The Multiple Structures of Vaterite

For many years the nature of the disordered structure of vaterite, a calcium carbonate polymorph that plays a significant role in biomineralisation, has been debated. In the past year, two independent studies (Angew. Chem., Int. Ed., 51, 7041; CrystEngComm, 14, 44) have proposed new, yet different models on the basis of electron diffraction experiments and ab initio calculations, respectively. Here, it is shown that there are at least three effectively isoenergetic models that are equally likely to describe the vaterite structure at room temperature. These distinct models, each consisting of multiple structures, can explain the disorder of vaterite in terms of different orientations of the carbonate anions, multiple stacking sequences of the carbonate layers, and possible chiral forms. Hence, vaterite is not a single “disordered” structure but should instead be considered as a combination of different forms, each of which can exhibit rapid interchange between multiple structures that exhibit similar average properties.