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As in most biominerals, the oyster shell is a composite of inorganic crystals and an organic matrix. Its mineral is primarily calcitic calcium carbonate and its matrix is made up largely of highly acidic phosphoproteins. Among molluscs, this type of matrix may be confined to those which form the foliated microstructure. These phosphoproteins form a continuum of molecular sizes ranging from modest molecular weight soluble proteins to insoluble complexes. The fact that similar sequences or even peptides can be identified among the various size classes suggests that all the classes may have common precursors. The primary structure of the major reverse phase HPLC fraction is roughly analogous to the phosphoryns of tooth dentin, in that it contains domains enriched in aspartic acid,

other domains

enriched in phosphoserine, and a terminal hydrophobic domain. The proteins have high affinities for crystal nuclei and calcite crystals, but not calcium, at high ionic strength. From these activities, it is hypothesized that the insoluble complexes bind nuclei, initiating crystal or crystal layer growth, and that the soluble components adsorb to crystal surfaces and, thereby, both control crystal morphology and limit crystal growth