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

• Published in: Physics and chemistry of minerals 2008-06, Vol.35 (5), p.287-297
• Main Authors: Akaogi, M., Kojitani, H., Morita, T., Kawaji, H., Atake, T.
• Format: Article
• Language: English
• Subjects: Crystallography and Scattering Methods; Earth and Environmental Science; Earth Sciences; Enstatite; Enthalpy; Geochemistry; High temperature; Ilmenite; Low temperature; Mathematical analysis; Mineral Resources; Mineralogy; Original Paper; Perovskites; Physical properties; Relaxation method (mathematics); Thermal relaxation; X-ray diffraction
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-008-0222-x

Low-temperature isobaric heat capacities ( C p ) of MgSiO 3 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 MgSiO 3 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 MgSiO 3 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.