Low-loss electron energy-loss spectroscopy and dielectric function of biological and geological polymorphs of CaCO3

Previous work on microstructural characterization has shown variations in t erms of defects and organization of nanostructures in the two polymorphs of calcium carbonate, calcite, and aragonite in mollusc shells. Large variati ons in mechanical properties are observed between these sections which have been attributed to variations in composite microstructure as well as intri nsic properties of the inorganic phases. Here we present local low-loss ele ctron energy-loss spectroscopic (EELS) study of calcitic and aragonitic reg ions of abalone shell that were compared to geological (single-crystal) cou nterpart polymorphs to reveal intrinsic differences that could be related t o organismal effects in biomineralization. In both sets of samples, local d ielectric function is computed using Kramer-Kronig analysis. The electronic structures of biogenic and geological calcitic materials are not significa ntly different. On the other hand, electronic structure of biogenic aragoni te is remarkably different from that of geological aragonite. This differen ce is attributed to the increased contribution from single electron excitat ions in biogenic aragonite as compared to that of geological aragonite. Fur thermore, an apparent bound characteristic of the Re(1/epsilon) is observed for biological samples which suggests a "quasi-plasmon"-like nature of the collective excitations. Implications of these changes are discussed in the context of macromolecular involvement in the making of the microstructures and properties in biogenic phases.