10:15 AM - 10:30 AM
[SMP33-06] Drop-solution enthalpy measurement of MgSiO3 majorite
Keywords:garnet, majorite, enthalpy, thermodynamic stability, thermodynamic calculation
MgSiO3 Mj samples for the drop-solution calorimetry was the same as those used in the heat capacity measurement, which were synthesized at 19 GPa and 2173 K using a high-pressure apparatus at Ehime University, GRC. The sintered polycrystalline samples were crushed into powder. The drop-solution calorimetry was performed using a Calvet-type high-temperature calorimeter (SETARAM, HT-1000). The powdered sample with weight of about 4 mg was compressed into a pellet and it was dropped into the lead borate solvent (2PbO·B2O3) placed in the calorimeter which was kept at 978 K. The summation of heat content from room temperature to 978 K and solution enthalpy at 978 K, namely drop-solution enthalpy (ΔHd-s), was measured. To hasten the solution of the samples, the solvent was stirred by bubbling with Ar gas.
ΔHd-s (MgSiO3 Mj) was determined to be 69.57±0.77 kJ/mol from the average of five data. This value is the middle between 80.0±2.5 kJ/mol (Yusa et al., 1993) and 59.5±3.2 kJ/mol (Saikia, 2008). From the difference between ΔHd-s (MgO) (33.74±0.99 kJ/mol, Kojitani et al. 2012) +ΔHd-s (SiO2 quartz) (40.05±0.36 kJ/mol, Akaogi et al. 1995) and our ΔHd-s (MgSiO3 Mj), the formation enthalpy from the oxides was obtained as 4.2±1.3 kJ/mol. The thermodynamic calculation of the P-T phase diagram by adopting the present enthalpy value suggests that the stability field of MgSiO3 Mj would expand to lower temperature region than that determined by previous high-pressure high-temperature experiments and thermodynamic calculations.