Crystallization of biogenic Ca-carbonate within organo-mineral micro-domains. Structure of the calcite prisms of the Pelecypod Pinctada margaritifera (Mollusca) at the submicron to nanometre ranges

Atomic force microscopy (AFM) and transmission electron microscopy (TEM) were used to investigate the the Structure of the calcite prisms from the pearl-oyster shell Pinctada margaritifera. The AFM analysis shows that the prisms are made of densely packed circular micro-domains (in the 0.1 mu m range) surrounded by a dense cortex. The TEM images and diffraction patterns allow the internal structure of the micro-domains to be described. Each of them is enriched in Ca-carbonate. Hosted in distinct regions of each prism. some are fully amorphous, and some others fully crystallized as Subunits of a large calcite single crystal. At the border separating the two regions, micro-domains display a crystallized core and an amorphous rim. Such a border probably marks out an arrested crystallization front having propagated through a previously bio-controlled architecture of the piling of amorphous micro-domains. Compared to recent data concerning the stepping mode of growth of the calcite prisms and the resulting layered organization at the mu m-scale, these results give unexpected views regarding the modalities of biocrystallization.