Incredible internal strains within a biogenic single crystal viewed by X-ray diffraction tomography

Abstract

The dorsal arm plates (DAPs) of the Ophiocoma Wendtii brittle star are highly functional single crystalline biominerals whose optimized structure and nanostructure enable them to fullfill mechanical and optical functions in the organism. Here, a large DAP bulk piece is characterized by means of synchrotron X-ray Diffraction Tomography (XRDT). This non-destructive crystallographic characterization revealed an astounding feature: the presence of very high compressive strains which relax when the mineral is cracked or grinded into a powder. Thus, previous destructive characterization techniques did not allow their detection. We attribute the compressive strains to the previously identified high-Mg calcite particles, which are coherently included and thereby compress the low-Mg calcite matrix. The measured slice contained both the bulk DAP sample as well as DAP powder. The data generated by the bulk piece could be separated from those by the powder, and the latter was used to calibrate and interprete the former. This study reveals yet another awe-inspiring feature of a biogenic structure, highlights the importance of non-destructive crystallographic characterization for biominerals, and exemplifies the potential of XRDT use in studying a single crystalline material, as well as the advantage of complementary measurement of bulk and powder for data calibration and interpretation.

Recent developments in X-ray diffraction/scattering computed tomography for materials science

“…calcite, quartz) provides information on diagenesis. The utility of XRD-CT in unveiling strain directionality in biogenic crystals is also recently demonstrated in a comparable work on a non-osseous Mg-calcite material at ID16B, ESRF in a study showing extremely high internal strains in the single crystal dorsal arm plates of the brittle star Ophiocoma Wendtii [147]. One of the best XRD-CT resolutions recorded in bone to date is a sub-120 nm dataset obtained by Palle et al [148] (figure 5) using a 12.7 keV 32.3 × 30.5 nm 2 beam on an ex vivo sample of cortical bone taken from a human iliac crest at ID13, ESRF, almost an order of magnitude smaller than the previous record low in bone reported by Wittig et al [149] around a year prior at DESY.…”

“…calcite, quartz) provides information on diagenesis. The utility of XRD-CT in unveiling strain directionality in biogenic crystals is also recently demonstrated in a comparable work on a non-osseous Mg-calcite material at ID16B, ESRF in a study showing extremely high internal strains in the single crystal dorsal arm plates of the brittle star Ophiocoma Wendtii [ 147 ].…”