Abstract

Stishovite, a high-pressure high-density polymorph of SiO2, isostructural with rutile, was synthesized at pressures between 75 and 125 kb and temperatures greater than 800°C. Significant conversion to the rutile form was obtained in a modified girdle apparatus at 125 kb; trace amounts were obtained in a belt-type apparatus between 75 and 100 kb. Stishovite is colorless, acicular in habit, and uniaxial positive with e = 1.845 ± .005, ω = 1.800 ± .005. The optic axis is commonly highly inclined to the morphological axis. The transformation of coesite (4-fold coordination) to Stishovite (6-fold coordination) exemplifies the decrease in molar refraction with increasing coordination predicted by Dachille and Roy.

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