Biomineralization of Barite in the Shell of the Freshwater Asiatic Clam Corbicula fluminea (Mollusca: Bivalvia)

Rosette-shaped clusters of barite crystals were found on the depositional surface and in the inner, complex crossed-lamellar shell layer of Corbicula fluminea collected from the Maurice River, New Jersey. Morphological analyses revealed that barite crystallized directly from the extrapallial fluid onto the inner shell layer in summer and was covered by inner shell-layer aragonite in fall. Ba, S, and Sr were detected in energy-dispersive X-ray analyses or rossetes, while X-ray diffraction of the heavy fraction of finely ground shells revealed the presence of a barite mineral composed of $\sim75 mol% BaSO_4$ and $25 mol% SrSO_4$. It is suggested that barite formation resulted from the purging of soluble Ba from the tissue into the extrapallial fluid where it crystalized onto the shell in a biologically inert form.