THERMODYNAMIC FUNCTIONS AND PROPERTIES OF MGO AT HIGH COMPRESSION AND HIGH-TEMPERATURE

The complete thermodynamic functions for MgO are presented for the temperature range 300–2000 K and the pressure range 0–150 GPa, both as tables and as graphs. Careful attention is given to the temperature and pressure dependence of the coefficient of thermal expansion α and the isothermal bulk modulus KT, which are the major corrections to the thermodynamic functions in extreme conditions. Our equations efficiently use the fact that the product αKT varies only slightly with either temperature or with volume, although scrupulous care is taken to account for the actual numbers. The corrections are now possible due to recent measurements of the bulk modulus up to 1800 K. The Birch–Murnaghan equation of state is used to account for certain terms in the expressions for internal energy and enthalpy. The parameters used in the Birch–Murnaghan equation of state are given as functions of temperature. The parameters which presently limit the calculations of thermodynamic functions to even higher temperatures and pressures and limit the applications of this method to other minerals are: (1) the uncertainty of the thermal expansivity at temperatures above 1500 K, and (2) the uncertainty of the temperature dependence of (∂KT/∂T)P at temperatures above 800 K. © 1990, American Institute of Physics for the National Institute of Standards and Technology. All rights reserved.