Low-temperature heat capacities, entropies and high-pressure phase relations of MgSiO3 ilmenite and perovskite

Low-temperature isobaric heat capacities (C-p ) of MgSiO3 ilmenite and perovskite were measured in the temperature range of 1.9-302.4 K with a thermal relaxation method using the Physical Properties Measurement System. The measured C-p of perovskite was higher than that of ilmenite in the whole temperature range studied. From the measured C-p , standard entropies at 298.15 K of MgSiO3 ilmenite and perovskite were determined to be 53.7 +/- 0.4 and 57.9 +/- 0.3 J/mol K, respectively. The positive entropy change (4.2 +/- 0.5 J/mol K) of the ilmenite-perovskite transition in MgSiO3 is compatible with structural change across the transition in which coordination of Mg atoms is changed from sixfold to eightfold. Calculation of the ilmenite-perovskite transition boundary using the measured entropies and published enthalpy data gives an equilibrium transition boundary at about 20-23 GPa at 1,000-2,000 K with a Clapeyron slope of -2.4 +/- 0.4 MPa/K at 1,600 K. The calculated boundary is almost consistent within the errors with those determined by high-pressure high-temperature in situ X-ray diffraction experiments.