Comparison of the Crystallographic Structure of Semi Nacre and Nacre by Electron Backscatter Diffraction

Abstract

Aragonite nacre is commonly found in the shells of molluscs, while semi nacre has been observed in the shells of organisms from two phyla: the Brachiopoda and the Bryozoa. Calcite semi nacre and aragonite nacre display similar morphologies with laminae composed of tablets that grow by screw dislocation. This study uses electron backscatter diffraction to compare the crystallography of calcite semi nacre from the shell of the Craniid brachiopod NoVocrania huttoni with aragonite nacre from the shell of the bivalve mollusc Mytilus edulis. Semi nacre and nacre are similar in morphology but ultimately have different crystallography. In N. huttoni calcite semi nacre, the c-axis lies parallel to the laminae of the secondary layer of the shell. In comparison, the c-axis of aragonite nacre in M. edulis is orientated perpendicular to the laminae. The crystallography of calcite semi nacre in the secondary layer of N. huttoni is similar to the crystallography of skeletal ultrastructures produced by bryozoans, while it differs from the crystallography described in other brachiopod genera.

Mineral bridges in nacre

“…Our study, together with previous evidence (Di Masi and Sarikaya, 2004;Dalbeck et al, 2006;England et al, 2007;Metzler et al, 2007;Gilbert et al, 2008;Gries et al, 2009b) leads us to conclude that there is an exact match in orientation between a given tablet and the one(s) that grow(s) on top of it. This feature evidently calls for the necessary communications of aragonite crystals across the interlamellar membrane.…”

Nacre biomineralisation: A review on the mechanisms of crystal nucleation

“…It was subsequently demonstrated by electron diffraction that in the bivalve nacre, domains of 3 to 10 crystals in a stack share the same orientation of their a-axis [16,53]. These observations were corroborated by follow-up studies both on bivalves and gastropods, using a variety of techniques such as synchrotron X-ray diffraction [54], electron back-scattered diffraction (EBSD) [55][56][57], X-ray absorption near-edge structure spectroscopy (XANES) and X-ray photoelectric emission spectromicroscopy (XPEEM) [58,59] and TEM [60]. For gastropods, the crystallographic alignment has been described to persist over a range of ~10 to 40 tablets [58].…”

Biomineralization: Elemental and Organic Influence in Carbonate Systems

“…The trace element distribution is not constant in rhynchonelliform brachiopods with higher concentrations of Mg, S, and Sr in the primary layer and the outer part of the secondary layer. 185 Strontium and magnesium reach fairly constant concentrations in the inner part of the shell, with the concentration of magnesium relating to water temperature. 189 …”