Molecular dynamics study of polymorphism in calcium carbonate systems

Calcium carbonate is a material of great significance to construction, pharmaceutical and agricultural industries as well as a vital component for marine life. Of the crystalline forms of calcium carbonate, calcite and aragonite are the most stable. Despite only a small difference in thermodynamics stability, calcite is disproportionally precipitated over aragonite. In biomineralisation however, calcite and aragonite are selectively deposited by organisms to fulfil different purposes. This suggests that control over the selected polymorph during nucleation of calcium carbonate is possible.
The presence of magnesium ions, commonly found in seawater, is known to promote precipitation of aragonite by inhibiting the nucleation of calcite. High temperature favours aragonite and recently, high carbonate content has been shown to nucleate aragonite over calcite at ambient conditions. The mechanisms behind either are not well understood. To investigate the early stages of nucleation when polymorph selection occurs, molecular dynamics simulations have been utilised.
In this work, both kinetic and thermodynamic factors effecting polymorph selection in calcium carbonate have been studied. Configurations of amorphous calcium carbonate with differing ion ratios or water content have been created. We have implemented a cluster analysis technique based on the Manhattan distance metric to identify calcite- or aragonite-like ion clusters within our trajectories. Few polymorph-like clusters were found in all systems but calcite was always the dominant phase.
Values for the interfacial free energy of calcite and aragonite with water have been calculated using a recently developed method. Previous calculations of these values generally contain only the enthalpic contribution, neglecting entropy. The calculation technique utilising an Einstein crystal reference state allows the calculation of both contributions. We found that the entropy contribution varies substantially across the surfaces. It is therefore imperative for entropy to be included in future calculations of interfacial free energies.