Hierarchically structured scleractinian coral biocrystals.

Microscopic (AFM and FESEM) observations show that scleractinian coral biomineral fibers in extant Desmophyllum and Favia, and fossil Jurassic Isastrea are composed of nanocrystalline grains of about 30-100 nm in size. In contrast to these findings, SR diffraction data on the same coral materials exhibit narrow Bragg peaks suggesting much larger crystallite size. These seemingly contradicting results of microscopic and diffraction studies are reconciled within a new, minute-scale model of scleractinian biomineral fibers. In this model, nanocrystalline aragonite units are interconnected by mineral bridges and form aggregates usually larger than 200 nm. Most likely, the size of the aggregates is resulting from physiological biomineralization cycles that control cellular secretion of ions and biopolymeric species. Intercalation of biopolymers into crystal lattice may influence consistently several structural parameters of the scleractinian coral bio-aragonite in all studied samples: (i) the lattice parameters and internal strains of the bio-aragonite are larger than in mineral aragonite, (ii) lattice parameter elongations and internal strains reveal directional anisotropy with respect to crystallographic axes.