Molecular dynamics simulations were used to study the pressure dependence of structure, self-diffusion and viscosity in CaAl2Si2O8 melt from 23 to 15183MPa at 1999K. The change with pressure of the relationship between self-diffusion and shear viscosity was also studied and related to the microscopic structure of the melt. There existed a strong correlation between self-diffusion and melt structure. With increasing pressure, the formation of five-fold coordination of Si4+ and Al3+ promoted diffusion while the density increase hindered diffusion. The two contrary effects resulted in the self-diffusion of Si4+, O2- and Al3+ varying slowly with pressure at first. Then as the pressure exceeded 5GPa, the effect of density increase surpassed that for the formation of five-fold coordination, so there was a rapid decrease in self-diffusion. The self-diffusion of Ca2+, a network-modifying ion, decoupled from other ions and decreased monotonicallly with increasing pressure. The self-diffusion of these ions was in the order: DCa > DAl > DO > DSi at pressures below 5GPa. It was found that the jump distances of Si4+' and O2- could be calculated from the amount of non-bridging oxygen in the melt. This made it possible to calculate the self-diffusion of oxygen and silicon from shear viscosity or vice versa, by measuring the non-bridging oxygen percentage of the melt.

Pressure Dependence of Viscosity and Self-Diffusion in CaAl2Si2O8 Melt: a Molecular Dynamics Study. Zhao, Y.J., Zhang, Y.G., Guo, G.J., Refson, K.: Acta Petrologica Sinica, 2004, 20[3], 737-46