Ks. Katti et al., Low-loss electron energy-loss spectroscopy and dielectric function of biological and geological polymorphs of CaCO3, MICROS MICR, 5(5), 1999, pp. 358-364
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.