Abstract
The low-temperature isobaric heat capacities (C p) of β- and γ-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°298) of β- and γ-Mg2SiO4 are 86.4 ± 0.4 and 82.7 ± 0.5 J/mol K, respectively. Enthalpies of transitions among α-, β- and γ-Mg2SiO4 were measured by high-temperature drop-solution calorimetry with gas-bubbling technique. The enthalpies of the α−β and β−γ transitions at 298 K (ΔH°298) in Mg2SiO4 are 27.2 ± 3.6 and 12.9 ± 3.3 kJ/mol, respectively. Calculated α−β and β−γ transition boundaries were generally consistent with those determined by high-pressure experiments within the errors. Combining the measured ΔH°298 and ΔS°298 with selected data of in situ X-ray diffraction experiments at high pressure, the ΔH°298 and ΔS°298 of the α−β and β−γ transitions were optimized. Calculation using the optimized data tightly constrained the α−β and β−γ transition boundaries in the P, T space. The slope of α−β transition boundary is 3.1 MPa/K at 13.4 GPa and 1,400 K, and that of β−γ boundary 5.2 MPa/K at 18.7 GPa and 1,600 K. The post-spinel transition boundary of γ-Mg2SiO4 to MgSiO3 perovskite plus MgO was also calculated, using the optimized data on γ-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.
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Acknowledgments
We thank M. Matsui, A. Oganov and T. Tsuchiya for valuable discussion, and H. Nakayama and K. Ishii for infrared spectroscopy. We are grateful to anonymous referees for constructive comments. This work was supported in part by the Grant-in-Aid of the Scientific Research (A) (No. 15204049) of the Japan Society for the Promotion of Science to M. Akaogi.
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Akaogi, M., Takayama, H., Kojitani, H. et al. Low-temperature heat capacities, entropies and enthalpies of Mg2SiO4 polymorphs, and α−β−γ and post-spinel phase relations at high pressure. Phys Chem Minerals 34, 169–183 (2007). https://doi.org/10.1007/s00269-006-0137-3
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DOI: https://doi.org/10.1007/s00269-006-0137-3