Microstructure of Brachiopod Shells - An Inorganic/Organic Fibre Composite with Nanocrystalline Protective Layer
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Microstructure of Brachiopod Shells - An Inorganic/Organic Fibre Composite with Nanocrystalline Protective Layer E. Griesshaber1, W. Schmahl1, R. Neuser1, R. Job2, M. Bluem3, U. Brand4 1. University of Bochum, Department of Geology, Mineralogy and Geophysics, Bochum, Germany 2. University of Hagen, Electrical Engineering and Information Technology, Hagen, Germany 3. University of Bochum, Department of Material Science, Bochum, Germany 4. Brock University, Department of Earth Sciences, Brock University, St. Catharines, Canada ABSTRACT 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 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. INTRODUCTION The architecture of inorganic biological materials is of interest as a prototype for the design of optimized materials. Therefore, an understanding of biomineralisation processes - the biologic control of crystal growth - may lead to new fabrication techniques and thus more suitable materials. Carbonate shells of marine organisms form an intriguing object for material science, since they are low-cost and low-weight composite structures of organic and inorganic components. Even though the knowledge of natural processes driving biomineralisation of calcium carbonate is still rudimentary, some notions exist on linkages of carbonate shell formation to protein substrates and templates [1-3]. We have investigated the microstructural (ultrastructural in biological terminology) and chemical characteristics of the shell of the modern brachiopods Megerlia truncata and Terebratalia transversa by SEM, electron back scattering diffraction (EBSD) and microhardness indentation. Although the structure of brachiopod shells was extensively studied previously [4-11], crystallographic methods had then not yet progressed to allow an investigation of crystallographic properties of the ultrastructure. However, electron backscattering diffraction (EBSD) has now reached such a high level of resolution, signal-tonoise ratio and automation [12] that studies of biomineralization products such as brac
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