Diffusion and Viscosity of Anorthite and Silica Liquids from First Principles Molecular Dynamics Simulations

We have carried out first principles molecular dynamics simulations of CaAl2Si2O8 (anorthite) and SiO2 liquids as a function of pressure and temperature within density functional theory. Along the 3000 K isotherm, the self-diffusion coefficients of Al/Si and O vary anomalously - they initially increase with pressure, reach a maximum (5 to 15 GPa), and then decrease upon further compression. The calculated melt viscosity also shows an anomalous behavior with a local minimum around similar pressure. We find that anorthite liquid is much more mobile and shows weaker anomaly than silica liquid because of its high content of nonbridging oxygens (NBO) and oxygen triclusters (O3). The predicted pressure variations can be associated with structural changes consisting of the pressure-induced maximum in the abundance of pentahedral states (fivefold Al/Si-O coordination) and rapid increase in the O3 abundance in both liquids.