The Ultrastructure of Brachiopod Shells - A Mechanically Optimized Material with Hierarchical Architecture
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The Ultrastructure of Brachiopod Shells - A M echanically Optimized Material with Hierarchical Architecture E. Griesshaber 1), K. Kelm 2), A. Sehrbrock 3), R. Job 4), W. W. Schmahl 5), W. Mader 2) 1) 2) 3) 4)
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University of Bochum, Dept. of Geology, Mineralogy and Geophysics, Bochum, Germany University of Bonn, Department of Inorganic Chemistry, Bonn, Germany Research Center CAESAR, Bonn, Germany University of Hagen, Department of Electrical Engineering and Information Technology, Hagen, Germany University of Munich, Department of Earth Sciences, Munich, Germany
ABSTRACT Brachiopod shells consist of low-magnesium calcite and belong to one of the most intriguing species for studies of marine paleoenvironments, variations in oceanographic conditions and ocean chemistry [6, 7, 11 – 13]. We have investigated the ultrastructure together with nano- and microhardness properties of modern brachiopod shells with transmission electron microscopy (TEM), scanning electron microscopy (SEM), nanoindentation and Vickers microhardness analyses. Brachiopod shells are structured into several layers, a thin, outer, hard, protective primary layer composed of randomly oriented nanocrystalline calcite, which is followed inward towards the soft tissue of the animal by a much softer shell segment (secondary layer) built of long calcite fibres, stacked parallely into blocks. The hardness distribution pattern within the shells is non-uniform and varies on scales as small as a few tens of microns. Our results show that the hardness of this biomaterial is controlled by two predominant features: (1.) The morphological orientation of the calcite fibres (not by the crystallographic orientation of the fibres), and (2.) the amount and distribution pattern of organic material between and within the calcite crystals. INTRODUCTION AND AIM OF THE STUDY Magnesian calcites of biogenic origin are hierarchically structured, multifunctional materials. They are organic–inorganic composites, where mineralization occurs through biologic control and mediation [1, 2, 3]. The organically induced fabrication of the hard tissues and the structuring on different scale levels are among the key driving processes which enable biogenic hard tissues to develop highly evolved functional materials with unique engineering properties suitable for adaptation to distinct environmental conditions [1, 2, 3]. We have investigated the nano- and microhardness properties of the modern brachiopod Megerlia truncata with nanoindentation and Vickers microhardness analyses together with the distribution pattern of inter- and intracrystalline organic material with TEM and micro-Raman spectroscopy [16] in order to detect the predominant causes of fracture toughness and the variation of hardness within the shells.
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Figures 1a, 1b, 1c, 1d. TEM images of the secondary shell layer of Megerlia truncata. We have observed the incorporation of organic material both, between the calcite fibres (Figs. 1a and 1b) and within the fibres (Figs. 1c, 1d and 1e).
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