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Crystallographic texture and microstructure of terebratulide brachiopod shell calcite; an optimized materials design with hierarchical architecture

Erika Griesshaber, Wolfgang W. Schmahl, Rolf Neuser, Thomas Pettke, Michael Bluem, Joerg Mutterlose and Uwe Brand
Crystallographic texture and microstructure of terebratulide brachiopod shell calcite; an optimized materials design with hierarchical architecture
American Mineralogist (May 2007) 92 (5-6): 722-734

Abstract

We analyzed the microstructure, microchemistry, and microhardness variations across the architectural elements of the shells of the brachiopod species Megerlia truncata and Terebratalia transversa with scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), laser-ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS), and Vickers microhardness indentation (VMHI). The brachiopod valves consist of two principal layers of distinct calcite biomineralization: a thin, nanocrystalline, outer, hard protective layer with VMHI values exceeding 200 HV and a much thicker, inner, secondary layer of a hybrid organic-inorganic fiber composite material. The secondary layer is further structured into two sublayers, an outer part with VMHI values varying between 110 and 140 HV, and a softer inner part (70<HV<110). Whereas the size of the calcite crystals within the primary layer varies between a few tens of nanometers and 2 mu m, calcite crystals within the secondary layer are fibrous, commonly reaching lengths exceeding 150 mu m. Cross sections of these fibrous crystals are spade shaped, their dimensions being about 5X20 mu m. The fibers are aligned parallel to each other. They are single crystals with their morphological fiber axes pointing almost parallel to the shell vault. The crystallographic orientation of the morphological fiber axes, however, is arbitrary within the a-b plane of the calcite lattice, whereas the c-axis (hexagonal unit-cell setting) is perpendicular to the morphological fiber axes and thus parallel to the radial vector of the valve vault. This morphology strongly indicates that fibrous growth is controlled by confinement within a cell in an organic matrix and not by attachment of biomolecules to specific crystallographic faces. We observe inhomogeneous Sr (super 2+) and Mg (super 2+) concentrations in the shell calcite within the 0.1-0.9 wt% range. Design of the shell appears to be highly optimized for mechanical performance. Crystal morphology and orientation as well as incorporated organic matter are structured hierarchially at different length levels forming a hybrid organic-inorganic fiber composite architecture.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 92
Serial Issue: 5-6
Title: Crystallographic texture and microstructure of terebratulide brachiopod shell calcite; an optimized materials design with hierarchical architecture
Affiliation: Ruhr-Universitaet Bochum, Institut fuer Geologie Mineralogie und Geophysik, Bochum, Federal Republic of Germany
Pages: 722-734
Published: 200705
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 73
Accession Number: 2007-065427
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: Ludwig-Maximilian University Munich, DEU, Federal Republic of GermanyETH Zentrum, CHE, SwitzerlandBrock University, CAN, Canada
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200733
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