Computer simulation of the structural and physical properties of silicate crystals and melts at high temperatures and high pressures

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Crystals and melts in the Na_2O-CaO-MgO-Al_2O_3-SiO_2 (NCMAS) system are important components of the earth's crust and mantle. Both molecular dynamics (MD) and lattice dynamics (LD) methods are used to calculate the properties of crystals, while the MD technique is applied to simulate melts. The interionic potential is taken to be the sum of pairwise additive Coulomb, van der Waals attraction, and repulsive interactions. In addition, in order to take account of many-body forces in crystals and melts, the breathing shell model is developed for simulation, in which the repulsive radii of 0 ions are allowed to deform isotropically under the effects of other ions in the system concerned. The net charges of the ions are constrained to be 2q(Na)=q(Ca)=q(Mg)=2/3q(A1)=1/2q(Si)=-q(0) to apply the potential to both crystals and melts with any composition in the NCMAS system. Required energy parameters, including the oxygen charge, repulsive radii, van der Waals coefficients of ions, and the oxygen breathing parameters, were derived empirically to reproduce the observed temperature-pressure-volume (T-P-V) equations of state of a wide structural. variety of crystals in the NCMAS system, as well as the measured volumes of enstatite, wollastonite, diopside, albite, and anorthite melts at high temperatures. In spite of a wide variety of crystals and melts studied, the LD and MD simulations are quite successful in reproducing accurately the measured structural and physical properties of both crystals and melts in the NCMAS system.