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Micro- and Nano-Crystal Orientations in Shells Print
Thursday, 12 April 2012 11:11

A research group from the University of Wisconsin and the ALS developed a new method termed Polarization-dependent Imaging Contrast (PIC) mapping to visually display the orientation of micro- and nano-crystals in calcium carbonates. PIC-mapping uses the ability to manipulate the polarization of synchrotron light from undulators, and enhances the capabilities of x-ray absorption spectroscopy and microscopy to include linear dichroism effects. By acquiring spectra at the same photon energy but at different polarizations, and using a photoelectron emission spectromicroscope (PEEM), PIC-mapping can determine the angular orientation of micro- and nano-crystals. PEEM instruments are already present at most synchrotrons, hence  PIC-mapping is readily available. Quantitative PIC-mapping was demonstrated for the first time in this article on geologic calcite (CaCO3), and also used to investigate the prismatic layer lining the outside of a mollusk shell, the Japanese white pearl oyster Pinctada fucata. The data reveal multiply oriented nanocrystals within calcite prisms, previously thought to be monocrystalline. The subdivision into multiply oriented nanocrystals, spread by more than 50°, may explain the excellent mechanical properties of the prismatic layer, known for decades but never explained.


crystalline structure

The nanocrystalline structure of the prismatic layer in the Japanese pearl oyster Pinctada fucata. Left: Visible polarized-light micrograph of a cross-section of the mollusk shell. At the bottom the embedding epoxy is visible, at the center is the prismatic layer, at the top is the nacre layer of the shell. In the prismatic layer, differently oriented prisms appear in different colors. Notice that each prism is not single-crystalline, but qualitatively multiply-oriented. Right: the PIC-map of one prism, from the box area on the left, shows that domains of calcite nanoparticles have different orientations, displayed by different gray levels. Quantitatively, the various nanocrystals in this prism are spread by more than 20°.



Work performed on ALS Beamline 11.0.1

Citation:  P. U. P. A. Gilbert, A. Young, and S. N. Coppersmith, "Measurement of c-axis angular orientation in calcite (CaCO3) nanocrystals using X-ray absorption spectroscopy," PNAS 108, 11350-11355 (2011)