3D Calcium carbonate polymorphs imaging with stimulated Raman scattering in biominerals.

We present a highly sensitive coherent Raman microscopy approach, which allows for the tridimensional (3D) imaging of a series of carbonate polymorphs in marine organisms. CaCO₃ biomineralization occurs from the transformation of metastable amorphous precursors and other crystalline phases into a final crystalline phase. Understanding biomineralization pathways requires identifying this physico-chemical temporal sequence. Our approach exploits the different vibrational signatures of amorphous calcium carbonate, aragonite, calcite, Mg-calcite or hemi-hydrated calcium carbonate. This optical method enables the production of spatially and spectrally resolved images of the different compounds. When applied on the growing edge of post-mortem samples of both Pinctada margaritifera pearl oyster shell and Stylophora pistillata coral, it allows for inferring a temporal crystallisation sequence. We thus highlight the existence of intermediate crystalline phases, involving magnesian calcite or hemi-hydrated calcium carbonate, respectively.