Thermochemical measurements on Mg2SiO4 wadsleyite and SiO2 stishovite and assessment of their self-consistent thermodynamic datasets for Gibbs energy expressions and equations of state

Abstract Drop-solution enthalpies of Mg2SiO4 wadsleyite and SiO2 stishovite were determined to be 141.38 ± 1.13 and 4.05 ± 0.38 kJ·mol−1, respectively, by performing drop-solution calorimetry with lead borate solvent at 978 K. Isobaric heat capacity of Mg2SiO4 wadsleyite was also measured using differential scanning calorimetry in the temperature range of 300−820 K. In addition, their self-consistent thermoelastic parameters of thermal expansivity (α), isothermal bulk modulus at the standard state (KT0) and its temperature derivatives [(∂KT0/∂T)P] were reassessed by combining the least squares fitting of a third-order Birch-Murnaghan equation of state with Grüneisen relation equation α = γth,0CV/(KT0V), together with those for Mg2SiO4 forsterite, Mg2SiO4 ringwoodite and SiO2 coesite, where isochoric heat capacity (CV) was calculated using the Kieffer model and thermal Grüneisen parameter at the standard state (γth,0) was constrained from measured α data. The obtained thermodynamic parameters were used to calculate forsterite–wadsleyite and wadsleyite–ringwoodite phase boundaries in Mg2SiO4 and coesite–stishovite phase boundary in SiO2. Finally, the present self-consistent thermodynamic data sets were applied to thermodynamic calculations of phase boundaries in the MgSiO3 system among [ringwoodite + stishovite], [wadsleyite + stishovite], and akimotoite by varying the standard enthalpies of formation for ringwoodite and stishovite within their uncertainties. The calculation results suggest that the stability field of the MgSiO3 akimotoite phase spreads to a lower pressure region by 2−4 GPa than what has been accepted so far.