Equations of state and phase equilibria of stishovite and a coesitelike phase from shock‐wave and other data

Shock-wave, static-compression (X ray), ultrasonic, thermal expansion, and thermodynamic data are simultaneously inverted to determine the equations of state of stishovite and a coesitelike SiO2 phase. All the stishovite data except the thermal expansion data are satisfied by a Mie-Gruneisen-type equation of state having a zero pressure bulk modulus K of about 3.50 ± 0.1 Mb, a pressure derivative dK/dP of 3.3 ± 1, and a Gruneisen parameter, initially 1.25 ± 0.1, that decreases slowly with compression. The volume coefficient of thermal expansion at ambient conditions is found to be 13 ± 1 × 10−6/°K, in comparison with 16.4 ± 1.3 measured by Weaver. Some Hugoniot data of Trunin et al. for very porous quartz have densities very close to the density of coesite. However, a calculation of the coesite-stishovite phase line shows that the coesitelike phase persists to about twice the predicted transition pressure at 10,000°K. It is suggested that the discrepancy can be explained if this phase is interpreted as a liquid of about coesite density.

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