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.