10:45 AM - 11:00 AM
[SMP38-07] Low-temperature heat capacity measurement of MgSiO3 majorite
Keywords:majorite, heat capacity, entropy, thermal relaxation method
It is thought that garnet is a major Earth’s mantle constituent mineral. High pressure high temperature experiments have cleared that pyroxene component dissolves into the garnet phase with increasing pressure. MgSiO3 majorite (Mj) which does not contain Al is one of the important endmembers of such the silicate garnet containing pyroxene component. No entropy data of MgSiO3 Mj based on measured heat capacity existed though it is necessary for thermodynamic treatment of MgSiO3 Mj. In this study, we performed low-temperature heat capacity (Cp) measurement of synthetic MgSiO3 Mj using the thermal relaxation method and determined the entropy from the lattice vibrational contribution at standard state.
MgSiO3 Mj sample for calorimetry was synthesized by holding MgSiO3 glass at 19 GPa and 2173 K for one hour using a high-pressure apparatus at Ehime University, GRC. Since the recovered sample showed the coexistence of MgSiO3 akimotoite with MgSiO3 Mj at lower temperature region, MgSiO3 akimotoite was removed. After the cleaning, it was confirmed that the MgSiO3 Mj sample was a single phase using a micro-focused X-ray diffractometer and a micro-Raman spectrometer. Low-temperature Cp measurement was made using a Physical Property Measurement System (PPMS) apparatus (Quantum Design) in a temperature range of 2-306 K with a temperature step of 2 K. The weight of the sample was 14.415 mg.
Low-temperature Cp data measured in this study are compared with previous ones measured above 150 K using DSC method by Yusa et al. (1993). Our data is consistent within the errors with those of the latter around 300 K. While, in a temperature range of 150-200 K, the present values are about 5% larger than those of the latter. Entropy from the lattice vibrational contribution at standard state (S298.15,vib) was determined to be 65.35(2) J/mol.K by integrating Cp/T in an interval of 0-298.15 K. In addition, configurational entropy (Sconf) is calculated as 1.76 J/mol.K by assuming that the degree of Mg-Si disorder at the octahedral sites is 15%. Therefore, the standard entropy of MgSiO3 Mj is obtained to be 67.11 J/mol.K from the summation of S298.15,vib and Sconf. The present result indicates that the standard entropy of MgSiO3 Mj is larger than those previously estimated, e.g., 60.3 J/mol.K by Fabrichnaya et al. (2004).
MgSiO3 Mj sample for calorimetry was synthesized by holding MgSiO3 glass at 19 GPa and 2173 K for one hour using a high-pressure apparatus at Ehime University, GRC. Since the recovered sample showed the coexistence of MgSiO3 akimotoite with MgSiO3 Mj at lower temperature region, MgSiO3 akimotoite was removed. After the cleaning, it was confirmed that the MgSiO3 Mj sample was a single phase using a micro-focused X-ray diffractometer and a micro-Raman spectrometer. Low-temperature Cp measurement was made using a Physical Property Measurement System (PPMS) apparatus (Quantum Design) in a temperature range of 2-306 K with a temperature step of 2 K. The weight of the sample was 14.415 mg.
Low-temperature Cp data measured in this study are compared with previous ones measured above 150 K using DSC method by Yusa et al. (1993). Our data is consistent within the errors with those of the latter around 300 K. While, in a temperature range of 150-200 K, the present values are about 5% larger than those of the latter. Entropy from the lattice vibrational contribution at standard state (S298.15,vib) was determined to be 65.35(2) J/mol.K by integrating Cp/T in an interval of 0-298.15 K. In addition, configurational entropy (Sconf) is calculated as 1.76 J/mol.K by assuming that the degree of Mg-Si disorder at the octahedral sites is 15%. Therefore, the standard entropy of MgSiO3 Mj is obtained to be 67.11 J/mol.K from the summation of S298.15,vib and Sconf. The present result indicates that the standard entropy of MgSiO3 Mj is larger than those previously estimated, e.g., 60.3 J/mol.K by Fabrichnaya et al. (2004).