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Amorphous Calcium Carbonate is the amorphous and least stable polymorph of calcium carbonate Amorphous Calcium Carbonate (or ACC) is so unstable under normal conditions that aside from several specialized organisms it is not found naturally. ACC have been known to science for over 100 years ago, when a non diffraction formula of calcium carbonate was discovered by Sturcke Herman.

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 ikite. 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. That is why ACC is not found naturally.

ACC in biology
Several organisms have developed methods to stabilize ACC by using specialized proteins for various purposes, for instance, ACC is being used by fresh water crustacean to store calcium during their molt cycle, in specialized storage organs called Gastroliths.

Synthetic ACC
Many methods have been devised for synthetically producing ACC since it's discovery at 1989, however, only few syntheses successfully stabilized ACC for more than several weeks. Huang et al., managed to stabilize ACC using polyacrylic acid for several months, while Loste et al., showed that magnesium ions con increase ACC stability as well. But only recent discovery that phosphorylated amino acids can produce long term stable ACC have opened the door for production commercialization.

ACC bioavailability
Calcium carbonate is being used as a calcium supplement worldwide, however, it is known that it's bioavailability is very low, only ~20-30%. Recent preclinical bioavailiability analyses performed by meiron et at., suggests that ACC is ~40% more bioavailable than crystalline calcium carbonate. Amorphical Ltd. has a written patent over the ACC formulation and is now in the middle of clinical trials in order to test the material's bioavailability in humans.