: SiO2 tectosilicate (framework silicate) Silica minerals stability diagram
Cristobalite is a polymorph 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
Hardness: 6½
Streak: White
Colour: Colorless, white, also blue grey, brown, grey, yellow
Environments:

Volcanic igneous environments
Metamorphic 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, where it is associated with tridymite, quartz, sanidine, pyroxene, fayalite and magnetite, and also in basalt. It is also found in thermally metamorphosed sandstone.

Alteration

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.

Common impurities: Fe,Ca,Al,K,Na,Ti,Mn,Mg,P