Experimental investigation of aluminum-silica aqueous complexing at 300°C

Equilibrium constants for aqueous Al–Si complexes at 300°C and saturated water vapour pressure (86 bar) were generated experimentally from the difference between the solubility of boehmite in silica-free and silica-bearing solutions. At strongly acidic (pH<2) and near-neutral to basic (pH>4.5) conditions aluminum silica aqueous complexes form according to Al3++H4SiO40=AlH3SiO42++H+ and Al(OH)4−+H4SiO40=Al(OH)3H3SiO4−+H2O for which the logarithm of the equilibrium constants was found to be 3.23±0.25 and 2.32±0.20, respectively. No solubility increase following addition of aqueous silica was observed in the slightly acidic range (2.5<pH<4), indicating that Si complexation with Al(OH)2+, Al(OH)2+, and Al(OH)30, is negligible. The equilibrium constant obtained for AlH3SiO42+ formation at 300°C was combined with published values for this complex at 25 to 150°C [Pokrovski, G.S., Schott, J., Harrichoury, J.-C., Sergeyev, A.S., 1996. The stability of aluminum silicate complexes in acidic solutions from 25 to 150°C. Geochim. Cosmochim. Acta 60, 2495–2501.] to generate standard partial molal thermodynamic properties at 25°C and 1 bar and HKF (revised Helgeson–Kirkham–Flowers model; [Tanger, J.C., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: revised equations of state for the standard partial molal properties of ions and electrolytes. Am. J. Sci., 288, 19–98; Shock, E.L., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and temperatures: correlations algorithms for ionic species and equation of state predictions to 5 kbar and 1000°C. Geochim. Cosmochim. Acta, 52, 2009–2036; Shock, E.L., Oelkers, E.H., Johnson, J.W., Sverjensky, D.A., Helgeson, H.C., 1992. Calculation of the thermodynamic properties of aqueous species at high pressures and temperatures. J. Chem. Faraday Trans. 88 (6), 803–826.] equation of state parameters for this complex. Calculations indicate that Al–Si complexing can increase Al solubility of up to 1/2 an order of magnitude at 500°C and 1 kbar. As a consequence, Al–Si complexes should be taken into account to accurately model chemical equilibria in hydrothermal and metamorphic fluids.