EXPERIMENTAL INVESTIGATION OF ALUMINUM-SILICA AQUEOUS COMPLEXING AT 300-DEGREES-C

Equilibrium constants for aqueous Al-Si complexes at 300 degrees C and saturated water vapour pressure (86 bar) were generated experimentall y from the difference between the solubility of boehmite in silica-fre e and silica-bearing solutions. At strongly acidic (pH < 2) and near-n eutral to basic (pH > 4.5) conditions aluminum silica aqueous complexe s form according to Al3+ + H4SiO40 = AlH3SiO42+ + H+ and Al(OH)(4)(-) H4SiO40 = Al(OH)(3)H3SiO4- + H2O for which the logarithm of the equil ibrium constants was found to be 3.23 +/- 0.25 and 2.32 +/- 0.20, resp ectively. No solubility increase following addition of aqueous silica was observed in the slightly acidic range (2.5 < pH < 4), indicating t hat Si complexation with Al(OH)(2+), Al(OH)(2)(+), and Al(OH)(3)(0), i s negligible. The equilibrium constant obtained for AlH3SiO42+ formati on at 300 degrees C was combined with published values for this comple x at 25 to 150 degrees C [Pokrovski, G.S., Schott, J., Hanichoury, J.- C., Sergeyev, A.S., 1996. The stability of aluminum silicate complexes in acidic solutions from 25 to 150 degrees C. Geochim. Cosmochim. Act a 60, 2495-2501.] to generate standard partial molal thermodynamic pro perties at 25 degrees C and 1 bar and HKF (revised Helgeson-Kirkham-Fl owers model; [Tanger, J.C., Helgeson, H.C., 1988. Calculation of the t hermodynamic and transport properties of aqueous species at high press ures and temperatures: revised equations of state for the standard par tial molal properties of ions and electrolytes. Am. J. Sci., 288, 19-9 8; Shock, E.L., Helgeson, H.C., 1988. Calculation of the thermodynamic and transport properties of aqueous species at high pressures and tem peratures: correlations algorithms for ionic species and equation of s tate predictions to 5 kbar and 1000 degrees C. Geochim. Cosmochim. Act a, 52, 2009-2036; Shock, E.L., Oelkers, E.H., Johnson, J.W., Svejensky , D.A., Helgeson, H.C., 1992. Calculation of the thermodynamic propert ies of aqueous species at high pressures and temperatures. J. Chem. Fa raday Trans. 88 (6), 803-826.] equation of state parameters for this c omplex. Calculations indicate that Al-Si complexing can increase Al so lubility of up to 1/2 an order of magnitude at 500 degrees C and 1 kba r. As a consequence, Al-Si complexes should be taken into account to a ccurately model chemical equilibria in hydrothermal and metamorphic fl uids. (C) 1998 Elsevier Science B.V. All rights reserved.