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?
