Ion dynamics computer simulations have been applied successfully to silicate and borate glasses as well as MgSiO3 and Mg2SiO4. Many liquid silicates, like silica itself, have component diffusion coefficients that show anomalous P dependences. Diffusivity maxima occur at P = 200-300 kbar and are correlated with a prevalence of five-fold coordination of silicon ions. The authors speculate that the interior viscosity of a molten silica-rich planetary body would be much smaller than the viscosity at the surface, since it seems that (at least to P = 150 kbar), increasing P acts in the same way as increasing T to decrease viscosity. For silica-poor liquids (such as the Earth's mantle) P and T act in opposite directions at all P, giving less dependence of viscosity with depth in a primitive, molten planetary body. -E.O.
ASJC Scopus subject areas