Synchrotron Radiation μ-XRF Imaging Reveals Mn Zoning in Freshwater Pearls

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.