DISORDERED DOLOMITE FOUND IN THE CENTER OF A NATURAL PEARL FROM A CASSIS SP. MOLLUSK

Biomineralization of mollusk shells is a complex combination of biochemical and physiological processes. Bivalvia and Gastropoda are two classes of species where majority of world’s pearls can form, and their shells are predominantly made of aragonite and calcite, two most thermodynamically stable forms of calcium carbonate. Other varieties of calcium carbonate structures such as vaterite, amorphous calcium carbonate, and high magnesium-calcite have also been found in these materials, although with much less frequency.

Natural pearls are produced without human intervention, mainly due to various irritations from the surrounding environment to their mantle tissues. Pearls usually possess similar mineral compositions to the host shells, which means they are also dominated by aragonite and calcite. In this study, we report a natural pearl from a Cassis species mollusk containing granular central structures. Raman spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), energy dispersive X-ray spectroscopy (EDS), powder X-ray diffraction (XRD) and in-situ XRD analyses were carried out in order to characterize the mineral composition in the center region of this pearl. Our results showed that this pearl’s center was made of mostly disordered dolomite (Ca_0.53Mg_0.47CO₃) mixing with small amount of aragonite and high magnesium-calcite. To the best of our knowledge, this is the first time disordered dolomite was conclusively identified inside of a pearl and such information provided invaluable insights into the internal growth structures and formation of natural pearls.