Calcium carbonate (CaCO3) is a prominent mineral in nature that exists in two primary polymorphs, calcite and aragonite. Both can be found in biominerals as well as in marine and geological settings, even though aragonite has a lower thermodynamic stability and higher solubility product when compared to calcite. The balance between aragonite and calcite, after exposure to aqueous environments, is often investigated in terms of factors that alter dissolution-recrystallization kinetics and/or thermodynamic stability, such as Mg2+ ions, temperature, pressure, and dissolved CO2. Even so, the mechanisms that govern dissolution-recrystallization rates have not been fully resolved. In this work, we use a simple model system (aqueous suspensions of mixed calcite and aragonite powders) that allows us to follow quantitatively the kinetics of the aragonite-to-calcite conversion, which we describe as a thermodynamic pump. Using 13C solid-state NMR and infrared spectroscopies, we follow how the relative proportions of the two polymorphs change during immersion, and we derive a lower bound for the rates of aragonite dissolution and of calcite recrystallization. This model system further serves as a platform to investigate how phosphate-bearing additives dissolved in the aqueous suspension affect the aragonite-to-calcite pump: there is surface incorporation of these additives, along with a significantly slowed pump rate. Our study lays groundwork for developing strategies to understand and regulate the aragonite-calcite balance in non-equilibrium environments.

Aragonite-to-calcite thermodynamic pump: a dissolution and recrystallization study

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