CALCULATION OF DISSOCIATION-CONSTANTS AND THE RELATIVE STABILITIES OFPOLYNUCLEAR CLUSTERS OF 1-1 ELECTROLYTES IN HYDROTHERMAL SOLUTIONS ATSUPERCRITICAL PRESSURES AND TEMPERATURES

Monte Carlo calculations (PITZER and SCHREIBER, 1987), electrostatic t heory (OELKERs and HELGESON, 1990), and revised interpretation of the concentration dependence of conductance data (OELKERs and HELGESON, 19 91) indicate that monatomic ions and neutral ion pairs predominate in single 1 : 1 electrolyte solutions only in the dilute concentration ra nge at supercritical pressures and temperatures where the dielectric c onstant of H2O (epsilon) is less than or similar 15. To assess geologi cally realistic degrees of formation of polynuclear clusters at higher concentrations, the logarithms of overall dissociation constants (log beta) for triple, quadruple, quintuple, and sextuple complexes of mon atomic ions in 16 single 1 : 1 electrolyte solutions at pressures from 500 to 4000 bars and temperatures from 400 to 800-degrees-C were gene rated from the results of Monte Carlo calculations reported by GILLAN (1983) for the restricted primitive model using hard sphere diameters (delta(H)) Obtained by regression of the logarithms of dissociation co nstants (log K) for diatomic neutral ion pairs reported by QUIST and M ARSHALL (1968), FRANTZ and MARSHALL (1984), and OELKERs and HELGESON ( 1988). In addition, the logarithms of dissociation constants for polyn uclear clusters involving more than one type of cation in the system N aCl-KCl-HCI were estimated by assuming a linear dependence Of delta(H) on cluster stoichiometry. The calculated and estimated values of log beta typically decrease monotonically with increasing cluster size at a given pressure and temperature. Speciation calculations generated fr om the log beta and log K values using activity coefficients for both charged and neutral aqueous species (OELKERS and HELGESON, 1990, 199 1 ) indicate that polyatomic clusters of monovalent ions are major solu te species in supercritical aqueous solutions with 1 : 1 solute concen trations > 0.5 m at low pressures and high temperatures where the diel ectric constant of H2O is low, which is in general agreement with conc lusions reached by PITZER and SCHREIBER ( 1987). It follows by analogy that polyatomic clusters involving Si, Al, Ag, Cu, Fe, S, and other e lements in geologic systems may increase substantially the solubilitie s of rock- and ore-forming minerals in concentrated hydrothermal solut ions at supercritical temperatures and pressures.