A Review of the Structures of Vaterite: The Impossible, the Possible, and the Likely

The importance of the metastable vaterite form of CaCO₃ is increasingly appreciated as an intermediate between amorphous calcium carbonate and more stable crystalline polymorphs in biogenic calcification pathways. Thus, there is a need to be able to identify nanocrystals of vaterite unambiguously, despite uncertainties about its exact crystal structure. The structures that have been proposed for vaterite are all based upon a common average structure with orientationally disordered carbonate groups. However, there is much ambiguity in the literature about the actual superstructure, and hence diffraction behavior of vaterite, some misinterpretation of data, and some claims that cannot be correct. This critical review of the literature aims to resolve the issue. The orientationally disordered (2H) structure is unlikely to occur, and the most plausible structures all share a √3 × √3 superstructure in the xy plane. Most experimental evidence is consistent with occurrence of 2M and 6H polytypes, but ab initio modeling suggests that these do not occur in their highest-symmetry forms, but have space-group symmetries C121 and P3₂21. Stacking faults in the xy plane are very common, and frequently result in packets of 4- or 6-layer periodicity, which can be regarded as nanoscale intergrowths of 4M and 6A₁ polytypes.