THE ZNSIO3 CLINOPYROXENE-ILMENITE TRANSITION - HEAT-CAPACITY, ENTHALPY OF TRANSITION, AND PHASE-EQUILIBRIA

ZnSiO3 clinopyroxene stable above 3 GPa transforms to ilmenite at 10-12 GPa, which further decomposes into ZnO (rock salt) plus stishovite at 20-30 GPa. The enthalpy of the clinopyroxene-ilmenite transition was measured by high-temperature solution calorimetry, giving ΔH0=51.71 ±3.18 kJ/mol at 298 K. The heat capacities of clinopyroxene and ilmenite were measured by differential scanning calorimetry at 343-733 and 343-633 K, respectively. The Cp of ilmenite is 3-5% smaller than that of clinopyroxene. The entropy of transition was calculated using the measured enthalpy and the free energy calculated from the phase equilibrium data. The enthalpy, entropy and volume changes of the pyroxene-ilmenite transition in ZnSiO3 are similar in magnitude to those in MgSiO3. The present thermochemical data are used to calculate the phase boundary of the ZnSiO3 clinopyroxene-ilmenite transition. The calculated boundary, {Mathematical expression}. 10-3, is more tightly constrained than those determined previously by high-pressure high-temperature experiments, and can be used as a pressure calibration at 10-12 GPa and 1000-2000 K. The disproportionation of ZnSiO3 ilmenite to ZnO (rock salt) plus stishovite is calculated to occur at about 25 GPa at 1000-2000 K. © 1990 Springer-Verlag.