A proposed equation of state of stishovite

The available shock-wave data for solid α quartz in the stishovite pressure regime are reduced to a 25°C isotherm and an adiabat, centered at standard conditions, using recent standard density, enthalpy, and coefficient of thermal expansion data. The calculated isothermal bulk modulus, 3 Mb, as determined from the Birch-Murnaghan equation, depends critically on the value of (dK/dP)T at zero pressure and to a yet unknown extent on the form of the equation of state. The high-temperature value of Grüneisen's ratio (0.8 to 0.9) along the a quartz (stishovite regime) Hugoniot was obtained from the pressure offsets of the fused quartz and porous quartz Hugoniot. The high value for γ obtained from thermochemical data at standard conditions (1.5±0.3) suggests that a marked decrease in the value of γ to 0.8 occurs with increasing temperature.