The fascinating world of mesocrystals

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In this last lecture, we focused on mineral or organo-mineral hybrid edifices with complex macroscopic shapes, partly resulting from a construction process mediated by mesoscopic assemblies or mesocrystals. This "chemistry of shapes" is fascinating. It makes it possible to synthesize materials with extraordinary ultra-structures in the shape of embedded bi-tetrahedra, cauliflower, leaves, coral, earthworms and so on. We have illustrated our lesson with two examples, whose mechanisms - although not fully elucidated - have been studied extensively by different authors via numerous characterization techniques.

The first example concerns the study of the growth of crystals of fluorohydroxyapatite (FHAP), a calcium phosphate, the mineral component of human compact bone, in the presence of a biopolymer, gelatin. Tracking the various stages in the formation of these fluoroapatite-gelatin composites reveals a cascade of processes. In the first stage, hybrid mesocrystals made up of calcium phosphate nanocrystals assemble in the presence of the biopolymer to form hexagonal rods that are precursors to more complex structures. This co-organization process is followed by fractal morphogenesis generated by the poisoning of certain sites, the orientation of which is induced by the intrinsic electric field of the hexagonal bar. This dendritic growth leads to the formation of cauliflower-like structures, which in terminal mode lead to the formation of single spheres, followed by core-crown spheres. In this example, organic matter has multiple functionalities. Through selective adsorption processes, it enables surface energies to be modified, which defines the number and location of calcium phosphate nucleation sites, determines the orientation of crystal growth and the aggregation mode of FHAP nanoparticles. However, it is the electric field of the mineral phase that is responsible for fractal morphogenesis. The question raised by this study and commonly accepted in many scientific communities is: do mineralization processes need to be combined with organic or biological matter to generate sinuous, curved shapes, or even more complex forms such as leaves, earthworms, helices? In other words, should the observation of minerals with curved, complex shapes be systematically associated with a biotic character and therefore with the hypothetical presence of life?

The second example in this lesson, the synthesis of biomorphs, invalidates this hypothesis. In fact, biomorphs are complex systems with hierarchical structures built from very simple mineral chemistry. They are mineral-mineral composites composed of barium carbonate and silica. In a basic environment, in the presence of silicates, a barium salt and carbon dioxide dissolved in the air in the form of carbonates, the crystallization of alkaline-earth carbonates _MCO3 (M = Ba, Sr, Ca) with curved and varied shapes can be achieved in a few hours. The formation mechanism is complex and has only been studied for biomorphs with the simplest resulting shapes (leaves, earthworms, helicoids, tulips, etc.). Under basic synthesis conditions, a barium carbonate nucleation step generates anisotropic nanoparticles whose growth is halted by the precipitation of silica on their surface. These reactions, carbonate and silica precipitation, are both controlled by the local acidity of the medium. They take place in alternation: a more basic environment leads to the precipitation of alkaline carbonate and thus the consumption of carbonate anions. The medium then becomes locally more acidic, leading to the formation of silica, whose precipitation causes the pH to rise, favoring carbonate precipitation. A succession of inhibited precipitations is thus observed, each giving rise to a set of anisotropic _BaCO3 nanocrystals coated with a thin layer of silica. These elongated nanocrystals form mesoscopic aggregates, analogous to those observed in certain liquid crystals, which propagate as a function of time in the form of a linguiform wave. Real-time monitoring of the evolution of these systems by microscopy shows that it is the hydrodynamics of this aquo-mineral wave (formation of lips resembling those observed in surf waves, of equal or different height and curvature) and its azimuthal and longitudinal progression speeds that are at the origin of the final shapes obtained (leaves, ground glasses, helicoids).

Finally, we have shown that the morphologies of certain biomorphs (totally mineral, therefore) are close to certain living forms such as G. Ferruginea bactérium, the Stentor protist or the Ebola virus. So let's avoid jumping to conclusions, because appearances can be deceptive: biomorphs are biomimetic but abiotic. The earthworm or helical filament shape observed in some fossil minerals cannot therefore be systematically attributed to the presence of an ancient life form.