High temperature solution calorimetry of the α-, β-, and γ-Mg2SiO4 polymorphs gives ΔH1000(αβ)=7610±680 cal mol1 and ΔH1000(βγ)=1630±900 cal mol1. Based on the phase equilibrium data of Suito (1977) and appropriate thermal expansivity, compressibility, and heat capacity data, ΔS1000=2.5±0.5 and −1.5±0.9 cal mol−1K−1 for the αβ and βγ transitions, respectively. Infrared and Raman spectra have been obtained for the three phases, and the lattice vibrational thermodynamic properties of the Mg2SiO4 polymorphs have been calculated using the model approach developed by Kieffer (1979c). A range of models consistent with the infrared and Raman data and compressional and shear wave velocities give entropies and heat capacities consistent with reported heat capacities (available only at 350–700 K for β- and γ-Mg2SiO4) and with the entropies of transition calculated above. From the vibrational calculations ΔS1000(αβ)=2.8±0.6 cal mol1K1 and ΔS1000(βγ)=1.3±0.9 cal mol1K1. These two approaches to calculating ΔS° (calorimetry plus phase equilibria compared to vibrational calculations) offer means of constraining the P–T slopes of phase transitions at very high pressure, where experimental determinations suffer from serious uncertainties. The thermochemical data for α, β, and γ-Mg2SiO4 are used to construct the P,T diagram for these phases. The slopes of the α–β, β–γ, and α–γ boundaries are calculated to be positive and a triple point is predicted to be near 500 (±150) K and 120 (±10) kbar.

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