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S C I E N T I F I C H I G H L I G H T S

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Visualising lattice distortions in biogenic crystals across scales

Biologically formed minerals have a morphology and crystallographic properties that are significantly different from their abiotically formed counterparts. Dark-field X-ray microscopy was used to study lattice distortions in biogenic calcite from the shell of P. nobilis mollusk, from the nano- to the macroscale. The results offer new insights into the biomineralisation process.

Understanding how living organisms form biological assemblies can provide unprecedented knowledge in the field of biomineralisation and biological materials, and offer explicit tools for designing and generating synthetic functional structures. However, despite the fact that numerous state-of-the-art characterisation methods have been employed to generate information on various biomineralisation processes, until now, a detailed multi- scale correlative study of biogenic minerals had never been achieved.

To study lattice distortions in biogenic calcite, individual prisms from the shell of P. nobilis mollusk were investigated using the newly developed Dark Field X-ray Microscopy (DFXM) technique at beamline ID06-HXM. DFXM provided detailed information on the local crystallographic orientation and lattice spacing of biogenic calcite in the prisms, demonstrating intricate patterns of lattice incoherence and distortion in 3D (Figures 110a-c). However, most importantly, it was possible to correlate

between these two local phenomena on the scale of the entire prism (Figures 110e-d).

The 3D reconstruction of the local d-spacing between (10 14) lattice planes of the first 25 µm was compared to the lattice distortion behaviour (Figures 110a-c). Specifically, the local degree of rotation around the [10 10]-axis and around its perpendicular, the [1 210]- axis, were calculated. Lattice spacing (Figure 110a) and orientation maps (Figures 110b-c) demonstrate a distinct striation pattern where the lattice rotates back and forth by approximately a tenth of a degree, and the d-spacing, d, between the {10 14} lattice planes alternates between 3.028 Å and an increased value of 3.041 Å. The observed striations follow specific crystal directions that run parallel to the [10 10] direction of calcite.

In order to link changes in lattice spacing and orientation along the prism s long-axis, values averaged over lines drawn perpendicular to the direction of growth for every longitudinal slice through the prisms were calculated for three datasets: d-spacing between (10 14) planes (Figure 110a), rotation around the [10 10]-axis (Figure 110b) and around the [1 210]-axis (Figure 110c). Astonishingly, when plotting lattice spacing against the measured lattice rotation along the long axis of the prism, a distinct correlation between lattice distortions and rotation around the [10 10] axis is observed. The banding only slightly affects the rotation around the [1 210] axis (Figure 110e). These data suggest that the registered rocking of the crystallographic orientation

Fig. 110: Analysis of lattice orientation and spacing in a P. nobilis prism using DFXM. a-c) Longitudinal sections of the first 25 µm of a P. nobilis prism showing d-spacing, and lattice rotation around the [10 10] axis and the [1 210] axis,

respectively. The isometric scale bars are 10 µm. d,e) Lattice spacing along the direction of growth (black) calculated as the average value of all pixels on lines perpendicular to the direction of growth as indicated in (a), in comparison with rotations

around the [10 10] axis (d) and the [1 210] axis (e).