Apatite X-ray linear dichroism reveals mesoscale crystal orientations in parrotfish bone and teeth*

Apatites are an important biomineral, being a major component of bone, dentin, enamel, and enameloid in a variety of organisms. Yet, the use of x-ray linear dichroism in the study of these minerals remains largely unexplored. We observed x-ray linear dichroism at the Calcium L and K edge in geologic hydroxyapatite (Ca₅(PO₄)₃OH), fluorapatite (Ca₅(PO₄)₃F), and their biogenic counterparts in parrotfish bone, dentin, and enameloid. Furthermore, we explain this phenomenon with detailed models of the apatite crystal and calculated spectra, in excellent agreement with experimental spectra¹. This discovery enables visualization of the nano- to micro-scale structure of apatite crystals in human teeth and bone using Polarization-dependent Imaging Contrast (PIC) maps². Illustrative new PIC maps of parrotfish enameloid show a most complex arrangement of fluorapatite crystals, bundled and interwoven³, which makes parrotfish enameloid uniquely capable of biting stony corals.

(1) C. A. Stifler et. al. PRL 2017, under review.
(2) PUPA Gilbert et al. PNAS 108, 2011. DOI: 10.1073/pnas.1107917108
(3) MA Marcus et al. ACS Nano in press 2017, DOI:10.1021/acsnano.7b05044