Pressure dependence of self-diffusion in the NaAlO ₂ -SiO ₂ system; compositional effects and mechanisms

University of California, Department of Geological Sciences, Santa Barbara, CA, United States

Molecular Dynamics (MD) simulations were used to study the relationship between structure and transport properties in five liquids (NaAlO ₂ , Na ₂ Al ₂ SiO ₆ , NaAlSiO ₄ , NaAlSi ₂ O ₆ , and NaAlSi ₃ O ₈ ) in the system NaAlO ₂ -NaAlSi ₃ O ₈ at temperatures ranging from 4000 to 6000 K and pressures from 0 to 55 GPa. Seventy simulations were carried out in the microcanonical ensemble using a simple pair-wise additive potential with Coulombic interaction and Born-Mayer repulsion. Detailed study of the coordination of O and network forming cations provides a master set of coordination environment or speciation curves. These master curves were applicable to all compositions and temperatures and were most explicit when compression (V _r /V; where V _r is the molar volume at a reference pressure) was used as the independent variable. The universality implied that coordination environments for network atoms O, Al, and Si depend weakly upon Si/Al, T/O, or Na/T atomic ratios for the compositions studied. Self-diffusion coefficients, computed from analysis of mean-square displacements, were used to evaluate the activation enthalpy (H _a = E _a +PV _a ) for self-diffusion for each species. The activation energy (E _a ) for Na was independent of composition, whereas E _a for O, Si, and Al increased as Si/Al increased. Activation volume (V _a ) at pressure <15 GPa was positive for Na and negative for O, Si, and Al and decreased with increasing Si/Al for all species. An extension of the Adam-Gibbs-DiMarzio configurational entropy theory taking explicit account of (super [2]) O and (super [3]) O mixing explained both the variation of the pressure-derivative of the shear viscosity as a function of composition and the disappearance of "anomalous" viscosity behavior at P> approximately 25 GPa for all compositions in the system NaAlO ₂ -NaAlSi ₃ O ₈ .

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