Crystalline arrangement and nanostructure of aragonitic crossed lamellar layers of the Meretrix lusoria shell

The crystallographic microstructure of Meretrix lusoria shells was investigated using scanning electron microscopy (SEM), X-ray diffractometry (XRD), and transmission electron microscopy (TEM). Crystallite sizes were determined by XRD analysis as 72 nm, which was quite similar to the 70 nm as measured by SEM. The shell comprised aggregates of hexagonal plates of aragonite (500 nm wide, 70 nm high) and organic matter. These plates were fourth-order units of an aragonitic crossed order lamellar structure. Subsequent TEM images showed the hexagonal plates’ nanostructure. The electron diffraction pattern of the fourth-order units revealed a consistent orientation of the hexagonal plates. The fourth-order lamellae (hexagonal crystallites) were piled up in the [0 0 1] direction to produce slender prisms (third-order lamellae), arranged mutually parallel, thereby forming a broad tablet (second-order lamellae). The second-order lamellae were piled up in different directions to form the first-order lamellae. The orientation level obtained from XRD and SEM images showed that the crossed lamellar layer was piled up curvilinearly, forming semi-circular growth lines. X-ray diffraction patterns of the cross-sections of the middle layer (vertical and parallel to the growth line) showed that the c axes of aragonite have a disposition of about 20° to the growth direction.