Within the framework of Fitter's specific interaction model, interaction parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solubility measurements. The values, at 25 degrees C, of the Pitzer interaction parameter (lambda(SiO2(aq)-i)) determined in this study are the following: 0.092 (i = Na+), 0.032 (K+), 0.165 (Li+), 0.292 (Ca2+, Mg2+), -0.139 (SO42-), and -0.009 (NO3-). A set of polynomial equations has been derived which can be used to calculate lambda(SiO2(aq)-i) for these ions at any temperature up to 250 degrees C, A linear relationship between the aqueous silica-ion interaction parameters (lambda(SiO2(aq)-i)) and the surface electrostatic field (Z(i)/r(e,i)) of ions was if no measurements are be used to estimate, in first approximation, lambda(SiO2(aq)-i) available. From this parameterisation, the calculated activity coefficient of aqueous silica is 2.52 at 25 degrees C and 1.45 at 250 degrees C in 5 m NaCl solution. At lower concentrations, e.g. 2 m NaCl, the activity coefficient of silica is 1.45 at 25 degrees C and 1.2 at 250 degrees C. Hence, in practice, it is necessary to take into account the activity coefficient of aqueous silica (gamma(SiO2(aq)not equal 1)) in hydrothermal solutions and basinal brines where the ionic strength exceeds 1. A comparison of measured [Marshall, W.L., Chen, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solubility behaviour in aqueous mixed electrolyte solutions to 300 degrees C. Geochim. Cosmochim. Acta 46, 289-291.] and computed amorphous silica solubility, using this parameterisation, shows a good agreement. Because the effect of individual ions on silicate and silica polymorph solubilities are additive, the present study has permitted to derive Fitter interaction parameters that allow a precise computation of gamma(SiO2(aq)) in the Na-K-Ca-Mg-Cl-SO4-HCO3-SiO2-H2O system, over a large range of salt concentrations and up to temperatures of 250 degrees C. (C) 1997 Elsevier Science B.V.
