S C

IE N

T IF

IC H

IG H

LI G

H T

S X

-R A

Y N

A N

O P

R O

B E

7 7 I H I G H L I G H T S 2 0 2 2

corresponding to growth regions where early-stage crystalline units, referred to as prisms, form and grow. The oyster shell prisms often exhibit ring-like morphological patterns associated to the growth cycle involved in the biomineralisation process.

By combing several state-of-the-art, highly resolved quantitative microscopy methods, the physico-chemical descriptions of the early prisms and ring structure could be described, shedding light on the amorphous-to-crystalline transition (Figure 66). Owing to its sensitivity to chemical bonds, coherent Raman microscopy [2] was used to target specific chemical species. It was found that the ring-like structural features correlate with a lack of calcite content and an increase of amorphous calcium carbonate and protein content. Optical birefingent microscopy based on vectorial ptychography was applied to retrieve the spatially resolved full Jones matrix (i.e., the matrix that expresses how light is distorted by the optically anisotropic material) of the birefingent calcite prisms [3], directly related to

Fig. 66: Multi-modal analysis of the growth region of a Pinctada margaritifera shell. a) White light microscopy of the growth region. b) Same region as seen with coherent Raman microscopy. c) Zoom-in view exhibiting annular features in early mineralised units (prims). d) Composite chemical map of the same prism, showing the relative amounts of calcite (blue), amorphous calcium carbonate (red) and the amount of proteins (green). e) Optical retardance map of a prism obtained with vectorial ptychography. f) Strain map of an early prism, obtained from X-ray nanodiffraction. In (d-f), the arrows point towards annular features, which highlight the presence of a chemical, optical and crystalline contrast.

Fig. 67: Temporal model of prism formation in early stages. a) Initial early prism formation resulting from the

transformation of a mixture of amorphous CaCO3 and organics into calcite, following a radial progression and

a radial exclusion and thus transport of organics towards the unit edge leading to (b) a mostly crystalline prism. c) A subsequent layer forms underneath the first layer,

following the same transformations as described in (a). The crystal quality under the edge of the previously formed

disc is modified locally (strain and tilts), in the vicinity of the organic-rich region. d) Subsequent layers form, following

the same transformations as described in (a) and (b). Their lateral extend is limited by the nearest neighbours,

resulting in the formation of interfaces where organics accumulate. e) The prism grows by self-replication.

the crystalline properties of the biomineralising units. It showed that the rings are associated with a large crystalline disorder. Finally, X-ray nanoprobe diffraction, performed at beamline ID13 [4], allowed the measurement of the strain distribution within the prisms, observed to be significantly larger in the vicinity of the rings with respect to the other parts of the prisms.