HIGH-TEMPERATURE HEAT-CAPACITY AND PREMELTING OF MINERALS IN THE SYSTEM MGO-CAO-AL2O3-SIO2

Relative enthalpy measurements have been made from 800 to 1700 or 1800 K for periclase, lime, spinel, diopside, pseudowollastonite, and anorthite. Significant premelting effects have been observed for diopside, pseudowollastonite, and anorthite, showing that only part of the structural changes that take place near the melting point can be quenched to room temperature. With the heat capacities derived from these measurements and other data for SiO2 polymorphs, corundum, forsterite and pyrope, the validity of proposed C(p) equations has been tested, particularly with respect to extrapolations at higher and lower temperatures. For high-temperature extrapolations, the best results have been obtained with an equation of the form C(p) = k0 + k(ln) ln T + k1/T + k2/T2 + k3/T3. Comparisions of experimental heat capacities with values calculated from polyhedral modeling show that significant deviations from additivity can be found above 1000 K, likely as a result of anharmonic factors. This raises the possibility that enthalpies and entropies of formation are temperature dependent under mantle conditions. Finally, the difficulties of extrapolating differential scanning calorimetry data for mantle minerals are emphasized.