We investigated the ultrastructure of the modern calcitic brachiopods Megerlia truncata (Linnaeus) and Terebratalia transversa (Sowerby) with SEM, electron backscattering diffraction and microhardness indentation. The outer, primary shell layer can be regarded as a nanocrystalline thin film that forms a hard protective coating around the inner, much softer secondary layer that can be expressed as an inorganic/organic fibre composite. The fibrous, curved growth of the secondary shell layer crystals occurs in arbitrary directions perpendicular to the <0 0 0 1> triad symmetry direction of calcite and is most likely obtained by simple confinement to a protein sheath rather than by biomolecular adsorbates blocking growth of any specific crystal face. The curvature of the fibres is caused by rearrangements of the secreting cell array during growth, whereby the existing crystal lattice is not distorted. It serves as a substrate for continued crystal growth. Thus biologically mediated calcite crystallization is a purposeful process and seems to be significantly different to the inorganic crystallization of calcite.
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Griesshaber, E., Schmahl, W., Neuser, R. et al. Microstructure of Brachiopod Shells - An Inorganic/Organic Fibre Composite with Nanocrystalline Protective Layer. MRS Online Proceedings Library 844, 93 (2004). https://doi.org/10.1557/PROC-844-Y9.3