Simulations were used to study the pressure-dependence of structure and self-diffusion in NaAlSi3O8 melt from 50 to 19862MPa at 2001.5K. Four-coordinated Al and Si transformed to five and six-coordinated complex with increasing pressure. The content of [5]Al reached a maximum at about 15GPa whereas the maximum in [5]Si did not appear up to 20GPa. There was a marked difference between the self-diffusion coefficients of network-modifying and network-forming ions. In contrast to the self-diffusion coefficient of Na, which decreased monotonically with increasing pressure, the self-diffusion coefficients of Si, Al and O all initially increased, then decreased, with increasing pressure. In the simulation, a peak in the self-diffusion of network-formers occurred at 8 to 10GPa and differed from the pressure at which [5]Al and [5]Si attained their maximum contents. Furthermore, it was found that the self-diffusion of all ions was negatively and linearly correlated with the lifetimes of the bonds between ions and oxygen.

Pressure Dependence of Self-Diffusion in NaAlSi3O8 Melt: a Molecular Dynamics Study. Zhao, Y.J., Zhang, Y.G., Guo, G.J., Refson, K.: Acta Petrologica Sinica, 2004, 20[6], 1461-8