The complex crystalline structure of an oyster shell

In many living organisms, biomineralisation processes regulate the production of the mineralised tissues such as bones, teeth and shells. Deciphering these mechanisms is of crucial importance for materials science, as it will provide bio-inspired strategies for the synthesis of nanostructured inorganic materials using soft chemistry and environmentally friendly processes. Such understanding is also important forpaleoclimatology where biomineral proxies are used to perform paleoclimate reconstructions. The calcium carbonate biomineral is one of the most striking examples because theories arising from classical crystallisation (involving monomer-by-monomer addition) cannot explain the production of highly regulated calcareous crystalline biomineral structures such as those observed in sea urchin or pearl oyster. The production of this major constituent of the Earth’s crust is still poorly understood.

The present study was motivated by an apparent contradiction observed in biomineral structures: while calcareous crystallising species present a remarkable architectural diversity at the macro and micro-scales, their sub-micrometric scale is characterised by a granular, but crystalline, structure. Hence, a proper description of the crystalline features at this mesoscale level, i.e., over a few sub-micrometric (50-500 nm) granules, is a key to building realistic scenarios of biomineralisation. However, none of the currently used experimental approaches (electron- or X-ray-based diffraction microscopies) is able to provide access to the detailed 3D crystalline granule arrangement.

In 2011, we proposed a new approach, named Bragg ptychography, to image the crystalline properties of complex materials in 3D [1]. This microscopy method exploits the partially redundant information obtained by scanning a finite-size beam spot transversally to the sample, while measuring the corresponding 3D far-field intensity coherent diffraction pattern in the vicinity of a Bragg reflection. The 3D structure of the crystalline sample is further retrieved by inversion algorithms, which deliver a highly-resolved image beyond the limit imposed by the size of the beam spot.

Figure 1. a) Optical image of a pearl oyster shell (Pinctada margaritifera) highlighting the investigated region (white rectangle). b) Enlarged view of the shell border showing the microscopic structure of the shell, constituted of juxtaposed calcite prisms. c) X-ray Bragg diffraction pattern acquired with the ID13 X-ray nanobeam and integrated over the full prism area. The single-crystalline behaviour of the shell produces well-defined Bragg diffraction spots.

This cutting-edge synchrotron-based X-ray microscopy method was implemented at beamline ID13 and used to reveal the details of the mesocrystalline organisation in calcite prisms, the generic mineral units of the pearl oyster shell (Figure 1). While these prisms are usually described as single crystals, the 3D image proves the existence of large iso-oriented and iso-strained crystalline domains, slightly different one from the other (Figure 2). These original results call for specific non-classical crystallisation pathways: the highlighted mesocrystalline properties support recent biomineralisation models, involving partial fusion of oriented attached nanoparticle assembly and/or liquid droplet precursors.

Figure 2. Results of the 3D Bragg ptychography microscopy reconstruction, extracted from the detailed measurement of one single Bragg peak with a coherent X-ray nanobeam. a) The retrieved volume (in yellow-grey) represents a small portion of the prism volume. b) 3D Rotation and c) strain maps, showing the existence of crystalline domains within the single-crystalline-like biomineral.

This study has been performed in the framework of a 4-year ANR grant. It constitutes the starting point of an ERC project, aiming at defining the physical, chemical and biological conditions needed to produce synthetic biominerals on demand. We expect that the unique properties of the upgraded ESRF source combined to the new (and fast) Bragg ptychography microscopy [2] will enable us to achieve this goal.

Principal publication and authors
Revealing crystalline domains in a mollusc shell “single-crystalline” prism, F. Mastropietro (a), P. Godard (a), M. Burghammer (b), C. Chevallard (c), J. Daillant (d), J. Duboisset (a), M. Allain (a), P. Guenoun (c), J. Nouet (e), V. Chamard (a), Nature Materials (2017); doi: 10.1038/NMAT4937.
(a) Aix-Marseille Univ, CNRS, Centrale Marseille, Institut Fresnel, Marseille (France)
(b) ESRF, Grenoble (France)
(c) NIMBE, CEA, CNRS, Université Paris-Saclay, Gif-sur-Yvette (France)
(d) Synchrotron SOLEIL, Gif-sur-Yvette (France)
(e) GEOPS, Univ. Paris-Sud, CNRS, Université Paris-Saclay, Orsay (France)

References
[1] P. Godard et al., Nature Communications 2, 568 (2011).
[2] S.O. Hruszkewycz, et al., Nature Materials 16, 244 (2017).

Top image: AFM image of the surface of a Pinctada margaritifera prism showing the granular structure.