Several specimens of Mercenaria mercenaria and Guekensia demissa collected alive in early August from tidal marshes on St. Catherines Island, Georgia, were found to have a prominent brass-yellow material on part of the outer surface of their shells. On some individuals this may occur as a surficial coating, for a portion flaked off one specimen revealed an apparently unaffected outer shell surface. In another specimen, however, thin sections and sections examined by scanning electron microscopy (SEM) showed the material extended into the shell, following planes of weakness, such as growth lines and microborings, and replaced aragonite.

In thin section, the material is opaque to transmitted light and appears much like framboidal pyrite under oblique reflected light. Studies by SEM reveal a massive outer region grading into loosely packed masses of spheroids about 0.1µm in diameter. Energy dispersive X-ray analysis identified iron and sulfur as the principal components of this material. Preliminary X-ray diffraction studies on mechanical preparations of the outermost region of the shell were hampered by the low concentration of the material relative to aragonite; despite this, the strongest pyrite peak (311) was detected in both diffractometer and powder camera results. No other iron sulfides could be identified.

These observations strongly suggest that we are observing the process of calcium carbonate replacement by pyrite in a modern sedimentary environment.

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