Peter Möller, Marco De Lucia 

At temperatures below 50°C, the Mg2+/Ca2+ values in groundwater and brines, irrespective of their origin - either carbonaceous or siliceous rocks/sediments - show a large spread. As shown by equilibria of surface layer composition of calcite in solutions containing Mg2+ , log10 (aMg2+ /a Ca2+) vary between minus infinity and +2.3, thereby covering thermodynamical equilibria between the minerals calcite, aragonite, dolomite and huntite. Log10 (aMg2+ /a Ca2+ ) 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 never equilibrates with the ambient solution due to extremely low ion mobility in the lattice. However, the thermodynamical equilibria are usually based on the composition of bulk minerals, therefore their estimates of equilibria between carbonates, i.e., log 10(a Mg2+/a Ca2+) in solution, differ significantly from values established by the chemical composition and structure of the surface layer of carbonates.