The impact of Mg2+ ions on equilibration of Mg-Ca carbonates in groundwater and brines

Abstract

At temperatures below 50 °C, the log10 (aMg2+/aCa2+) values in groundwater and brines, irrespective of their origin – either carbonaceous or siliceous rocks/sediments – cover the range between −1.5 and +1.0. Calculations of thermodynamic equilibria between the minerals calcite, aragonite, dolomite and huntite suggest a spread of log10 (aMg2+/aCa2+) between minus infinity and +2.3. Log10(aMg2+/aCa2+) in solution of dissolving ordered dolomite at 25 °C fits the thermodynamical equilibrium between disordered dolomite and calcite and nearly corresponds to that of pure calcite with a dolomitic surface layer due to exchange of Ca2+ against Mg2+ in Mg2+-containing solutions. This observation suggests that the solubility of Mg-Ca carbonates is controlled by the composition of their monomolecular surface layers in equilibrium with the ambient aqueous phase. Incongruently dissolving minerals such as dolomite attain equilibrium between individual surface compositions of different carbonates. The bulk composition of these carbonates hardly if ever equilibrates with the ambient solution due to extremely low ion mobility in the lattice. Because the thermodynamical equilibria are based on the composition of bulk minerals, their estimates of equilibria between carbonates, i.e., log10 (aMg2+/aCa2+) in solution, differ significantly from values established by the chemical composition and structure of the surface layer of carbonates.