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Investigations of voids in the aragonite platelets of nacre

: Gries, K.; Kröger, R.; Kübel, C.; Fritz, M.; Rosenauer, A.


Acta biomaterialia 5 (2009), No.8, pp.3038-3044
ISSN: 1742-7061
ISSN: 1878-7568
Journal Article
Fraunhofer IFAM ()

Nacre, the inner iridescent layer of mollusc shells, is a typical example of a material that is formed by such biomineralization processes. Nacre is composed of the CaCO3-polymorph aragonite and a small amount (about 5 wt.%) of organic matter. The authors studied the structure of the aragonite platelets of Haliotis laevigata nacre, using conventional transmission electron microscopy, Z-contrast, electron tomography, energy-dispersive X-ray analysis (EDX)and electron energy-loss spectroscopy (EELS). The authors observed faceted voids several nanometers wide within the aragonite platelets. The electron tomography investigations showed that the voids are distributed more or less randomly in the studied specimen and allowed an estimation of the order of magnitude of the width and the volumetric content of the voids. The voids are located within the aragonite platelets, so that they are not an artifact of preparation. Furthermore, a first proposal for a three-dimensional model of the voids was made. EDX and EELS measurements revealed that the voids include a larger amount of carbon in comparison to the bulk material. The questions why these voids are formed and how have still to be answered. A speculative function of the voids could be the decreased propagation of cracks in the shell providing an increased fracture toughness.