In situ nanoscale observations of the dissolution of {101¯4} dolomite cleavage surfaces

Abstract

Knowledge of the kinetics and mechanisms of carbonate dissolution is essential, for instance, to determine the contribution of carbonate-fluid reactions to the global carbon cycle and CO 2 sequestration strategies, as well as to design new methods that mitigate the effects of weathering processes on carbonate stones. There is a significant lack of understanding of the molecular-scale reaction mechanisms of dolomite (CaMg(CO 3) 2), particularly in comparison to other common carbonates such as calcite (CaCO 3). Here we present a systematic in situ Atomic Force Microscopy (AFM) study of dolomite dissolution in the pH range 3-10 aimed at improving our understanding of the nanoscale processes governing dolomite-fluid interactions. The results of this study indicate that the overall dolomite dissolution rate is controlled by the removal of dolomite layers by spreading and coalescence of shallow etch pits, nucleated at point defects and/or in defect-free areas. Our results also suggest that at all pH (and particularly at pH <5) and under conditions of relatively slow solution flow, dolomite dissolves via a dissolution-precipitation reaction with the formation of an Mg-rich surface precipitate. As a consequence, the effluent solution shows Ca/Mg ratios >1. This explains why in the past the dolomite dissolution was considered to be " incongruent". In situ, direct observations of the reacting mineral surfaces are important to unambiguously ascertain the kinetics and mechanism of mineral dissolution. In fact, direct observations allow quantification of the kinetics of the process from the measurement of etch pit spreading rates, which are unaffected by the formation of a secondary precipitate whose existence has been neglected in the past. © 2011 Elsevier Ltd.