Bromate incorporation in calcite and aragonite

The presence of bromine as a trace-element in calcium carbonate speleothems constitutes a useful proxy of past volcanic activity, thus helping to provide input parameters for climate model simulations and risk assessment. However, the chemical nature of bromine-containing impurities in the calcium carbonate phases forming speleothems is not understood, which limits interpretation of experimental measurements on speleothems. We present here a computer simulation study, based on quantum mechanical calculations, of the incorporation of bromine as BrO3 oxyanions in CaCO3 polymorphs calcite and aragonite. We discuss how the relative distributions of bromate oxyanions and charge-compensating alkali-metal cations are determined by the interplay between an impurity binding effect (which is stronger for aragonite than for calcite, and changes in the order Li < Na < K) and a configurational entropic effect that tends to disassociate the impurities. For concentrations above parts-per-million, bromate impurities can be expected to be paired, in nearest-neighbour configurations, with the compensating cations. Bromate substitution, compensated by sodium or potassium cations, is predicted to be metastable with respect to phase separation of the impurities as solid NaBrO3 or KBrO3 phases, respectively, but the solubility limits of BrO3 in calcite and aragonite are still higher than those calculated for tetrahedral oxyanions (SO4)2 and (MoO4)2 that are used as alternative volcanic records in speleothems.