A Proposed Equation of State of Stishovite

The available shock-wave data for solid .a quartz in the stishovite pressure regime are reduced to a 25øC 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 Bireh-Murnaghan equation, depends critically on the value of (dK/dP)•, at zero pressure and to a yet unknown extent on the form of the equation of state. The high-temperature value of Griineisen's ratio (0.8 to 0.0) along the a quartz (stishovite regime) I-Iugoniot was obtained from the pressure offsets of the fused quartz and porous quartz Hugoniot. The high value for 7 obtained from thermochemical data at standard conditions (1•5 q0.3) suggests that a marked decrease in the value of 7 to 0.8 occurs with increasing temperature.