Influence of crystallographic orientation of biogenic calcite on in situ Mg XANES analyses

Micro X-ray absorption near-edge spectroscopy at the Mg K-edge is a useful technique for acquiring information about the environment of Mg²⁺ in biogenic calcite. These analyses can be applied to shell powders or intact shell structures. The advantage of the latter is that the XANES analyses can be applied to specific areas, at high (e.g. micrometre) spatial resolution, to determine the environment of Mg²⁺ in a biomineral context. Such in situ synchrotron analysis has to take into account the potential effect of crystallographic orientation given the anisotropy of calcite crystals and the polarized nature of X-rays. Brachiopod shells of species with different crystallographic orientations are used to assess this crystallographic effect on in situ synchrotron measurements at the Mg K-edge. Results show that, owing to the anisotropy of calcite, in situ X-ray absorption spectra (XAS) are influenced by the crystallographic orientation of calcite crystals with a subsequent potentially erroneous interpretation of Mg²⁺ data. Thus, this study demonstrates the importance of crystallography for XAS analyses and, therefore, the necessity to obtain crystallographic information at high spatial resolution prior to spectroscopic analysis.