Synchrotron Radiation μ-XRF Imaging Reveals Oscillatory Zoning of Freshwater Pearls

Pearls and shells are common biominerals characterized with a highly organized laminar structure that is constructed by inorganic aragonite platelets alternating with organic material films. Formed in the bodies of mollusks, the uptake of trace elements into pearls and shells is under both biological control and environmental control. Thus, the spatial and temporal resolution of the trace element distributions could reflect the life activities of the mother mollusks and the chemistry of the environments. However, elemental maps in pearls and shells have not received due attention from previous studies. In this study, the elemental profile in cultivated pearls is carefully investigated using synchrotron radiation μ-X-ray fluorescence imaging. A variety of trace elements such as Ti, V, Mn, Fe, Ni, Cu, Sr and Ba has been detected. Most elements are distributed evenly in these pearls, with exceptions of Mn, Fe and Ba that show oscillatory zoning in freshwater pearls, coupled with the pearl growth pattern. Mn zoning is ubiquitous in all the freshwater pearls, with different numbers, width, positions, and Mn concentrations, etc. in individual. In some pearls, the Mn zoning are cooperative with the Fe zoning. It is suggested that the oscillatory zoning in freshwater pearls is a dissipative structure, and its formation results from metabolic rate changes of the mother mollusks after pearl-mantle-inserting operation. The presence of the Mn oscillatory zoning is expected to be a key character of freshwater pearls, as no such zoning is found in seawater pearls. Seawater pearls are featured with extremely low Mn concentrations. It is thus, speculated that marine Pinctada martensii have different requirements for trace elements in the course of biomineralization in comparison with freshwater Hyriopsis cumingii, and could maintain relatively stable metabolism during pearl growth. In view that pearls and shells show highly similar crystal chemistry, it is reasonable to believe that the elemental profile in shells may also be affected by the mother mollusks' activities. So the geochemical signature of shells should be treated with caution to reconstruct environments.