The prediction of mineral solubilities in natural waters: A chemical equilibrium model for the NaKCaMgClSO 4H 2O system at temperatures below 25°C

Abstract

A low temperature thermochemical model for the system NaKCaMgClSO 4H 2O is presented. Aqueous species and standard chemical potentials of solid-solution reactions are modeled from published data for binary and ternary solutions. The temperature range below 25°C (to near −60°C) is emphasized, although the model parameters are fitted to merge smoothly with those of higher temperature models at temperatures between 25 and 100°C. Binary and ternary specific ion interaction terms vary independently with temperature and are modeled using freezing point depression and mineral solubility measurements. The standard chemical potential of the ice-water reaction is fitted independent of the model (from vapor pressure and free energy data). Remaining standard chemical potentials of solidsolution reactions are fitted along with the specific ion interaction terms. Model predictions are tested against published data for minerals formed and brine compositions obtained by chilling seawater to the eutectic (about −54°C). The model predicts the sequence of solid phases observed to precipitate from chilled seawater (mice-mirabilite-hydrohalite-sylvite-MgCl 2 · 12H 2Oantarcticite). For all but mirabilite model temperatures are within the uncertainty of the measured temperature. The compositions of brines predicted by the model also closely follow the observed compositions. The model allows accurate predictions of the freezing points of simple and complex solutions in the system. Low temperature phase equilibria and mineral solubilities may also be predicted. The model may be used to determine the composition of brines in fluid inclusions in the multicomponent system based on low temperature phase equilibria.