Crystallographic texture of crossed-lamellar structure in <i>Cymbiola nobilis</i> shell

Abstract

The crossed-lamellar structure is known to exhibit a fantastic microarchitecture and excellent fracture resistance. Despite a similar morphology, this structure belongs to a family of homeomorphic microstructures with complex and varied crystallographies. In the present study, the crystallographies of different parts in Cymbiola nobilis shell were studied. The width of the first building blocks becomes increasingly thicker from the top to bottom of inner and middle layers. All the crystallites tend to align their c-axes towards the shell surface, and two twinned crystals share the c-axis at the top of inner and middle layers, while at the bottom the 001 pole figure shows two orientation components with c-axes inclined, and four preferential b-axis orientations distributed into two pairs. Texture analysis provides important information about the nature of shell growth, and suggests that the shape of the building blocks may be controlled by the crystallographic arrangement during the aragonite deposition.©2017 The Ceramic Society of Japan. All rights reserved.Key-words : Cymbiola nobilis shell, Inner layer, Middle layer, Crossed-lamellar structure, Crystallographic orientation, Texture [Received December 30, 2016; Accepted March 13, 2017] It is well known that Nature exhibits a rich diversity, and ³8.74 million species of eukaryotes exist on Earth.1) Actually, the biodiversity is reflected not only by the number of species, but also by the types of microstructures of biomaterials, which usually extend over several distinct length scales. For example, in mollusk shells there are several commonly recognized microstructures, including nacreous, foliated, prismatic, homogeneous, crossed-lamellar and complex crossed lamellar structures, as well as various combinations of these microstructures.2) In recent years, mimicking the structural features of shells has become a fascinating and thriving area.3)7) However, our knowledge is limited on how mother Nature is used to fabricate the shell structures, which is of vital importance for the design of highperformance bioinspired materials.In general, biological shells are hierarchically structured. For example, one of the most complicated hierarchical structures is the crossed-lamellar structure. 8) Specifically, this structure is normally stacked by three-order lamellae, i.e., the 1st-order lamellae are composed of the 2nd-order lamellae that further consist of the parallel 3rd-order lamellae. 9)12) With a relatively small amount of organic (0.11.0 wt.%), 13),14) a nanometer-sized growth control of the crossed-lamellar structure is achieved. Chateigner et al. 15) used X-ray diffraction (XRD) to determine the orientation distributions on 46 kinds of Mollusc shells, and pointed out that the microstructures with a similar morphology may have different crystallographic textures. Rodriguez-Navarro et al. 13) reported that there were two sets of twin laws at the lower (the 3rd-order lamellae) and higher (the 1st-order lamellae) length scales in the crossed-lamellar structure in...

Deep origin of the crossed‐lamellar microstructure in early Cambrian molluscs

“…A, schematic diagram showing a complex hierarchical organization with four levels, based on Ji et al . (2017). B–C, Cymbiola sp., B, fracture surface showing stacked second‐order lamellae; C, etched inner surface showing the first‐order lamellae with barb‐like interlocks.…”