Structure, thermodynamic and transport properties of CaAl 2Si 2O 8 liquid. Part I: Molecular dynamics simulations

Abstract

Molecular dynamics simulations for liquid CaAl 2Si 2O 8 have been carried out at 72 state points spanning ranges in density ( ρ: 2398–4327 kg/m 3), temperature ( T: 3490–6100 K) and pressure ( P: 0.84–120 GPa) relevant to geosystems. The atomic scale structure of the melt is determined by analysis of nearest neighbor coordination statistics as a function of T and P. Dramatic structural change occurs as pressure increases especially for 0 < P <20 GPa at all temperatures. Changes in structure are encapsulated by examining the coordination of Si, Al, Ca and O around oxygen and vice versa. Si and Al change from predominantly fourfold at low- P to dominantly sixfold for P >∼ 20 GPa. Pentahedrally coordinated Si and Al in distorted trigonal bipyramids attain abundance maxima corresponding to ∼60% of total (Si, Al)O n at 3–5 GPa and weakly depend on T. The coordination of Ca by oxygen increases from 7 to 10 for 0 < P < 20 GPa and changes slowly for P > 20 GPa at 3500 K. Similar behavior is seen at 6000 K except that the interval of rapid changes occurs at higher pressure. Oxygen with only one nearest Si or Al neighbor (i.e., non-bridging oxygen, NBO) decreases whereas oxygen with two or three nearest neighbors of Si, Al or Ca increases as pressure increases. Changes in melt structure are reflected in the variation of thermodynamic and transport properties of the liquid. Values of the self-diffusivities of Ca, Al, Si and O are fit to a modified Arrhenian expression and compare well to limited laboratory data. Self-diffusivities are best fit using ‘low P’ and ‘high- P’ expressions, identical in form but with different parameters, with activation energies and activation volumes in the range 150–200 kJ/mol and +5 to −1 cm 3/mol, respectively. Green–Kubo calculations for liquid shear viscosity are presented and compare well with limited laboratory results. Application of the Eyring model to determine the characteristic size and number of atoms in the activated cluster based on independently computed D and η suggests that the activated cluster decreases from ∼8 to ∼3 atoms from low to high pressure while its characteristic size shrinks from ∼14 Å to ∼3 Å providing insight into dynamics of atom mobility and possible cooperative behavior. The equation of state and variation of internal energy with T and V are used in Part II ( Ghiorso et al., 2009) to derive a comprehensive thermodynamic description of liquid CaAl 2Si 2O 8. This is best accomplished by allowing for EOS expressions broken into high and low pressure intervals consistent with coordination statistics and MD-derived transport properties.