Thermodynamics of Magnesium Binding at Calcite Kink Sites and Implications for Growth inhibition

The standard dissolution free energies of magnesium binding at each of the eight distinct calcite cation kinks were computed from classical molecular dynamics simulations using alchemical methods to transform magnesium into calcium at each site. The preferred calcite kink site for magnesium binding is found to correspond to one of the two symmetry inequivalent cation sites at the acute step along the kink direction that forms an obtuse angle between the end of the row and terrace (AO_a according to our notation system). Incorporation of magnesium at this site is found to inhibit growth along the step edge. However, this effect occurs not due to destabilization of the addition of the next carbonate immediately adjacent to Mg but instead by altering the thermodynamics of the subsequent cation. The binding free energy of this calcium site is reduced by more than 10 kJ/mol once magnesium has been incorporated in the step edge. Our results support and provide a mechanistic explanation for the experimental observation that acute steps are more affected by the presence of magnesium in the growth solution relative to obtuse steps.