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 ΔH9730 and ΔS9730 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).

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