Biomineral Architectures: the red coral model – CoRo

The following techniques will be used: optical microscopy, scanning and transmission electron microscopy, Electron Backscattered Diffraction, synchrotron XRD, electron microprobe, SIMS, and nanoSIMS. We will also characterize the porosity of the skeleton, another important part of biomineral structures, by isothermal manometry. Experimental syntheses of calcium carbonate by Hydrothermal Carbonation are planned to explore the relationships between calcite crystal surfaces and organic compounds.

Biominerals commonly display complex hierarchical mesocrystalline organizations and bring new insights on properties of mesocrystals. Two types of biomineral structures found in the red coral have been studied: the axial skeleton and the sclerites.
1 - In the red coral skeleton, ‘building blocks’ are arranged into eight hierarchical levels of similarly (but not identically) oriented modules. Each module is made of an organized array of sub-units, and is at the same time a sub-unit of a larger module. This crystallographic organization is observed down to a few nm. Thus, the concept of ‘mesocrystalline’ organization applies to the red coral skeleton; we add to this concept the notion of ‘multilevel modularity’. EBSD and TEM studies show that the degree of crystallographic misorientation between the building blocks decreases with decreasing module size. Thus, the transition from imperfect crystallographic order at mm scale to nearly perfect single crystalline domains at nm scale is progressive.
2 - Sclerites (also called spicules) are small grains of Mg-rich calcite found in the living tissues of Corallium rubrum and other octocorals; they are particularly appropriate to decipher the principles of crystallographic organization in biominerals and explore the relationships between morphology and crystallography. Sclerites are made of well separated submicrometer crystalline units (?80 nm); on the other hand, EBSD studies show that these crystalline units are similarly oriented with only a low degree of misorientation between them. Thus, the concept of ‘mesocrystal’ applies also to sclerites. High resolution EBSD data show that quite unexpectedly slight misorientations of crystallites are not at random but result from rotations around the three equivalent a axes of the calcite hexagonal unit cell. This observation leads to the concept of ‘misorientation ordering’.

The project is still in its initial stages.
Characterization of natural objects is in progress with the following techniques (SEM, TEM, EBSD, XRD, IR, Raman).
Experimental aspects are in their preliminary phases (setting up of new equipments and experimental procedures.

1 article accepted for publication : Ordered Misorientations and Preferential Directions of Growth in Mesocrystalline Red Coral Sclerites. Journal of Crystal Growth.

1 article in preparation : Magnesium and sulfur distribution in the Red Coral.

1 invited conference : Order and disorder in biominerals: the example of the Mediterranean red coral. Materials Research Society, Avril 2012, San Francisco

Precious red corals from the Mediterranean (Corallium rubrum) and the western Pacific (Corallium Japonicum and Corallium Elatius) represent excellent model organisms to better understand bio assisted crystal growth and the organization of structures in complex systems at the organic/inorganic interface.
The aim of this project is to decipher the hierarchy of chemical, physical and morphological organization of red coral skeletons and their sclerites made of Mg calcite using crystallography as a main guide. For this purpose the following techniques will be used: optical microscopy, scanning and transmission electron microscopy, Electron Backscattered Diffraction, synchrotron XRD, electron microprobe, SIMS, and nanoSIMS. We will also characterize the porosity of the skeleton, another important part of biomineral structures, by isothermal manometry. Experimental syntheses of calcium carbonate by Hydrothermal Carbonation are planned to explore the relationships between calcite crystal surfaces and organic compounds. Two main types of experiments will be carried out: (1) overgrowth of sub-micrometric calcite particles on pre-existing carbonate surfaces and (2) nucleation-growth of calcite sub-micrometric particles in presence of natural or synthetic organic molecules in the solution.
This project involves two partners: CINaM – Marseille specialized in the characterization of physico-chemical properties of organic and inorganic materials from the nano to the macroscale, and (2) ISTerre – Grenoble for the experimental side of the project. This project will also benefit from current collaborations with different oceanographic Institutions [ICM Barcelona, COM Marseille, CSM Monaco] and other laboratories [Biogéosciences Dijon and Geological and Planetary Division Caltech]. This project is part of the COST action TD0903 ‘Understanding and manipulating enzymatic and proteomic processes in biomineralization’.