“Biominerals are naturally occurring biomaterials combining organic and inorganic compounds organized in a multi-scale structure, ranging from the macroscopic level down to nanometer. Understanding this organization and its formation mechanisms implies to unveil the chemical composition and structure at the nanoscale. Of particular relevance is the characterisation of the distributions of the mineral and the organic compounds. Conventional transmission electron microscopy (CTEM) cannot achieve this, since it cannot easily distinguish the chemical nature of the compounds composing the material. Compared to other spectroscopic approaches, STEM-EELS (scanning transmission electron microscopy coupled with Electron Energy Loss Spectroscopy) offers the advantage of an outstanding spatial resolution (down to nanometer) in both chemical analysis and imaging. Moreover, the use of a latest-generation STEM microscope equipped with an electron monochromator and a direct-detection camera offers the possibility to detect very weak signals with a spectral resolution down to 7 meV, close to those obtained from X-ray based approaches This high spectral resolution gives access to the crystallinity of calcium carbonate phases through the measure of the crystal field splitting (CFS) on Calcium L23-edge.

In this talk, I present our work concerning the compositional and structural characterization of different biominerals. For instance, I will discuss the study of the calcite secreted by the giant acorn barnacle Austromegabalanus psittacus. Based on EELS results, we develop a model by which the separation of the crystalline and amorphous phases takes place upon crystallization of the calcite from a precursor ACC. The organic biomolecules are expelled from the crystal lattice and concentrate in the form of pellicles, where they stabilize minor amounts of ACC/nanocalcite. In this way, we change the previously established conception of biomineral structure and growth.”