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Abstract
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.