Surface-water interactions in the dolomite problem

Atomistic computer modelling methods are used to investigate the effect of the strong magnesium-water interaction on the crystal growth of dolomite MgCa(CO3)(2). We find that the main dolomite cleavage plane under aqueous conditions is less stable than its counterparts in either pure calcite CaCO3 or magnesite MgCO3. Molecular dynamics simulations of the incorporation of CaCO3 and MgCO3 at calcite growth steps show that, on thermodynamic grounds, magnesium is easily incorporated into a growing calcite crystal, but will then inhibit incorporation of further calcium, which is required for dolomite formation. In addition, segregation of surface magnesium ions into the bulk calcite crystal is energetically unfavourable when the surface is hydrated, but will occur if the water is removed. However, incorporation of magnesium into the bulk crystal of ikaite, a calcium carbonate hexahydrate and precursor of calcite, is energetically feasible and highly site specific, with an optimum uptake of 50% magnesium. Hence, we suggest that hydrous Ca-Mg carbonate phases may be successful as precursors in the laboratory preparation of dolomite.