New Crystallographic Relationships in Biogenic Aragonite: The Crossed-Lamellar Microstructures of Mollusks

Abstract

Crossed-lamellar microstructures are the most common shell-forming biomaterials in mollusks. Because of their complex hierarchical 3D arrangement and small crystallite size, previous crystallographic studies are scarce and have centered on particular species with no comprehensive analysis available. To evaluate the crystallographic diversity of the crossed-lamellar microstructures, we have studied a large set of bivalve and gastropod species with crossed-lamellar layers using X-ray diffraction and electron backscatter diffraction. From the number, distribution, and relationships of maxima, we have classified pole figures into nine different recurring crystallographic patterns. According to their crystallographic equivalences, these patterns can be grouped into five groups. A first division is established according to whether there is one or two main orientations for the c-axis of aragonite. In the latter case, each orientation corresponds to one of the two sets of alternating first-order lamellae. The two main orientations of the c-axis diverge by rotation within the plane of the first-order lamellae around either a common a- or b-axis. We also show how some patterns may derive from others. Patterns with two c-axis orientations represent crystal relationships until now completely unknown in biogenic and abiogenic aragonite and are most likely produced by particular proteomic pools

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

“…5(b). Almagro et al 8) concluded that for a/b/2 patterns each 001 maxima and its associated 100 and 010 maxima corresponded Journal of the Ceramic Society of Japan 125 [5] 419-422 2017 exclusively to one of the adjacent 1st-order lamellae, as confirmed by their EBSD analyses. Thus, the a-and b-axes have a 64°r otation with respect to the corresponding ones in the adjacent 1st-order lamellae.…”

“…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 Conus marmoreus shell. Almagro et al 8) performed XRD and electron backscatter diffraction (EBSD) examinations on the samples of 20 species from Bivalvia and another 20 species from Gastropoda, and identified a detailed crystallographic diversity including nine types of texture patterns, which can be classified into five main groups with significantly different crystallographies. They also pointed out that the crossed-lamellar structure was not a single microstructure but rather a family of homeomorphic microstructures with complex and varied crystallographies.…”

“…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.…”

“…4. Almagro et al 8) assigned the resulting patterns a threedigit code (a/b/c) to indicate the number of the preferential orientations for the a-, b-, c-axes. They distinguished nine patterns (including 3/3/1, 2/2/1,¨/¨/1, 1/2/2, 5/6/2, 2/1/2, 6/5/2, 4/4/2 and 2/4/2) from 49 sets of 100, 010, 001 PFs among 20 species in Bivalvia and another 20 species in Gastropoda.…”

Cymbiola nobilis shell: Toughening mechanisms in a crossed-lamellar structure

“…While extensive studies have been focused on the simpler “brick-and-mortar” shell microstructure named nacre1718, there have been limited experimental efforts on the crossed-lamellar structure19 although this type of structure consists of more than 90% species within the Mollusca20. This structure is hierarchically assembled by ordinary brittle inorganic calcium carbonate with only 0.1–1 wt.% organic matrix2122.…”