Equations of State and Phase Boundaries of SiO₂ Polymorphs Under Lower Mantle Conditions

SiO2 is an important modally abundant component of terrestrial planets, while there are few studies that quantitatively determine the properties of silica under simultaneously high-temperature and high-pressure conditions. In this study, thermal properties of stishovite, CaCl2-type SiO2 and seifertite have been obtained by using extensive first-principles simulations. The quasi-harmonic approximation and intrinsic anharmonic effects were considered, and a generalized rescaling method was adopted to eliminate systematic errors in the simulation results. Based on the simulation data, we have established new equations of state for the SiO2 polymorphs and determined their phase boundaries. Anharmonicity has been demonstrated to show noticeable effects on their thermal properties and phase boundaries. With the new results, we found that SiO2 polymorphs show increasing buoyancy in the subducted basaltic slabs at depths greater than similar to 800-900 km. In the normal lower mantle, they are readily stable over a broad range of depths approximately from 1,500 km to 2,600 km. This implies that SiO2 polymorphs may contribute to the lateral heterogeneity in the deep lower mantle as observed by small-scale seismic scatterers.