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. 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. ==In biology==

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. ==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. 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. The best effective method to stabilize ACC lifetime is by forming it in the presence of magnesium and/or phosphorus. Also, ACC crystallisation pathways have been observed to depend on its Mg/Ca ratio, transforming to aragonite, 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, and phosphorylated amino acids can produce long term stable ACC have opened the door for production commercialization. Synthetic ACC have been found to be conductive. Chemical environment of carbon and proton in synthetic ACC are found to be similar to that its crystalline polymorph which is monohydrocalcite. ==Highly porous ACC==

Highly porous ACC has been synthesized using a surfactant-free method. In this method CaO is dispersed in methanol under a pressure of carbon dioxide in a sealed reaction vessel. ACC with surface area over 350 m2/g was synthesized using this method. Highly porous ACC appeared to be made up aggregated nanoparticles with dimensions less than 10 nm in size. Highly porous ACC was also found to be stable in ambient conditions for up to 3 weeks with most of its porosity retained. ==Applications and uses==

Bioavailability: Since 2013 a company named Amorphical Ltd. sells an ACC dietary supplement. Calcium carbonate is 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. Drug delivery: Due to the ability to tune the size and morphology of the amorphous calcium carbonate particles (as well as other calcium carbonate particles), they have huge applications in drug delivery systems. Highly porous ACC showed the ability to stabilize poorly soluble drug molecules in its extensive pore system and could also enhance the drug release rates of these drugs. Environmental remediation: Improving environmental remediation efforts through gaining insight into the roles of earth materials in biogeochemical cycling of nutrients and metals through better understandings of the properties of environmental mineral phases involved in elemental uptake and release Material science: Improving nanomaterials design and synthesis such as improving photovoltaic, photocatalytic, and thermoelectric materials for energy applications or improving biomedical cementations. Also improving framework material development for CO2 capture, H2 storage, emissions control, biomass conversion, molecular separations, and biofuel purification. ==References==