Eggshells of chickens are formed from amorphous calcium carbonate nanoparticles which are transformed into ordered crystals by protein mediation.

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Eggshell this a thin yet relatively strong material due to its composite makeup of ordered calcium carbonate crystals and protein. A layer of crystalline calcite clusters overlays partially aligned calcite columns. The mineral begins as amorphous calcium carbonate nanoparticles, then is transformed to ordered crystals by the presence of C-type lectin-type proteins. The proteins attach themselves to the nanoparticles, initiating crystal transformation, and then detach while crystal growth continues.

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"The use of biomolecules in nature to direct crystal growth leads to a degree of polymorph and morphology control that far surpasses anything currently accessible in a laboratory. Examples include the intricate nano- and microcrystalline structures found in mollusk shells, coccoliths, and eggshells, which imbue the shells with important physical properties...Eggshells have an intricate structure that consists of two domains attached to an inner membrane. The first domain is an array of small polycrystalline calcite clusters (mammillary caps) attached to the membrane that surrounds the albumin; the second domain (pallisade layer) consists of elongated calcite crystals with partial alignment. Experiments have identified various proteins associated with eggshell formation. One class, C-type lectin-type proteins, is found only within the mineral region and is important in controlling calcite deposition. In vitro studies with OC-17 (chicken) and ansocalcin (goose) have shown that these proteins promote calcite formation and define the crystal morphology."(Freeman et al. 2010:5135)

"The results show that the chicken eggshell protein ovocleidin-17 can facilitate a transition from amorphous particle to calcite crystal. Intriguingly, strong binding was observed only with smaller nanoparticles (192 formula units); with a larger particle size (300 formula units) the protein consistently desorbed from the calcite phase. Strong binding is regained for very large crystalline surfaces, but in this case mediated by structured surface water...[T]he results lead us to propose that OC-17 acts as a catalyst by binding to amorphous calcium carbonate nanoparticles, transforming them to calcite nuclei, and then desorbing as the calcite begins to grow, thus leaving the OC-17 available to bind to another ACC nanoparticle (see Figure 5)." (Freeman et al. 2010:5136-5137)

Journal article
Structural Control of Crystal Nuclei by an Eggshell ProteinAngewandte ChemieJune 9, 2010
Colin L. Freeman, John H. Harding, David Quigley, P. Mark Rodger

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