Crystallographic orientation inhomogeneity and crystal splitting in biogenic calcite

Abstract

The calcitic prismatic units forming the outer shell of the bivalve Pinctada margaritifera have been analysed using scanning electron microscopyelectron back-scatter diffraction, transmission electron microscopy and atomic force microscopy. In the initial stages of growth, the individual prismatic units are single crystals. Their crystalline orientation is not consistent but rather changes gradually during growth. The gradients in crystallographic orientation occur mainly in a direction parallel to the long axis of the prism, i.e. perpendicular to the shell surface and do not show preferential tilting along any of the calcite lattice axes. At a certain growth stage, gradients begin to spread and diverge, implying that the prismatic units split into several crystalline domains. In this way, a branched crystal, in which the ends of the branches are independent crystalline domains, is formed. At the nanometre scale, the material is composed of slightly misoriented domains, which are separated by planes approximately perpendicular to the c-axis. Orientational gradients and splitting processes are described in biocrystals for the first time and are undoubtedly related to the high content of intracrystalline organic molecules, although the way in which these act to induce the observed crystalline patterns is a matter of future research.

Evidence of a Biological Control over Origin, Growth and End of the Calcite Prisms in the Shells of Pinctada margaritifera (Pelecypod, Pterioidea)

“…In accordance with the reported opinions of Wada, Wilbur, Taylor and Kennedy, Checa et al [24] admit that a direct crystallization allows explaining the polygonal morphology of the prisms. Polygonal organization of the calcite is generally attributed to physical mechanisms.…”

“…As typical for calcareous biominerals, these grains are covered by an irregular cortex (Figure 9h). From Dauphin [23] up to Checa et al [24], such a type of infra-micrometric organo-mineral structure has been recognized among various mollusk shell components and was extended to the skeletons of practically all other calcifying phyla [25]. Regarding the Pinctada's inital calcified structures it can be seen that overall orientation of the rods (themselves built by the submicrometric grains, Figure 9d-h) is consistent with the growth direction of the shell.…”

Ocean acidification reduces the crystallographic control in juvenile mussel shells

“…Since well constrained crystallographic control is a feature of biomineral formation (e.g. Checa et al, 2013;Pérez-Huerta, and Cusack, 2008;Schmahl et al, 2004), diminished crystallographic control may have equally detrimental effects to decreased structural integrity. This possibility should be tested in order to determine the potential influence of OA on shell integrity since reduced crystallographic control may be less apparent yet potentially as damaging as reduced structural integrity.…”

Evolution of Microstructure in Rolled Mg-Based Alloy. Textural Aspect / Ewolucja Mikrostruktury W Walcowanym Stopie Na Bazie Mg. Aspekt Teksturowy

“…Analysis of the EBSD results was performed using a dedicated software developed at IMMS PAS that allows elaborated correcting and filtering orientation maps and reveals accurate characteristics of disorientations between the grains [12,13].…”