Amorphous calcium carbonate

These deposits of tufa contain amorphous calcium carbonate along remains of algae and moss

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.[1]

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.[2][3] That is why ACC is not found naturally.

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 know to produce very stable ACC.[4]

Synthetic ACC

Many methods,[5][6] 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.[7] Also, ACC crystallisation pathways have been observed to depend on its Mg/Ca ratio, transforming to Mg-calcite,[8] monohydrocalcite[9] or dolomite[10] with increasing Mg content. Huang et al. managed to stabilize ACC using polyacrylic acid for several months,[11] while Loste et al. showed that magnesium ions can increase ACC stability as well.[12] But only the discovery that aspartic acid, glycine,[13] citrate [14] phosphorylated amino acids can produce long term stable ACC[15] have opened the door for production commercialization.

Bioavailability

Since 2013 a company named Amorphical Ltd. sells an ACC dietary supplement.[16][17]

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.[18]

References

  1. ↑ US patent 603225, Sturcke, Herman E., "Process of preparing amorphous carbonate of lime from residues", issued April 26, 1898
  2. ↑ Rodriguez-Blanco, J.D., Shaw, S. and Benning, L.G. (2011) The kinetics and mechanisms of Amorphous Calcium Carbonate (ACC) crystallization to calcite, via vaterite. Nanoscale, 3, 265-271. doi: 10.1039/c0nr00589d
  3. ↑ Bots, P., Rodriguez-Blanco, J.D., Roncal-Herrero, T., Benning, L.G. and Shaw, S. (2012) Mechanistic insights into the crystallization of amorphous calcium carbonate to vaterite. Crystal Growth and Design, 12, 3806-3814. doi: 10.1021/cg300676b.
  4. ↑ Hodson, M.E., Benning, L.G., Demarchi, B., Penkman, K.E.H., Rodriguez-Blanco, J.D., Schofield, P.F., Versteegh, E.A.A. (2015) Biomineralisation by earthworms – an investigation into the stability and distribution of amorphous calcium carbonate. Geochemical Transactions, 16, 4. doi:10.1186/s12932-015-0019-z
  5. ↑ Rodriguez-Blanco, J.D., Shaw, S. and Benning, L.G. (2008) How to make ‘stable’ ACC: protocol and preliminary structural characterization. Mineralogical Magazine, 72, 283-286. doi: 10.1180/minmag.2008.072.1.12.
  6. ↑ Rodriguez-Blanco, J.D., Shaw, S. and Benning, L.G. (2011) The kinetics and mechanisms of Amorphous Calcium Carbonate (ACC) crystallization to calcite, via vaterite. Nanoscale, 3, 265-271. doi:10.1039/c0nr00589d
  7. ↑ Cobourne, G., Mountjoy, G., Rodriguez-Blanco, J.D., Benning, L.G., Hannon, A.C. and Plaisier, J.R. (2014) Neutron and X-ray Diffraction and Empirical Potential Structure Refinement Modelling of Magnesium Stabilised Amorphous Calcium Carbonate. Journal of Non-Crystalline Solids, 401, 154-158. doi: 10.1016/j.jnoncrysol.2013.12.023
  8. ↑ Rodriguez-Blanco, J.D., Shaw, S., Bots, P., Roncal-Herrero, T., and Benning, L.G. (2012) The role of pH and Mg on the stability and crystallization of amorphous calcium carbonate. Journal of Alloys and Compounds, 536, Suppl1, S477-S479. doi: 10.1016/j.jallcom.2011.11.057.
  9. ↑ Rodriguez-Blanco, J.D., Shaw, S., Bots, P., Roncal-Herrero, T., and Benning, L.G. (2014) The role of Mg in the crystallisation of monohydrocalcite. Geochimica et Cosmochimica Acta, 127, 204-220. doi: 10.1016/j.gca.2013.11.034
  10. ↑ Rodriguez-Blanco, J.D., Shaw, S. and Benning, L.G. (2015) A route for the direct crystallization of dolomite. American Mineralogist, 100, 1172–1181. DOI:10.2138/am-2015-4963 http://www.minsocam.org/msa/ammin/toc/2015/open_access/AM100P1172.pdf.
  11. ↑ Shu-Chen Huang, Kensuke Naka & Yoshiki Chujo (2007). "A carbonate controlled-addition method for amorphous calcium carbonate spheres stabilized by poly(acrylic acid)s". Langmuir 23 (24): 12086–12095. doi:10.1021/la701972n.
  12. ↑ Loste, Eva; Wilson, Rory M.; Seshadri, Ram; Meldrum, Fiona C. (2003). "The role of magnesium in stabilising amorphous calcium carbonate and controlling calcite morphologies". Journal of Crystal Growth 254: 206–18. Bibcode:2003JCrGr.254..206L. doi:10.1016/S0022-0248(03)01153-9.
  13. ↑ Tobler, D.J., Rodriguez-Blanco, J.D., Dideriksen, K., Sand, K.K., Bovet, N. Benning, L.G. and Stipp, S.L.S. (2014) Effect of aspartic acid and glycine on amorphous calcium carbonate (ACC) structure, stability and crystallisation. Procedia Earth and Planetary Science, 10, 143–148.
  14. ↑ Tobler, D.J., Rodriguez-Blanco, J.D., Dideriksen, K., Bovet, N., Sand, K.K., Stipp. S.L.S. (2015) Citrate Effects on Amorphous Calcium Carbonate (ACC) Structure, Stability, and Crystallization. Advanced Functional Materials. DOI: 10.1002/adfm.201500400]
  15. ↑ Bentov, Shmuel; Weil, Simy; Glazer, Lilah; Sagi, Amir; Berman, Amir (2010). "Stabilization of amorphous calcium carbonate by phosphate rich organic matrix proteins and by single phosphoamino acids". Journal of Structural Biology 171 (2): 207–215. doi:10.1016/j.jsb.2010.04.007. PMID 20416381.
  16. ↑ The dietary supplement "DENSITY" homepage
  17. ↑ סידן, הדור הבא: התוסף שמצליח לבנות עצם מחדש hebrew article on Ynet about the new calcium dietary supplement, December 2013
  18. ↑ Meiron, Oren E; Bar-David, Elad; Aflalo, Eliahu D; Shechter, Assaf; Stepensky, David; Berman, Amir; Sagi, Amir (2011). "Solubility and bioavailability of stabilized amorphous calcium carbonate". Journal of Bone and Mineral Research 26 (2): 364–372. doi:10.1002/jbmr.196. PMID 20690187.
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