2, Empa, Swiss Federal Laboratories for Materials Science and Technology, St Gallen, , Switzerland
Within biomineralisation we frequently see the expression of both calcite and aragonite with the later being common in many marine organisms probably due to its enhanced material properties. These two polymorphs are very similar and differ only marginally in thermodynamic stability to favour calcite . Despite its abundance in the natural world, aragonite is difficult to synthesise in the lab, requiring Mg or other additives. An alternative method is to raise the solution temperature above 70OC  despite the fact that the actual thermodynamic stability is unchanged at this these temperatures. The lack of aragonite and even its general absence from a solution during crystallisation implies that the nucleus must be unstable and quickly dissolve. This is at odds with recent simulation work that demonstrates that the surfaces of aragonite may be more stable in water than their calcite counterparts .
Computational molecular dynamics provides an ideal tool to study this unusual phenomena. We have studied the interfacial energies of a range of calcium carbonate polymorphs in contact with water and amorphous calcium carbonate with varying water concentrations. We use this data to demonstrate the stability of the different calcium carbonate nuclei. Using a statistical analysis of the structure we are able to identify the first stages of nuclei formation and discuss the potential structural features of these. This investigation reveals that kinetics rather than thermodynamics may be dictating the phase selection.
 J.C. Jamieson J. Chem. Phys. 21 (1953) 1385
 T. Ogino et al. Geochim Cosmochim Acta 51 (1987) 275
 A.M. Bano et al Langmuir 30 (2014) 7513