Variability in the crystallographic texture of bivalve nacre

Abstract

The crystallographic texture of nacre in eleven bivalve species belonging to the superorders Opponobranchia, Pteriomorpha, and Paleoheterodonta, was studied. Our analysis confirmed a uniform orientation of the crystallographic c-axes of aragonite platelets in all the species studied, but a variable arrangement of a-and b-axes. New data suggest that the rate of alignment of a-and b-axes from girdle-like (unordered nacre) to a single crystal-like texture (ordered nacre) differs considerably among the analyzed species. Numerical evaluation of the data obtained from species with ordered nacre also revealed systematic differences in texture strength among the main crystallographic axes of aragonite platelets. The texture strength of a-axes (direction parallel to growth lines) is always weaker than that of b-and c-axes. Observed differences in shape of the growth lines on the surface of nacreous platelets, in twinning percentage as well as in rate of alignment of a-and b-axes suggest variations in nacre growth process in individual phylogenetic lineages of the Class Bivalvia. These facts may imply not only evolutionary changes in microstructure and crystallographic texture of the nacre but also in molecular mechanisms driving its production during the long molluscan evolution. • Key words: Bivalvia, nacre, crystallographic texture, phylogeny.

Experimental neutron pole figures of minerals composing the bivalve mollusc shells

“…A study of the mineral composition of Bivalvia mollusc shells and the orientational distribution of crystals in these shells may be useful in solving a number of fundamental problems. Previous attempts have been made to compare the mineral composition and texture of the shell crystals of Bivalvia molluscs from different families [11]. It should be noted that throughout this manuscript, we use the term 'texture' as synonym for the term 'crystallographic texture' , i.e.…”

“…The EBSD technique has been used to obtain crystallographic texture information about bivalve mollusc shells [11][12][13], and the pole figures for these shells have been measured with X-rays [14]. We use neutron time-offlight diffraction to measure pole figures.…”

“…Mytilus shells possess both calcite and aragonite phases with different content. The local crystallographic texture of Mytilus shells was studied in [11][12][13], and the global texture of Mya arenaria (Linnaeus, 1758) and Ostrea edulis (Linnaeus, 1758) has also been investigated.…”

Crystallographic texture determines mechanical properties of molluscan nacre

“…Two basic textural patterns (i.e. unordered and ordered) of the bivalve nacre have been documented (Frýda et al 2009(Frýda et al , 2010. The diagnostic character of the unordered nacre is a perfect preferred arrangement of [0 0 1] axis of individual platelets, and a random arrangement of their [1 0 0] and [0 1 0] axes.…”

Natural Pearls

“…Whether the shell's composition had to be replicated in order to replicate its aspect, was an open question 27 : gel media were associated with field observations of fast growing shell 28 , comparing the interaction of calcium carbonate with relevant organic media in vitro and in vivo [31,35] produced some interesting discussion on the nature of "organo-mineral complexes" that may direct shell patterns, but the composition of shell matrix was inaccessible 29 .…”