First principles thermodynamic assessment of the MgO–SiO₂ system

Application of the Polarized Continuum Model to molten oxides in the MgO–SiO₂ system combined with all-electron ab initio calculations of the thermodynamic and thermophysical properties of all the solid phases nucleating in the system permits the computation of the phase diagram topology at high pressure and temperature up to deep Earth’s conditions. The first principle parameterization reproduces satisfactorily the extrinsic stability fields of the various metasilicate and orthosilicate polymorphs at subsolidus conditions. The extrinsic stability field of Anhydrous-B (Mg₁₄Si₅O₂₄; Anh-B) with respect to a Mg₂SiO₄ + MgO assemblage opens up at pressures higher than 10 GPa and widens with temperature to form a triangular pressure–temperature stability field. Superimposing the mantle adiabat Anh-B appears to predate the Mg₂SiO₄ + MgO assemblage with increasing pressure in a range comprised from roughly 10 to 20 GPa. Interactions among components in the liquid are addressed through the Hybrid Polymeric Approach (HPA). The P = 1 bar mixing properties of the liquid are consistent with a simple acid–base interaction according to Lux-Flood notation and with some experimental evidence concerning the enthalpy of fusion of stoichiometric compounds along the binary system. Limited strain energy contributions, which arise from loss of vibrational entropy in the mixture, are responsible for the liquid–liquid miscibility gap experimentally observed at room conditions. Disappearance of the miscibility gap at high P (i.e. P > 5 GPa) is due to the progressively vanishing effect of strain energy, counterbalanced by quite limited (and P-dependent) excess volumes of mixing (V_exc). The metasilicate melts congruently at P > 0 GPa. Forsterite forms peritectically at P ≤ 5 GPa.