First Principles Molecular Dynamics Simulations of Diopside Liquid at High Pressure

Diopside (CaMgSi$_{2}$O$_{6})$ is a major component of basalt; the high-pressure end members, Mg-perovskite (MgSiO$_{3})$ and Ca-perovskite (CaSiO$_{3})$, make up more than 80 {\%} of the lower mantle. Despite its importance, most studies of diopside liquid have been performed at relatively low pressures and temperatures. In this study, we investigated CaMgSi$_{2}$O$_{6}$ liquid at lower mantle conditions by first principles molecular dynamics (FPMD) simulations based on density functional theory. The average Si-O coordination number increases nearly linearly from 4 to 6 with two-fold compression. The structure shows evidence of incipient exsolution with non-random clustering of Mg and Ca ions. Our results are well fitted by Mie-Gr\"{u}neisen equation of state with a Gr\"{u}neisen parameter that increases on compression. The variation of the diffusion coefficient with pressure and temperature is captured by the Arrhenius relation with activation energy and volume E* = 1.2 eV and V* = 1.25 {\AA}$^{3}$. The electronic properties of the CaMgSi$_{2}$O$_{6}$ liquid phase are similar as those of the MgSiO$_{3}$ liquid: there is no band gap and an extra peak appears at the Fermi level at low pressure.