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Amorphous calcium carbonate (ACC) is the amorphous and least stable polymorph of calcium carbonate. ACC is monohydrate and is so unstable under normal conditions that aside from several specialized organisms it is not found naturally. ACC has been known to science for over 100 years when a non diffraction formula of calcium carbonate was discovered by Sturcke Herman.
Contents
Stability
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 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.
In biology
Several organisms have developed methods to stabilize ACC by using specialized proteins for various purposes, for instance, ACC is being used by freshwater crustaceans to store calcium during their molt cycle, in specialized storage organs called gastroliths. Also, earthworms are known to produce very stable ACC.
Synthetic ACC
Many methods, have been devised for synthetically producing ACC since its discovery at 1989, however, only few syntheses successfully stabilized ACC for more than several weeks. The best effective method to stabilize ACC lifetime is by forming it in the presence of magnesium. Also, ACC crystallisation pathways have been observed to depend on its Mg/Ca ratio, transforming to Mg-calcite, monohydrocalcite or dolomite with increasing Mg content. Huang et al. managed to stabilize ACC using polyacrylic acid for several months, while Loste et al. showed that magnesium ions can increase ACC stability as well. But only the discovery that aspartic acid, glycine, citrate phosphorylated amino acids can produce long term stable ACC have opened the door for production commercialization.
Bioavailability
Since 2013 a company named Amorphical Ltd. sells an ACC dietary supplement.
Calcium carbonate is being used as a calcium supplement worldwide, however, it is known that its bioavailability is very low, only around 20–30%. ACC is roughly 40% more bioavailable than crystalline calcium carbonate.