Molecular dynamics investigation of viscosity, chemical diffusivities and partial molar volumes of liquids along the MgO–SiO 2 join as functions of pressure

Abstract

Molecular dynamics simulations are carried out to systematically address the effects of composition and pressure on melts along the MgO–SiO 2 join and elucidate the role of structural modifier content on silicate melt properties. The MgO–SiO 2 system shows non-ideal mixing with a negative excess volume of mixing at low pressures, but the mixing becomes closer to ideal at higher pressures. At atmospheric pressure, the viscosities and diffusivities vary by more than 3 orders of magnitude as the composition is varied along this join, with the low SiO 2 melts characterized by lower viscosities and higher diffusivities; these results are in quantitative agreement with experimental results for the dependence of viscosity and diffusivity on structural modifier content in a wide range of silicate systems. The transport properties of melts in this system converge at higher pressures; at pressures greater than ∼15 GPa the viscosity and diffusivities vary by less than an order of magnitude across the entire range of compositions. The relevance of equations that relate the viscosity and diffusivity is also addressed.