On the mechanism of calcium carbonate polymorph selection via confinement

Organisms deposit various biominerals in the course of their biomineralisation. The most abundant of these is calcium carbonate, which manifests itself in several polymorphs. While organisms possess the ability to control the specific polymorph deposited, the exact mechanism by which polymorph selection takes place is not yet fully understood. Because biominerals often grow within confined spaces, one of the suggested possibilities was that polymorph selection might be an outcome of confinement. Confining conditions have indeed been extensively shown to have a strong impact on the nucleation and crystal growth of calcium carbonate and, in particular, on its polymorph selection. However, despite numerous studies on the crystal growth of calcium carbonate in confined spaces, the mechanism of polymorph selection under confinement has not been elucidated. Herein, we discuss previously reported results and suggest a mechanistic explanation of the observed selective formation of calcite or aragonite or vaterite. We consider the possible effects of charged confining inner surfaces and of the sizes of the confining pores, and discuss whether the predominantly precipitating phase is amorphous calcium carbonate. We also discuss two possible scenarios of crystallization from amorphous calcium carbonate under conditions of confinement: via solid-state transformation or via a mechanism of dissolution-reprecipitation.