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.