Low-temperature heat capacities, entropies and enthalpies of Mg2SiO4 polymorphs, and α-β-γ and post-spinel phase relations at high pressure

The low-temperature isobaric heat capacities (C-p) of beta- and gamma-Mg2SiO4 were measured at the range of 1.8 - 304.7 K with a thermal relaxation method using the Physical Property Measurement System. The obtained standard entropies (S degrees(298)) of beta- and gamma-Mg2SiO4 are 86.4 +/- 0.4 and 82.7 +/- 0.5 J/mol K, respectively. Enthalpies of transitions among alpha-, beta- and gamma- Mg2SiO4 were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the alpha-beta and beta - gamma transitions at 298 K (Delta H degrees(298)) in Mg2SiO4 are 27.2 +/- 3.6 and 12.9 +/- 3.3 kJ/ mol, respectively. Calculated alpha - beta and beta - gamma transition boundaries were generally consistent with those determined by high-pressure experiments within the errors. Combining the measured Delta H degrees(298) and Delta S degrees(298) with selected data of in situ X-ray diffraction experiments at high pressure, the Delta H degrees(298) and Delta S degrees(298) of the alpha - beta and beta - gamma transitions were optimized. Calculation using the optimized data tightly constrained the alpha - beta and beta - gamma transition boundaries in the P, T space. The slope of alpha - beta transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of beta - gamma boundary 5.2 MPa/K at 18.7 GPa and 1,600 K. The post-spinel transition boundary of gamma-Mg2SiO4 to MgSiO3 perovskite plus MgO was also calculated, using the optimized data on gamma-Mg2SiO4 and available enthalpy and entropy data on MgSiO3 perovskite and MgO. The calculated post-spinel boundary with a Clapeyron slope of - 2.6 +/- 0.2 MPa/K is located at pressure consistent with the 660 km discontinuity, considering the error of the thermodynamic data.