On the interaction between calcite and dolomite: Insights from gas and aqueous geochemistry and mineralogical characterization

Abstract

The current models developed to determine the pore water chemistry of clayrocks consider that Ca, Mg and Fe are controlled by equilibrium reactions with pure carbonates. However, the Ca/Mg ratios measured in pore water are not always in agreement with calculations. Here we investigated the interaction between calcite and dolomite at ambient temperature and at 80 °C. During batch experiments, despite the fast equilibration of the solution composition (within weeks), the gaseous CO2 concentration increased during several months at ambient temperature while it reached a steady state at 80 °C. The observations made at ambient temperature are apparently in conflict (metastable system). However, nanoscale characterizations proved to be useful to evidence a mineralogical evolution during the experiments. Indeed, local equilibria in solution enabled the precipitation of secondary phases despite their undersaturation with respect to the bulk solution and the dissolution of pristine phases. These results suggest that the dissolution/precipitation kinetics are slow and ongoing despite the apparent stability of the system, something usually not considered in the prediction of sedimentary facies over a geological timescale.