THERMODYNAMIC PROPERTIES OF ALPHA-QUARTZ, COESITE, AND STISHOVITE AND EQUILIBRIUM PHASE-RELATIONS AT HIGH-PRESSURES AND HIGH-TEMPERATURES

Isobaric heat capacities of alpha-quartz, coesite, and stishovite were measured by differential scanning calorimetry at 183-703 K. The heat capacity data of coesite represent a significant revision of the previous data. The heat capacities and entropies were also calculated using Kieffer's model. By high-temperature solution calorimetry, transition enthalpy for the beta-quartz-coesite at 979 K was measured to be 1.27+/-0.39 kJ/mol. The enthalpies for the alpha-quartz-coesite and coesite-stishovite transitions at 298 K were measured to be 3.40+/-0.56 and 33.62+/-1.01 kJ/mol, respectively, by differential drop-solution calorimetry. The enthalpy of the coesite-stishovite transition obtained in this study is about 10-15 kJ/mol smaller than those in the previous studies. Phase relations in SiO2 at high pressures and high temperatures were calculated using these new thermodynamic data. The calculated boundary for the alpha-quartz-coesite transition is consistent with those determined experimentally. For the coesite-stishovite transition, the calculated boundary having a slope of 2.5+/-0.3 MPa/K disagrees with that by Yagi and Akimoto but is generally consistent with that by Zhang et al.'s new in situ X ray diffraction study.