In situ mapping of growth lines in the calcitic prismatic layers of mollusc shells using X-ray absorption near-edge structure (XANES) spectroscopy at the sulphur K-edge |
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Authors: | Y Dauphin J Cuif J Doucet M Salomé J Susini C Williams |
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Institution: | 1.Laboratoire de paléontologie, FRE2566 du CNRS, Batiment 504, Université Paris XI–Orsay, 91405 Orsay cedex, France,;2.LURE, Batiment 209D, BP 34, Université Paris XI–Orsay, 91898 Orsay cedex, France,;3.ID21, ESRF, BP 220, 38043 Grenoble cedex, France,;4.Department of Mineralogy, The Natural History Museum, Cromwell Road, London, SW7 5BD, UK, |
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Abstract: | The microstructure and composition, including chemical speciation of sulphur (S), of two mollusc shells were investigated using a combination of scanning electron microscopy, X-ray absorption near-edge structure spectroscopy (XANES) and electron probe microanalysis (EPMA). The shell of Pinna is composed of monocrystalline, and Pinctada, of polycrystalline, calcite prisms separated by organic-rich walls. Sulphur speciation information from XANES spectra using a scanning X-ray microscope showed that the protein S content of the interprismatic walls is higher than the SO4 content, whereas the reverse is true for the intraprismatic structures. High-spatial-resolution XANES maps for the different S species across adjacent calcite prisms confirm their distinctive distributions in the molluscan shells and illustrate the presence of narrow, submicron transverse growth features. On a larger scale, a series of wider growth zones, incorporating the submicron zones, are aligned parallel to each other and cross-cut many calcite prisms. EPMA element maps for magnesium (Mg) and S demonstrate that these growth increments are compositionally zoned, comprising alternating layers of high mineral (Mg-rich) and high organic (S-rich) components. Additionally, these maps confirm that the organic interprismatic walls have lower Mg and higher S than the intraprismatic structures. |
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