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.
Polarized Raman spectroscopy is a useful technique in studying orientation of molecular vibrations of crystals, which, however, has been applied insufficiently in crystallography, mineralogy, or geology. This present study is devoted to measure the transversal orientation of high‐quality cultivated pearls using polarized Raman spectroscopy. The pearls were cultivated by Pinctada fucata in Japan, consisting of aragonite dominantly. On the pearl surface, the aragonite micro‐crystals show irregularly hexagonal and pentagonal shapes, orienting their c axes towards the pearl surface. A number of aragonite micro‐crystals makes up a lamina with a noncircular contour, and a batch of laminae stacks up with a decreasing area from bottom to top to form a target pattern. So there is an abundant of target patterns spreading over the pearl surface. Using a marker on the pearl surface as a reference, angular variation of aragonite is quantifiable by measuring mode intensities of aragonite and depolarization ratio. Polarized Raman images are collected on the pearl surface, which show spatial differences in orientation of aragonite micro‐crystals on the mesoscopic scale. This transversal heterogeneity exists not only between different laminae but also between aragonite micro‐crystals on the same lamina. We suggest that the development of the stress around the aragonite micro‐grains lead to the angular variations of aragonite. The orientation of biominerals has been studied using transmission electron microscope (TEM) and electron back‐scattered diffraction (EBSD). The present study indicates another sensitive technique of polarized Raman spectroscopy to evaluate molecular orientation in biominerals.
Chemical zoning is commonly seen in inorganic minerals, and it holds implications for magmatic processes, hydrothermal evolution, or metal mineralization. However, few studies are available on oscillatory zoning in biominerals, which carries the most direct and unique information on the biomineralization mechanism. This study investigates spatial and temporal resolution of trace element distributions in pearls that were cultivated in the Hyriopsis cumingii mantle (non-nucleated pearls and nucleated Akoya pearls) or visceral mass (nucleated Edison pearls) for 1–5 years. Using synchrotron radiation micro-X-ray fluorescence imaging, we find a variety of trace elements such as Sc, Cr, Mn, Cu, Zn, Ge, and Ba. The types of trace elements are slightly different for individuals. For the first time, submillimeter-scale Mn zoning is identified ubiquitously, concentric with the pearl, and exhibits increased concentrations toward the pearl margin. In non-nucleated pearls and nucleated Akoya pearls, the Mn zones are superposed with a spatially damped pattern with a decrease in the interzones. In contrast, no damped trends are observed in the Mn zones in nucleated Edison pearls. This difference may be due to different cultivation sites within mollusks that have different requirements for Mn during pearl growth. We suggest a growth model of dissipative structure for the Mn zoning in pearls, which depends upon the coupling between the interface kinetics and the diffusion of chemical species in the environment. The trace elements (including Mn) substitute Ca in aragonite isomorphically, based on Raman imaging. The scanning electron microscopy images show a periodic structure of aragonite platelets and organic matter of pearls. Locally in the Mn zones, there are minor defects on platelets, which may arise from the enrichment of trace elements. This study would develop a new research field for chemical zoning in minerals and introduce a new angle in understanding trace element incorporation in biominerals and the biomineralization processes.
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