S T R U C T U R E O F M A T E R I A L S

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

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Learning from nature: revealing the links between structure and mechanical durability

The biomineralised tissues of nine marine organisms with skeletons composed of Mg-calcite were characterised using electron microscopy and powder X-ray diffraction techniques. The inclusion of high-Mg calcite nanoparticles within a low-Mg calcite matrix was found in skeletons with a Mg content higher than 14 mol%. This structural organisation enhances the mechanical durability of the skeletons.

Biominerals are inorganic components forming the skeletons of living organisms. They exhibit complicated structures and possess superior mechanical properties with respect to their synthetic counterparts and, for this reason, have drawn great attention from research scientists. This work reports a comparative study on the biomineralised skeletons of nine different organisms across different kingdoms and phyla. These organisms were selected despite their biological distinctions and were chosen based on the similar compositions of their skeletons, all composed of Mg-calcite.

A recent study showed that an average Mg content in the range of 14-48 mol% within the organisms skeletons permits spinodal decomposition of an amorphous Mg- calcite precursor, which upon crystallisation leads to the formation of high-Mg nanoparticles within a low-Mg calcite matrix [1]. The selected organisms were divided into two categories depending on the Mg content of their skeletons.

The first category, with a Mg content above 14 mol% (defined as the lower limit for spinodal decomposition), included organisms belonging to the animal kingdom, in particular, the brittle star Ophiactis savignyi (phylum Echinodermata), the soft coral Tubipora musica (phylum Cnidaria) and a starfish, Echinaster sepositus (phylum Echinodermata). A second category, with a Mg content lower than 14 mol%, comprised three organisms; namely spines of the sea urchins Phyllacanthus imperialis, Paracentrotus lividus and Heterocentrotus mammillatus (phylum Echinodermata). In addition, the discussion also included the data previously obtained for the case of two coralline red algae species belonging to the plant kingdom; Jania sp. and Corallina sp. (phylum Rhodophyta) [2], as well as previous data on the brittle star Ophiomastix wendtii (phylum Echinodermata) [3]. The skeletons (insets) of the studied organisms and their diverse microstructures, imaged using high-resolution scanning electron microscopy images, are presented in Figure 121.

Samples were characterised using state-of-the-art techniques, among them high-resolution transmission electron microscopy and synchrotron radiation high- resolution powder X-ray diffraction at beamline ID22, including isochronous annealing experiments as well as experiments performed in a high-pressured CO2 environment.

These experiments provided strong evidence of the presence of high-Mg calcite nanoparticles within the skeletons of various organisms with a Mg content above 14 mol% (the lower limit for spinodal decomposition).

Fig. 121: Six of the studied organisms demonstrating high-Mg calcite nanoparticles embedded within a low-Mg calcite matrix. All exhibit Mg concentration above 14 mol% within their skeletons.