The Strucutures and physical properties of ultrahigh-pressure silicate phases at high temperatures and high pressures in the lower mantle

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Cryatals and liquids in the system Na_2O-CaO-MgO-Al_2O_3-SiO_2 (NCMAS) are important constituents of the Earth's mantle. A transferable interatomic potential model has been developed for use in computer simulation of both crystals and liquids in this geophysically important system. 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)=2/3q(Al)=1/2q(Si)=-q(O) in order to preserve the requirement of transferability between phases with different composition in the NCMAS system. The reliability and applicability of the potential model has been tested using the molecular dynamics (MD) simulation, with the results that the simulation has succeeded in reproducing accurately (1) the observed structures and measured bulk moduli of a wide variety of 29 crystals in the NCMAS system, and (2) the observed temperature-pressure-volume equation-of-state parameters of the five silicate liquids with the composition, enstatite (MgSiO_3), wollastonite (CaSiO_3), diopside (CaMgSi_2O_6), anorthite (CaAl_2Si_2O_8) and albite (NaAlSi_3O_8). The MD technique is then successfully used to investigate (1) the equation-of state parameters of forsterite (Mg_2SiO_4) liquid at high temperatures, (2) the possible occurrence of the density inversion between diopside crystal and liquid at high temperatures and high pressures, and (3) the melting temperature of MgSiO_3 perovskite at ultrahigh pressures and high temoperatures prevailing in the lower mantle.