Pyrite with characteristic brass-yellow luster was found coating parts of the outer surface of living Mercenaria mercenaria and Geukensia demissa in a Georgia tidal marsh. Sections through one shell revealed additional pyrite replacing aragonite in parts of the outer shell layer, especially around fractures, borings, and concentrations of organic matrix. This is the first report of the precipitation of massive (metallic luster) pyrite in modern sediments and the first indication that pyritization can commence in a mollusk shell even before the death of the mollusk.

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