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

Bivalve shell microstructures are important traits that can be used for evolutionary and phylogenetic studies. Here we examine the crossed lamellar layers forming the shells of the arcoids, Arca noae, Glycymeris glycymeris and Glycymeris nummaria in order to better understand the crystallography of this complex biomaterial. Textural aspects and crystallography of the outer crossed lamellar layer of these species have been clarified using high-resolution electron microscopy and X-ray diffraction (XRD) techniques. These shells are made of aragonite crystals in a crossed lamellar arrangement with a high preferred crystal orientation (texture). The distribution of maxima in the pole figures implies that there is not a single crystallographic pattern within the measured area, but a continuous variation between two classes of distinct patterns. In the first of these, there is a set of four crystal orientations (referred to as upper set). These four crystal orientations are distributed in two pairs (which are coplanar), with the crystals of each pair being twinned on {110}. The pairs are tilted with respect to each other by approximately 20-40º around an axis perpendicular to the {110} common twinning plane. In the second pattern (referred to as lower set), the crystal orientations of each pair are rotated around a <–110> direction until the a-axes of the diametrically opposing crystal orientations of two different twinned pairs become parallel. As a result of this rotation, in the lower set the crystal orientations no longer form pairs twinned on {110}.These crystallographic relationships are unknown in inorganic aragonite. Our results are similar to those reported for the neogastropod Conus marmoreus. Thus, the common crossed lamellar crystallography in the arcoid bivalves and in C. marmoreus is a striking example of convergence in the development of crossed lamellar microstructures.

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