ISYEB Institut de Systématique, Evolution, Biodiversité, UMR 7205 CNRS MNHN Sorbonne-Université, EPHE Muséum National d'Histoire Naturelle, 75005 Paris, France.
Department of Materials Science and Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel.
J Struct Biol. 2019 Feb 1;205(2):121-132. doi: 10.1016/j.jsb.2019.01.004. Epub 2019 Jan 24.
Cross-sections of calcitic prismatic layers in mollusk shells, cut perpendicular to growth direction, reveal well-defined polygonal shapes of individual "grains" clearly visible by light and electron microscopy. For several kinds of shells, it was shown that the average number of edges in an individual prism approaches six during the growth process. Taking into account the rhombohedral symmetry of calcite, often presented in hexagonal axes, all this led to the long-standing opinion that calcitic prisms grow along the c-axis of calcite. In this paper, using X-ray diffraction and electron backscatter diffraction (EBSD), we unambiguously show that calcitic prisms in pearl oyster Pinctada margaritifera predominantly grow perpendicular to the c-axis. The obtained results imply that the hexagon-like habitus of growing crystallites may be not necessarily connected to calcite crystallography and, therefore, other factors should be taken into consideration. We analyze this phenomenon by comparing the organic contents in Pinctada margaritifera and Pinna nobilis shells, the later revealing regular growth of calcitic prisms along the c-axis.
珍珠贝 Pinctada margaritifera 棱柱层的横截面与生长方向垂直，通过光镜和电子显微镜清晰地显示出个体“晶粒”的多边形形状。对于几种贝壳，研究表明，在生长过程中，单个棱柱的平均棱数接近 6。考虑到方解石的菱面体对称性，通常呈六方轴，所有这些都导致了长期以来的观点，即方解石棱柱沿着方解石的 c 轴生长。在本文中，我们使用 X 射线衍射和电子背散射衍射（EBSD），明确地表明珍珠贝 Pinctada margaritifera 中的方解石棱柱主要垂直于 c 轴生长。所得结果表明，生长的晶体呈六边形状不一定与方解石结晶学有关，因此，应该考虑其他因素。我们通过比较珍珠贝 Pinctada margaritifera 和 Pinna nobilis 贝壳中的有机含量来分析这种现象，后者揭示了方解石棱柱沿 c 轴的规则生长。
