Vaterite

A rare CaCO₃ modification that is metastable below approx. 400°C. May be stabilised by sulphate (Fernández-Díaz et al., 2010).

Vaterite is actually composed of at least two different crystallographic structures that coexist within a pseudo–single crystal. The major structure (actually substructure) exhibits hexagonal symmetry; the minor structure, existing as nanodomains within the major matrix, is still unknown (Kabalah-Amitai et al., 2013). However, as suggested by Christy (2017) in his review, the most plausible polytypes to describe the structure, namely the 2M and 6H ones, actually "do not occur in their highest-symmetry forms", but are described by the space groups C121 and P3₂21, respectively.

The structure is disordered in terms of (1) different orientations of the carbonate groups, (2) different stacking sequences of the carbonate-comprising layers, and (3) possible chiral forms (Demichelis et al., 2013). The OD character leads to polytypism; the OD layer comprises Ca coordination polyhedra and halves of the carbonate groups, and the group symmetry of the layer is C2/m; the known stacking sequences include: P6₁22, P6₅22, C2/c, C2/c2/m2₁/m, and P3₁2 or P3₂2. The type of the OD layering is similar to those observed in bastnäsite-synchysite polysomatic series (Makovicky, 2016).

In 2019, Steciuk et al. identified the modulation and polycrystalline properties of vaterite for the first time using electron diffraction experiments. Based on Steciuk’s ideas, San et al. proposed a “polymorph coexistence” model through molecular dynamics simulations driven by a deep neural network potential function (DNN), achieving an error of only 0.3% compared to experiments and successfully explaining the phase transition behavior of vaterite. It is proposed that the pseudohexagonal symmetry of vaterite is actually caused by the slip stackings with different periodicities along a specific direction, and the slight difference in the orientation of carbonate groups leads to the phenomenon of polymorphism. This result was directly observed by Okumura’s team using scanning transmission electron microscopy (STEM) combined with annular dark-field imaging (ADF), which verifies the accuracy of the San model and may mark the end of the phase of controversy regarding the vaterite structure.

Not uncommon as a biomineral (other sources: rarely used in hard tissue). Exists in fish otoliths. As such, it is formed from the precursor - Unnamed (Amorphous Calcium Carbonate) - via dehydration (Bots et al., 2012), the transformation process being inhibited in the presence of PO₄^3- ions (Sugiura et al., 2016). Further dissolution-reprecipitation turns vaterite into calcite. Vaterite/calcite precipitates are known in the Cladosporium fungus, too (Ye et al., 2023).

May comprise a series with bästnasite group, known as bästnasite-vaterite homologous series.