The mollusc shell prismatic layer of Atrina rigida is composed of an assemblage of large and relatively perfect single calcite crystals, embedded in an organic matrix. A key to elucidating basic mechanisms of mineralization is understanding the structures of the matrix, the mineral and the relations between them. The matrix that envelopes each prism (the inter-prismatic matrix) is composed mainly of glycine-rich proteins, while the matrix inside each prism (intra-crystalline matrix) is composed of a network of chitin fibers. Prisms grow by deposition of mineral particles on the chitin fibers. The mineral particles are associated with highly acidic proteins from the Asprich family, which presumably stabilize an amorphous mineral precursor. We infer that once in contact with the already formed crystalline prism, the particles crystallize by epitaxial nucleation. In nacre, sheets of β-chitin are interspaced by silk-like proteins in a hydrated gel-like state. β-Chitin forms a scaffold onto which the acidic proteins are adsorbed. Some of these are organized into a crystal nucleation site, where nucleation of aragonite, supposedly from colloidal amorphous calcium carbonate particles, is induced. Comparing the mechanisms of growth of the nacreous and prismatic layers can help to understand the underlying strategies of formation of mineralized structures.
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