SOLUBILITY OF SILICA POLYMORPHS IN ELECTROLYTE-SOLUTIONS .1. ACTIVITY-COEFFICIENT OF AQUEOUS SILICA FROM 25-DEGREES TO 250-DEGREES-C, PITZERS PARAMETERIZATION

Within the framework of Fitter's specific interaction model, interacti on parameters for aqueous silica in concentrated electrolyte solutions have been derived from Marshall and co-authors amorphous silica solub ility measurements. The values, at 25 degrees C, of the Pitzer interac tion parameter (lambda(SiO2(aq)-i)) determined in this study are the f ollowing: 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 th ese 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, lamb da(SiO2(aq)-i) available. From this parameterisation, the calculated a ctivity 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 a nd 1.2 at 250 degrees C. Hence, in practice, it is necessary to take i nto account the activity coefficient of aqueous silica (gamma(SiO2(aq) not equal 1)) in hydrothermal solutions and basinal brines where the i onic strength exceeds 1. A comparison of measured [Marshall, W.L., Che n, C.-T.A., 1982. Amorphous silica solubilities, V. Prediction of solu bility behaviour in aqueous mixed electrolyte solutions to 300 degrees C. Geochim. Cosmochim. Acta 46, 289-291.] and computed amorphous sili ca solubility, using this parameterisation, shows a good agreement. Be cause the effect of individual ions on silicate and silica polymorph s olubilities are additive, the present study has permitted to derive Fi tter 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.