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Crystallogenesis and Mineralogy
Saint Petersburg, Russia
30 September-4 October 2007
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Crystal morphology and hybrid fibre composite architecture of phosphatic and calcitic brachiopod shell materials - an overview

RMS-DPI code:  2007-1-23-1
Scientific session:  Session 1. Fundamental problems of crystallogenesis
Author listing:  Schmahl W.W., Götz A., Griesshaber E., Kelm K., Sehrbrock A., Merkel C., Lüter C., Neuser R., Mader W.
Principal author:  Schmahl Wolfgang W.
Language:  English
Abstract - Summary 
(short description):
Brachiopods have existed throughout the phanerozoic and they developed shell materials employing the two principal mineral groups of hard biologic tissue: calcium carbonate and calcium phosphate, each with distinct hierarchical architecture. We investigated the shell material with scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The phosphatic shells consist of a predominantly chitin fibrous matrix, which is reinforced by isometric hydroxyapatite nanocrystals (diameter 10 to 50 nm) or similarly sized amorphous Ca-phosphate particles attached to the fibres [1]. The fibre composit is not unlike vertebrate bone (where the biologic polymer component is protein, however). A laminated structure is created by changing volume ratios of chitin and reinforcing particles. In contrast, the composit structure of calcitic brachiopod shells employs inorganic single-crystal fibres reinforced by thin intercrystalline and extremely thin intracrystalline organic membranes [2, 3, 4]. The fibrous calcite shows a pronounced crystallographic texture, where the calcite c-axis is perpendicular to the morphologic fibre axis, and the morphologic fibre axes are in arbitrary directions within the (001) plane [2, 4]. The non-adherence to morphologic trigonal symmetry excludes plane-specific inhibitors as a controlling agent of crystallogenesis. From EBSD maps the texture appears to originate from growth selection. Over lengths well exceeding hundred micrometers the calcite crystal fibres change crystallographic orientation continuously in the order of one degree. Dense nanocrystalline calcite is frequently employed as a thin, hard outer layer of the shells. In some genera it builds the whole shell. While the laminated organic fibre/inorganic nanoparticle strategy provides a degree of shell flexibility for the benthic organisms, the inorganic fibre/polymer membrane composit of the epibenthic organisms provides a high stiffness and hardness (exceeding that of purely inorganic calcite). In both material/microstructure strategies, the composit architecture provides fracture toughnes.
RMS-DPI code:  2007-1-23-1
Proceeedings volume 
in the RMS catalogue:
Crystallogenesis and Mineralogy
Pages:  44-45
File of proceedings 
 278 K
Status:  printed
Accepted:  20/05/2007
Published on-line:  21/12/2007
Permanent address of publication:  http://www.minsoc.ru/2007-1-23-1/
Contact author(s):  Schmahl, Wolfgang W.

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