Thermodynamic investigation on phase equilibrium boundary between calcium ferrite-type MgAl_2O_4 and MgO+α-Al_2O_3

摘要

Calcium ferrite-type MgAl_2O_4 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 MgAl_2O_4 and a mixture of MgO and α-Al_2O_3 with a mol ratio of 1:1 were measured as 109.18±1.10 and 140.41±0.85kJ/mol, respectively, by drop-solution calorimetry with 2PbO·B_2O_3 solvent at 978K. As determined by differences in the drop-solution enthalpies, the formation enthalpy of calcium ferrite-type MgAl_2O_4 from oxides at 298K (Δ_fH~°_(298)) was 31.23±1.39kJ/mol. To thermodynamically calculate the phase boundary between calcium ferrite-type MgAl_2O_4 and MgO+α-Al_2O_3, the Δ_fH~°_(298) was used together with the other thermochemical and thermoelastic data for calcium ferrite-type MgAl_2O_4, 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 26.6GPa at 1873K with a slope of -0.006GPa/K. This phase boundary is potentially applicable to a high-pressure calibration standard.