Among biologically produced minerals, Ca-carbonate prismatic units of Mollusc shells epitomise the paradox of biocrystals. The long-standing recognition of their biologically controlled sizes and shapes has raised questions about the sequence of "molecular tectonic" events [1] that lead to formation of crystalline objects so visibly growing apart from crystallographic rules. The modelling of biomineral growth, controlled from the nanometre to the millimetre scale, requires the understanding of the roles of the mineral and organic matrices and their interaction [2]. However, little information on the organic matrix is available, in particular no correlation has yet been established between its high sulphur content and the mineralising process. Sulphur can be found in both proteins and glucids, the two major organic compounds that compose organic matrices responsible for calcification in carbonate producing organisms [3]. Therefore, studying the chemical state of sulphur may be used as an indicator of the relationship between the mineralising matrices and the mineral ions.

A study was carried out on calcitic units that built the external layer of a Mollusc shell, the Pelecypod species Pinna nobilis, rather common in the Mediterranean Sea. These units appear as linear prisms, closely and very regularly packed side-by-side, of 50 to 75 micrometers in transverse sections and up to 3-4 millimetre in length (Figure 33). The prisms of Pinna offer remarkable advantages for the investigation of the organisation of biocrystals. They are the largest units that globally exhibit a single crystal like organisation, although they are typical polyclinic structures. Chemically, they are characterised by the highest sulphur content ever observed, about 0.3 to 0.4%, a chemical feature that could be related to its exceptional crystallographic coherence. The experiment was carried out on ID21 using the scanning X-ray microscope in fluorescence mode at the sulphur K-edge. The microscope used a Fresnel zone-plate as a focussing lens and delivered a microbeam of 0.25 x 0.25 µm2. Sulphur speciation analysis was performed in both intra- and inter-prismatic unit matrices. Mapping of these composites at two energies revealed for the first time the existence of at least two different sulphur compounds in the matrices of Pinna nobilis (Figure 34). The spectra measured in the organic and mineral phases were compared with XANES spectra of pure standard products (cystin, cystein and chondroitin sulphate). These preliminary results indicate clearly the absence ­ or the very low concentration ­ of sulphidic crosslinks, the predominance of sulphate in the intra-prismatic matrix and the high concentration of "amino-acid" type sulphur in the inter-prismatic matrix, in particular at the nodes of the hexagonal mesh of the matrix. Further analysis must be performed to clarify the exact role of the various sulphur compounds in this organic-inorganic interface, which drives the bio-mineralisation process.

[1] S. Mann, Nature, 365, 499 (1993).
[2] S. Weiner, L. Addadi, J. Mater. Chem., 7(5), 689, (1997).
[3] Y. Dauphin, J.P. Cuif, Annales des Sciences Naturelles, 2, 73, (1999).

Y. Dauphin (a), M. Salomé (b), J. Susini (b), J. Doucet (c), J.P. Cuif (a).

(a) Geology-Paleontology Laboratory, CNRS, University Paris XI (France)
(b) ESRF
(c) LURE, University Paris XI (France)