Shape, Microstructure, and Chemical Composition of Pearls from the Freshwater Clam Diplodon chilensis Native to South America

SIMPLE SUMMARY: Natural pearls of Diplodon chilensis, a freshwater clam native to southern South America, are reported and characterized for the first time. The finding also constitutes the first record of pearls in a species of the genus Diplodon. The pearls have different shapes and sizes, and were found in both, male and female specimens. The microstructure and chemical composition of pearls is consistent with those reported in other bivalve species. ABSTRACT: The capability to produce pearls is widespread in the phylum Mollusca, including bivalves of the superfamily Unionoidea. Here, we identified and characterized natural pearls formed by Diplodon chilensis, a freshwater clam native to southern South America, using samples obtained from two lakes located in the Chilean Patagonia. Pearls were studied using light and scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), Fourier transform infrared spectroscopy (FTIR), and Raman spectroscopy. Naturally formed pearls were found in both male and female D. chilensis specimens. Pearls are produced in different shapes, including spherical, ellipsoidal, buttoned, and bumpy, ranging in size from 200 µm to 1.9 mm. The internal microstructure is composed of irregular polygonal tablets, about 0.40 to 0.55 μm in thickness. EDX analysis showed that pearls are composed of calcium carbonate. FTIR and Raman spectra recorded several peaks attributable to the aragonite in pearls of this species, as has been shown in other mollusks. In addition to these results, pearls of different colors are illustrated.