The dielectric constant approach to speciation and ion pairing at high temperature and pressure

Abstract

Fluids in the Earth's crust commonly contain, in addition to the solvent, H₂O, high concentrations of other volatiles (such as CO₂, CH₄ or N₂) and/or salts (such as NaCl, KC1 or CaCl₂). The solubility of minerals in these fluids is of prime importance for understanding metasomatic processes. Many theoretical and empirical relations have been proposed to account for various aspects of solubility behaviour and speciation. Here we show that quartz solubility behaviour in solutions where the activity of H₂O is reduced from unity is easily modelled from knowledge of changes in the dielectric constant of the solvent. The approach may be generalized to all neutral aqueous species. We further demonstrate the usefulness of considering a modified form of the Fuoss equation¹ for the calculation of the dielectric-constant dependence of ion pairing at high temperatures and pressures. This approach thus allows the prediction of speciation in complex fluids at high temperature and pressure for reduced activities of H₂O.