VATERITE FOUND IN SALTWATER NATURAL PEARLS FROM PINCTADA SP. MOLLUSK

Pearls are produced by a variety of mollusk species as the result of biomineralization, which is a complex combination of biochemical and physiological processes. The majority of saltwater pearls formed by various Pinctada species (or “pearl oysters”) contain a nacreous surface, with aragonite being the main mineral structure, although occasionally calcite formation has also been observed. Vaterite, the least thermodynamically stable form of calcium carbonate, has only been previously found on the lackluster areas of the surfaces in freshwater cultured pearls, or near the center of freshwater tissue nucleated pearls regardless of their surface luster quality.

In this study, we report a group of natural saltwater pearls reportedly from the Pinctada radiata mollusk sourced from the pearl oyster bed (Heir Al Adan) near the water of Kuwait that contain vaterite. This rare polymorph of calcium carbonate was found on the surfaces and/or cross-sectional areas of these pearls. Raman spectroscopy and X-ray diffraction (XRD) analyses were carried out in order to characterize the structure of these pearls, which confirmed the presence of vaterite, along with aragonite and calcite. Additional analyses using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) will be further performed to provide additional morphological and trace elemental information. Our results showed that vaterite not only existed in freshwater pearls as previously reported, but also can be found in marine bivalve mollusks. To the best of our knowledge, this is the first time vaterite was conclusively identified in saltwater pearls and pearls of natural origin, and such information provided invaluable insights into calcium carbonate biomineralization of these unique organic gems.