Many biominerals, including mollusk and echinoderm shells, avian eggshells, modern and fossil bacterial sediments, planktonic coccolithophores, and foraminifera, contain carbonates in the form of biogenic aragonite or calcite. Here we analyze biogenic and geologic aragonite using different kinds of surface-and bulksensitive X-ray absorption near-edge structure (XANES) spectroscopy at the carbon K-edge, as well as highresolution scanning transmission X-ray microscopy (STXM). Besides the well-known main π* and σ* carbonate peaks, we observed and fully characterized four minor peaks, at energies between the main π* and σ* peaks. As expected, the main peaks are similar in geologic and biogenic aragonite, while the minor peaks differ in relative intensity. In this and previous work, the minor peaks appear to be the ones most affected in biomineralization processes, hence the interest in characterizing them. Peak assignment was achieved by correlation of polarization-dependent behavior of the minor peaks with that of the main π* and σ* peaks. The present characterization provides the background for future studies of aragonitic biominerals.
IntroductionThe calcium carbonates (CaCO 3 ) aragonite and calcite constitute the majority of biominerals, including the outer shells of mollusks: bivalves, gastropods and cephalopods; the endoskeletons of echinoderms, avian eggshells, corals, planktonic coccolithophores and foraminifera, and bacterial biominerals, comprising extant and fossil bacterial biofilms. 1,2 It is therefore of great interest to characterize all spectral components in X-ray absorption near-edge structure (XANES) spectra from aragonite, which is done here.One of the most interesting and studied aragonitic biominerals is mother-of-pearl, or nacre: a composite of 95% aragonite and 5% organic molecules. 1,2 Its remarkable toughnesssmany thousands of times more resistant to fracture than aragonite 3 shas stimulated many studies of nacre's mechanical performance, structure, proteins and glycoproteins, and the role that these might play in nacre formation. [4][5][6][7][8] We recently discovered that nacre exhibits X-ray linear dichroism, a phenomenon widely studied in man-made materials but never before reported in a biomineral. 8,9 Dichroism in carbonates generates a polarizationdependent imaging contrast (PIC), which is particularly useful to investigate the meso-scale architecture of nacre using X-ray photoelectron emission spectromicroscopy (X-PEEM) 10 and scanning transmission X-ray spectromicroscopy (STXM). 11,12 With X-PEEM we revealed that the 400-nm high, 5-µm wide aragonite tablets that form the layered abalone nacre structure are arranged in stacks of co-oriented tablets, staggered with respect to each other.We have previously extensively characterized the origin of PIC and X-ray linear dichroism with a series of pure-