Abstract
We have used first-principles molecular dynamics simulation to examine the high-pressure behavior of an aluminosilicate melt with albite stoichiometry (NaAlSi3O8). We have explored the equation of state and transport properties up to a pressure of 25 GPa and over a range of temperatures (2500-4000 K). Our results show that upon compression of up to 5 GPa, the initial densification in an albite melt occurs by the reduction of the T-O-T bond angle; however, the Si-O coordination remains virtually unchanged. Upon compression beyond 5 GPa, the densification occurs via changes in the T-O coordination. We also find that at lower isotherms, i.e., 2500-3000 K, the viscosity decreases upon compression and there is a concomitant enhancement of the diffusivity of the network-forming tetrahedral cations (silicon/aluminum) and oxygen anions. However, there is a reversal in the trend of viscosity and diffusivity upon further compression. For all of the temperatures explored in this study, at higher pressures, i.e., >10 GPa, the viscosity increases with increasing pressure, whereas the diffusivity decreases with increasing pressures. This behavior of the melt transport property at high pressures is expected, contrary to the observed behavior of melt transport at low pressures and low temperatures. The pressure-dependent anomalous transport properties are very likely related to the formation of the 5-fold coordinated aluminum ions that have a shorter lifetime compared to those of the more stable 4-fold and 6-fold coordinated units.