Calculation of the thermodynamic properties of aqueous silica and the solubility of quartz and its polymorphs at high pressures and temperatures

Abstract

Regression of experimental solubility data with theoretical equations providing for the intrinsic properties of SiO/sub 2/ and the calorimetric consequences of electrostriction collapse and solvation affords close approximation of the thermodynamic behavior of aqueous silica to 5 kb and 600/sup 0/C. The standard molal volume of aqueous silica (V/sup 0/) decreases as a sigmoid function of temperature at constant pressure but increases as pressure increases at constant temperature. In contrast, the standard molal heat capacity (C/sup 0//sub p/) maximizes with increasing pressure at intermediate temperatures. C/sup 0//sub p/, which is negative at low temperatures, exhibits a maximum also as a function of temperature at constant pressure. Owing to the behavior of the partial derivatives of the dielectric constant of H/sub 2/O with respect to pressure and temperature, both V/sup 0/ and C/sup 0//sub p/ approach -infinity at the critical point of H/sub 2/O, but at low temperatures where V/sup 0/ is positive, C/sup 0//sub p/ is negative and decreases dramatically with decreasing temperature at constant pressure. The broad extrema exhibited by curves representing the solubility of ..cap alpha..-quartz as a function of temperature at pressures less than or equal to a kilobar are a consequence of these variations in V/sup 0/ and C/sup 0//sub p/. At high temperatures and low pressures, changes in the thermodynamic properties of aqueous silica are controlled primarily by the electrostatic properties of H/sub 2/O, but at low temperatures the local solvent structure dominates the temperature and pressure dependence of V/sup 0/ and C/sup 0//sub p/. The geologic consequences of the thermodynamic behavior of aqueous silica can be predicted by combining the theoretical equations and regression coefficients with thermodynamic data for minerals and other aqueous species, which permits calculation of the distribution of SiO/sub 2/ among silicates and H/sub 2/O-rich hydrothermal solutions at high pressures and temperatures.