Thermodynamic investigation on phase equilibrium boundary between calcium ferrite-type MgAl2O4 and MgO + α-Al2O3

Calcium ferrite-type MgAl2O4 is an important endmember of the calcium ferrite phase, which is a high-pressure constituent mineral of mid-ocean ridge basalt at lower mantle conditions. Drop-solution enthalpies of the calcium ferrite-type MgAl2O4 and a mixture of MgO and α-Al2O3 with a mol ratio of 1:1 were measured as 109.18 ± 1.10 and 140.41 ± 0.85 kJ/mol, respectively, by drop-solution calorimetry with 2PbO·B2O3 solvent at 978 K. As determined by differences in the drop-solution enthalpies, the formation enthalpy of calcium ferrite-type MgAl2O4 from oxides at 298 K (ΔfH°298) was 31.23 ± 1.39 kJ/mol. To thermodynamically calculate the phase boundary between calcium ferrite-type MgAl2O4 and MgO + α-Al2O3, the ΔfH°298 was used together with the other thermochemical and thermoelastic data for calcium ferrite-type MgAl2O4, i.e., heat capacity and entropy estimated by the lattice vibrational calculation, thermal expansivity calculated from the Grüneisen equation, and isothermal bulk modulus and its pressure and temperature derivatives determined by Sueda et al. (2009). The calculated boundary gave a phase transition pressure of 26.6 GPa at 1873 K with a slope of −0.006 GPa/K. This phase boundary is potentially applicable to a high-pressure calibration standard.