Amorphous calcium carbonate

ACC is the sixth and least stable polymorph of calcium carbonate. The remaining five polymorphs (in decreasing stability) are: calcite, aragonite, vaterite, monohydrocalcite and ikaite. When mixing two supersaturated solutions of calcium chloride and sodium carbonate (or sodium bicarbonates) these polymorphs will precipitate from solution following Ostwald's step rule, which states that the least stable polymorph will precipitate first. But while ACC is the first product to precipitate, it rapidly transforms into one of the more stable polymorphs within seconds.^[7][8] When in pure CaCO₃, ACC transforms within seconds into one of the crystalline calcium carbonate polymorphs. This transformation from amorphous to crystalline is proposed to be a dissolution-reprecipitation mechanism.^[3] Despite ACC's highly unstable nature, some organisms are able to produce stable ACC. For example, the American Lobster Homarus americanus, maintains stable ACC throughout its yearly molt cycle.^[2] Studies of biogenic ACC have also shown that these stable forms of ACC are hydrated whereas the transient forms are not. From observations of spicule growth in sea urchins, it seems that ACC is deposited at the location of new mineral growth where it then dehydrates and transforms into calcite.^[2] Recent solid‑state NMR studies have shown that poly‑aspartate can integrate into amorphous calcium carbonate (ACC), adopting an α‑helix conformation and significantly stabilizing the phase against crystallization; additionally, water molecules within ACC were observed to reorient via 180° flips on a millisecond timescale.^[9]

Several organisms have developed methods to stabilize ACC by using specialized proteins for various purposes. The function of ACC in these species is inferred to be for the storage/transport of materials for biomineralization or enhancement of physical properties, but the validity of such inferences has yet to be determined. Earthworms, some bivalves species, and some gastropods species are known to produce very stable ACC.^[2][10] ACC is widely used by crustaceans to stiffen the exoskeleton as well as to store calcium in gastroliths during the molt cycle. Here, the benefit of utilizing ACC may not be for physical strength, but for its periodic need of the exoskeleton to be dissolved for molting.^[2] Sea urchins and their larvae utilize the transient form of ACC when forming spicules. The new material, a hydrated form of ACC, for the spicule is transported and deposited at the outer edges of the spicule. Then the deposited material, ACC·H₂O, rapidly dehydrates to ACC. Following the dehydration, within 24 hours, all of the ACC will have transformed into calcite.^[11]

Many methods,^[10][7][12] have been devised for synthetically producing ACC since its discovery at 1989, however, only few syntheses successfully stabilized ACC for more than several weeks. Contrary to earlier assumptions of rapid crystallization, recent observations confirm that poly-aspartate stabilized ACC can remain amorphous for years under ambient conditions, demonstrating remarkable long-term stability.^[13] The best effective method to stabilize ACC lifetime is by forming it in the presence of magnesium and/or phosphorus.^[14][15] Also, ACC crystallisation pathways have been observed to depend on its Mg/Ca ratio, transforming to aragonite,^[16] Mg-calcite,^[17] monohydrocalcite^[18] or dolomite^[19] with increasing Mg content. Huang et al. managed to stabilize ACC using polyacrylic acid for several months,^[20] while Loste et al. showed that magnesium ions can increase ACC stability as well.^[21] But only the discovery that aspartic acid, glycine,^[22] citrate,^[23] and phosphorylated amino acids can produce long term stable ACC^[24] have opened the door for production commercialization. Synthetic ACC have been found to be conductive.^[25] Chemical environment of carbon and proton in synthetic ACC are found to be similar to that its crystalline polymorph which is monohydrocalcite.^[26]