Computational Modeling of Crystals and Liquids in the System Na2O‐CaO‐MgO‐Al2O3‐SiO2

A transferable interatomic potential model has been developed for use in computer simulation of crystals and liquids in the system Na 2 O-CaO-MgO-Al 2 O 3 -SiO 2 (NCMAS). The potential energy of the system is taken as the sum of pairwise additive Coulomb, van der Waals, and repulsive interactions. The net charges on the Na, Ca, Mg, Al, Si, and O ions are constrained to be 2q(Na) = q(Ca) = q(Mg) = 2/3 q(Al) = 1/2 q(Si) = -q(O) in order to preserve the requirement of transferability between phases with different composition in the NCMAS system. To assess the potential model, the molecular dynamics (MD) method with the potential is applied to a wide structural variety of 29 crystals in the NCMAS system and the five silicate liquids with the compositions enstatite (MgSiO 3 ), wollastonite (CaSiO 3 ), diopside (CaMgSi 2 O 6 ), anorthite (CaAl 2 Si 2 O 8 ), and albite (NaAlSi 3 O 8 ). The MD simulated structures and bulk moduli of the 29 crystals and the MD values of the temperature-pressure-volume equation-of-state parameters for the five silicate liquids are found to compare well with the available experimental data. The MD technique is then used to simulate the pressure dependence of the molar volume of both MgSiO 3 perovskite and liquid at pressures up to 150 GPa and at the temperatures of 2500 and 3500 K, with the results that no volume reversal is found between MgSiO 3 perovskite and liquid at these temperature and pressure conditions. This indicates the perovskite melting curve has no maximum over the pressure range in the lower mantle.

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