Study of the MgGeO ₃ polymorphs (orthopyroxene, clinopyroxene, and ilmenite structures) by calorimetry, spectroscopy, and phase equilibria

Below 6 GPa, MgGeO₃ transforms with increasing pressure from an orthopyroxene, to a clinopyroxene, and finally to an ilmenite structure. Thermodynamic parameters for MgGeO₃ (opx → cpx) and MgGeO₃ (cpx → ilm) have been constrained by high-temperature solution calorimetric experiments, lattice vibrational calculations, and phase-equilibrium studies. The enthalpy and entropy of MgGeO₃ (opx → cpx) determined in this study are −3440 ± 681 J·mol⁻¹ and −1.5 to −3.0 J·mol⁻¹ · K⁻¹, respectively, which are comparable in magnitude to values for other phase transitions among pyroxenes and pyroxenoids. The values of Δ$H9730$ and Δ$S9730$ for MgGeO₃ (cpx → ilm) determined from calorimetry and lattice vibrational calculations are 7808 ± 561 J·mol ⁻¹ and −7.5 ± 1.5 J·mol⁻¹-K⁻¹, respectively. These values are consistent with new experimental reversals of the MgGeO₃ (cpx → ilm) phase boundary, P (in GPa) = 1.913 + 0.0018T (in K).