Crystallographic structure of the foliated calcite of bivalves.

The foliated layer of bivalves is constituted by platy calcite crystals, or laths, surrounded by an organic layer, and which are arranged into sheets (folia). Therefore, the foliated microstructure can be considered the calcitic analogue to nacre. In this paper, the foliated microstructure has been studied in detail using electron and X-ray diffraction techniques, together with SEM observations on naturally decalcified shells, to investigate the crystallographic organization on different length scales and to resolve among previous contradictory results. This layer is highly organized and displays a coherent crystallographic orientation. The surface of the laths of the foliated layer is constituted by calcite crystals oriented with their c-axis tilted opposite to the growth direction of the laths and one of its {101 4} rhombohedral faces looking in the growth direction. These faces are only expressed as the terminal faces of the laths, whereas the main surfaces of laths coincide with {101 8} rhombohedral faces. This arrangement was consistently found in all specimens studied, which leads us to the provisional conclusion that, unlike previous studies, there is only one possible crystallographic arrangement for the foliated layer. Future studies on other species will help to ascertain this assertion.