We created a microkinetic model for calcite growth, assuming kink formation via ion pair adsorption on steps is rate-limiting and that adsorption of single ions on the step can block available sites for kink formation. Our model explains how calcite grows as a function of solution composition, saturation index, pH and in the presence of impurity ions. Some implications consistent with our modeling are: * Ion pair attachment to steps is rate limiting at low saturation index, but direct attachment of polynuclear complexes becomes important for growth at higher saturation index. * Dehydration of the attaching unit is rate limiting. * Different water reactivity at the steps is responsible for the anomalous growth behavior of calcite growth as a function of solution composition observed at the acute steps for high pH. * Impurity ions adsorb on steps and block growth. They can also form ion pairs in solution, reducing the number of growth units. Taking these two effects into account allowed us to quantitatively reproduce macroscopic calcite growth inhibition by Mg2+ and SO42-. * When divalent cations adsorb on calcite steps, they are hydrolysed, even at neutral pH. * Bicarbonate is unstable at the calcite interface, regardless of surface site and thus most likely plays little to no role in the growth process. * Ion incorporation does not influence the calcite growth rate in itself, but we have modeled the thermodynamics of cation incorporation into the calcite surface and found that it significantly changes the surface chemistry, particularly the relative surface affinity for organic molecules compared to water. Our results provide potential insight into molecular scale mechanisms of biomineralization. Organisms may use a subtle interplay between organic molecule composition and inorganic ions to control mineral growth. It has also become possible to use computational methods to rationally design improved mineral growth inhibitors.

Modeling the calcite-water interface: Ion adsorption, ion incorporation and mineral growth

253rd National Meeting (2017)

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