Crystallography of the foliated calcite shell layers of bivalve molluscs

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Abstract
The foliated calcite of living scallops (Pectinacea), oysters (Ostreacea), window-pane and jingle shells (Anomiacea) consists of approximately 250 nm thick sheets (folia) formed of subparallel laths joined laterally. The surfaces of these very thin folia are not the basal pinacoid of calcite (0001), as has previously been stated, but in each species studied are one of the rhombohedral forms (1011), (1012) or (0112). One of these rhombohedra, (1012), is rarely found in non-biological calcite because it lies perpendicular to a direction of very fast crystal growth. Another form, (1011), is much more widespread; it occurs commonly in inorganic crystals, and it lies parallel to the secreting surface in the skeletal calcite of some scallops and oysters, in the pentaliths of some planktonic algae, in the tests of some forams, and in the outer layer of the hen eggshell. By contrast, the foliated calcite of some extinct articulate brachiopods appears to be composed of laths which are flattened parallel to (0001); this material therefore differs significantly from the foliated calcite of bivalve molluscs. The ability of anomiids to specify the rare calcite form (1012) may result from a precise stereochemical fit between the amino acid residues of the matrix protein and the spacing of calcium atoms in the surfaces of the underlying and overlying mineral layers. Analogous or homologous β-sheets may specify the two other types of foliated calcite.