On the equation of state of stishovite

Abstract

Because of the discovery and subsequent synthesis of stishovite, a high-density polymorph of silica, Wackerle's shock-wave data on quartz have been re-examined to see if the high-pressure phase he reports could indeed be stishovite. Although the temperatures used for synthesis are about 1000°C higher than the temperature calculated by Wackerle for the transformation, the fact that the pressures are nearly the same (about 140 kb) is the prime evidence that the two transformations are the same, since the small change in entropy for a solid-solid phase change and the large observed volume change indicate that the slope of the phase line, dP/dT, should be quite small. The large increase in density shown by the shock-wave data above the transformation is comparable to the increase in density of quartz to stishovite; this gives additional support for identification of the transformations. On the assumptions that Wackerle's high-pressure data, for both crystalline and vitreous quartz, correspond to stishovite, a Hugoniot equation of state has been found for stishovite. This material has a Gruneisen ratioV(∂P/∂E)υ = 0.9, a heat of formation relative to quartz of 1.5×1010 ergs/g, and a bulk sound speed of about 10 km/sec.