Thermodynamic assessment of hydrothermal alkali feldspar-mica-aluminosilicate equilibria

The thermodynamic properties of minerals retrieved from consideration of solid-solid and dehydration equilibria with calorimetric reference values, and those of aqueous species derived from studies of electrolytes, are not consistent with experimentally measured high-temperature solubilities in the systems K₂O- and Na₂O-Al₂O₃-SiO₂-H₂O-HCl (e.g., K-fs — Ms — Qtz — K⁺ — H⁺). This introduces major inaccuracies into the computation of ionic activity ratios and the acidities of diagenetic, metamorphic, and magmatic hydrothermal fluids buffered by alkali silicate-bearing assemblages. We report a thermodynamic analysis of revised solubility equilibria in these systems that integrates the thermodynamic properties of minerals obtained from phase equilibria studies (Berman, 1988) with the properties of aqueous species calculated from a calibrated equation of state (Shock and Helgeson, 1988). This was achieved in two separate steps.

First, new values of the free energies and enthalpies of formation at 25°C and 1 bar for the alkali silicates muscovite and albite were retrieved from the experimental solubility equilibria at 300°C and P_sat. Because the latter have stoichiometric reaction coefficients different from those for solid-solid and dehydration equilibria, our procedure preserves exactly the relative thermodynamic properties of the alkali-bearing silicates (Berman, 1988). Only simple arithmetic adjustments of −1,600 and −1,626 (±500">±500) cal/mol to all the K- and Na-bearing silicates, respectively, in Berman (1988) are required. In all cases, the revised values are within ±0.2%">±0.2% of calorimetric values. Similar adjustments were derived for the properties of minerals from Helgeson et al. (1978).

Second, new values of the dissociation constant of HCl were retrieved from the solubility equilibria at temperatures and pressures from 300–600°C and 0.5–2.0 kbars using a simple model for aqueous speciation. The results agree well with the conductance-derived dissociation constants from Franck (1956a,b) for temperatures from 300–550°C. Compared to the conductance-derived results of Frantz and Marshall (1984), our dissociation constants agree well at the highest densities, but are greater at lower densities. At the lowest density, at 600°C and 1 kbar, the discrepancy of 0.9 log units is within the overall uncertainties associated with our experimental results and those associated with deriving dissociation constants from conductance measurements in highly associated solutions (Oelkers and Helgeson, 1988). Finally, we also report an equation of state fit to the standard thermodynamic properties of the aqueous HCl molecule that is consistent with a wide array of independently determined dissociation constants of HCl and permits interpolation and extrapolation of the dissociation constant of HCl to 1000°C and 5.0 kbars.