Unnamed (Amorphous Calcium Carbonate)

An amorphous precursor of crystalline calcium carbonates, in particular of vaterite. Abbreviated as ACC. The least stable CaCO₃ polymorph; stabilized by some microorganisms (Hodson et al., 2015). A biomaterial; rapidly dehydrates to vaterite, which is later turned to calcite via dissolution-reprecipitation mechanism (Rodriguez-Blanco et al., 2011). The ACC-to-vaterite transformation is inhibited by PO₄^3- ions (Bots et al., 2012).

May spontaneously form in water by high saturation of essential ions; there it exists as nanospheres (50-400 nm) (Brečević et al., 1989).

Also a precursor of carbonate-rich hydroxylapatite in human bone, stabilized by inorganic polyphosphate; has osteogenic potential (Tolba et al., 2016).

May show short-range structural forms (related to calcite, aragonite, monohydrocalcite, but not necessarily to vaterite) and thus possibly various types exist (Cartwright et al., 2012).

May be precipitated by some molds, in relatively huge quantities.

Compare monohydrocalcite (the ACC may in fact turn into it when in aqueous solution, as shown in a biomimetic mineralization study by Wang et al., 2015).