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Crystallography and chemistry of the calcium carbonate polymorph switch in M. edulis shells

Paul Dalbeck, Jennifer England, Maggie Cusack, Martin R. Lee and Anthony E. Fallick
Crystallography and chemistry of the calcium carbonate polymorph switch in M. edulis shells
European Journal of Mineralogy (October 2006) 18 (5): 601-609


The control exerted by some invertebrates on the calcium carbonate polymorph produced is intriguing but not understood. Mytilus edulis shells, with the abrupt polymorph switch within their valves from an outer calcite to inner aragonite layer, are excellent examples of this phenomenon. Detailed crystallography of intact valves using Electron Backscatter Diffraction (EBSD) is considered in the context of quantitative chemical analyses by electron microprobe. Apart from the outer 40 mu m, individual crystals that comprise the calcite layer of M. edulis differ from each other in terms of misorientation by less than 10 degrees . Similar uniformity occurs in the inner aragonite layer with notable 'mineral bridging' between tablets of aragonite nacre. The first-formed aragonite laminae are submicron thickness and the subsequent laminae of uniform 1 mu m thickness. Variations in chemical composition through the two valves correspond in part with the distribution of the two polymorphs. Magnesium is present in notably higher concentrations within calcite than aragonite. However, the Mg (super 2+) concentration in calcite is not uniform and increases with growth before decreasing at the polymorph switch. Sodium concentrations decrease steadily through the calcite layer. The aragonite layer is compositionally more uniform. Sulphur is not a good proxy for organic content in this system because it does not reflect the higher organic content of the aragonite. Sector zoning is not responsible for the element distribution seen here while differences in crystal size and association with organic components remain as possible explanations.

ISSN: 0935-1221
EISSN: 1617-4011
Serial Title: European Journal of Mineralogy
Serial Volume: 18
Serial Issue: 5
Title: Crystallography and chemistry of the calcium carbonate polymorph switch in M. edulis shells
Affiliation: University of Glasgow, Department of Geographical & Earth Sciences, Glasgow, United Kingdom
Pages: 601-609
Published: 200610
Text Language: English
Publisher: Schweizerbart'sche Verlagsbuchhandlung (Naegele u. Obermiller), Stuttgart, Federal Republic of Germany
References: 33
Accession Number: 2007-008236
Categories: Mineralogy of non-silicates
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus.
Secondary Affiliation: Scottish Universities Environmental Research Centre, GBR, United Kingdom
Country of Publication: Germany
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute.
Update Code: 200704
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