Crystallography (Electron Backscatter Diffraction) and Chemistry (Electron Probe Microanalysis) of the Avian Eggshell

Abstract

Biomineralized structures are often used as indicators of environmental conditions in which they have grown. This study investigates the distribution of two trace elements, Mg and Na, commonly used in these environmental proxies, in the eggshells of seven different avian species to investigate the behavior of these elements in a mineralized system produced in a constant temperature environment where salinity is also strictly controlled. We used electron backscatter diffraction (EBSD) in conjunction with electron probe microanalysis (EPMA) to examine the eggshell structure to establish possible relationships between the crystallographic structures and the trace element distribution within each specimen as well as between species. A universal trend between species can be established for crystallographic structure and trace element distribution with some exceptions. Mg and Na vary across the eggshell profile in a carbonate system produced in a constant temperature environment, with links to crystallography remaining ambiguous. The reasons for variability in trace element distribution in such systems may therefore be more closely related to variation in organic distribution than merely environmental and physical crystallographic factors. The crystallographic data show the nucleation of small crystals on the mammillary caps with "fanning" of orientation until fusion of the mammillary caps to create large single crystals in the palisade layer with the elongation along the c-axis in the 〈0001〉 direction. The Mg and Na concentrations both decrease after nucleation but vary in their distribution and concentration between species after the nucleation stage of the shell.

Hierarchical structure of marine shell biomaterials: biomechanical functionalization of calcite by brachiopods

“…Assuming that the triad axis [001] of calcite is the fastest direction of growth, crystals with [001] parallel to the main growth direction of the shell will extend their size as long as Ca 2+ and carbonate are added, while the growth of other crystals with [001] more parallel to the plane of nucleation is impeded by their neighbors and their growth will stop. Texture and microstructure formation by competitive growth has been described for the calcite of the avian egg shell (Rodriguez-Navarro & Garcia-Ruiz, 2000; Dalbeck & Cusack, 2006).…”

A Howiesons Poort tradition of engraving ostrich eggshell containers dated to 60,000 years ago at Diepkloof Rock Shelter, South Africa

“…4, Bottom). The heterogeneity of calcite crystallite orientation in the ostrich eggshell (33,34) and the tool used for engraving can also influence the incision morphology.…”

Biomineralization: Elemental and Organic Influence in Carbonate Systems

“…30 Even in examples where the crystallographic orientation is constant, such as in avian eggshells, the distribution of elements such as magnesium is not homogeneous. 31,32 Although crystal lattice structure and partition coefficients account for some of the variation in chemical composition found in biominerals, they do not account for the exquisite biological control that leads to such diversity of ultrastructure. There is now accelerating interest in understanding the subtle control exerted by living systems on all aspects of biomineral formation.…”