First principles molecular dynamics simulations of diopside (CaMgSi2O6) liquid to high pressure

We use first principles molecular dynamics simulations based on density functional theory in the local density approximation to investigate CaMgSi2O6 liquid over the entire mantle pressure regime. We find that the liquid structure becomes much more densely packed with increasing pressure, with the mean Si–O coordination number increasing nearly linearly with volume from fourfold near ambient pressure to sixfold at the base of the mantle. Fivefold Si–O coordination environments are most abundant at intermediate compression. The properties of Mg and Si coordination environments are nearly identical to those in MgSiO3 liquid, whereas Ca is more highly coordinated with larger mean Ca–O bond length as compared with Mg. The density increases smoothly with increasing pressure over the entire range studied. The Grüneisen parameter increases by a factor of three on twofold compression. The density contrast between diopside composition liquid and the isochemical crystalline assemblage is less than 2% at the core mantle boundary, less than that in the case of MgSiO3. Thermodynamic properties are described in terms of a liquid-state fundamental thermodynamic relation.