Crystallographic characterization of the crossed lamellar structure in the bivalve Meretrix lamarckii using electron beam techniques.

To understand the formation mechanism of crossed lamellar structures in molluskan shells, the crystallographic structural features in the shell of a bivalve, Meretrix lamarckii, were investigated using scanning electron microscopy, electron backscattered diffraction, and transmission electron microscopy with a focused ion beam sample preparation technique. Approximately 0.5 μm-thick lamellae (the second-order units) are piled up obliquely toward the growth direction to form the first-order unit and the obliquity is inverted between adjacent units along the shell thickness direction. The first-order units originate around the center of the shell, initially growing parallel to the shell and subsequently curving toward the inner or outer surfaces. The lamellae consist of aragonite granular and columnar layers, which group together to adopt the same crystal orientation forming crystallographic units (crystallites). Multiple {110} twins are common both in the granular and columnar layers. The crystallite c-axis is parallel to the columns and is inclined at angles 0-50° from the lamellar normal (dispersing among individual lamellae), toward the shell growth direction. Probably, the directions of the a- and b-axes are random in the lamellae, showing no specific orientation.