No abstract

No abstract

No 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 ε = 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.

RECENT attempts to clarify the complex data on the polymorphism of MgSi03 (Brown and Smith, 1963; Boyd and Schairer, 1964) concluded that at atmospheric pressure protoenstatite is stable at high temperature, and clinoenstatite is a metastable phase produced by quenching protoenstatite in the stability field of orthoenstatite. Brown, Morimoto, and Smith (1961) presented crystal-chemical arguments iraplying that smaller atomic displacements are needed to form clinoenstatite instead of orthoenstatite from protoenstatite, thus explaining the metastable formation of clinoenstatite during quenching. Recent syntheses by Sclar, Carrison, and Schwartz (1964) have demonstrated a reversible phase boundary between clinoenstatite on the higher-pressurelower-temperature side and orthoenstatite at the lower-pressure-highertemperature side. They report that the phase boundary is given by T(~ = 538+3"3P (kilobars). Davis (1963), Boyd andEngland (1963), andBoyd, England, and Davis (1964) have synthesized MgSiO 3 polymorphs between 1300 and 1950 ~ C and pressures up to 46 Kb. Protoenstatite is stable from about 1000 ~ C to the melting point at atmospheric pressure, but at pressures above 8 Kb and temperatures above 1300 ~ C orthoenstatite is the only phase present. At pressures below 8 Kb an