Excessive Uptake of Trace Elements Results in an Abnormal Structure of Nacre

Abstract

Nacre of shells and pearls is a common biomineral produced by mollusks, containing a variety of trace elements. Little attention has been paid on the mechanisms of trace element uptake into nacre and its influence on the microstructure of nacre. This knowledge, however, is a prerequisite for our understanding of nacre biomineralization and holds implications for pearl cultivating technology, ecological environment, and materials sciences. Here, we give serious consideration on nacre with an abnormal microstructure of Edison pearls, discovering correlations between the trace element concentrations and the morphology and microstructure of the nacre. Edison pearls are a new breed of freshwater nucleated pearls. The scanning electron microscopy images show that the abnormal area in the nacre has a brick-and-mortar structure as the normal nacre, in which aragonite platelets alternate with organic materials. However, the aragonite platelets vary widely in thicknesses, littered with microcrystal cracks, and present clear color contrasts and a rugged surface. Much higher counts of trace elements are detected such as Mn, Fe, Zn, and Ba in the abnormal nacre, using synchrotron radiation micro-X-ray fluorescence imaging. The abnormal nacre exhibits Raman mode characteristic of multiple carbonate minerals, which suggests that these trace elements substitute for Ca in the aragonite lattice and distort the crystal lattice. In comparison with the peripheral normal nacre, there are lower concentrations of organic materials in the abnormal nacre and that may contribute to the formation of transversal and orthogonal platelet cracks. As a result, the abnormal nacre has a loose coupling structure. Formation of abnormal nacre has individual difference, and it is hypothesized that the metabolic abnormalities of visceral mass after operation procedure results in excessive uptake of trace elements and the consequent formation of abnormal nacre.