ABSTRACT

Sand-size particles of vivianite (Fe₃(PO₄)₂ · 8H₂O) have been identified by X-ray diffraction and scanning electron microscopy in surficial sediments of the tidal Potomac River near a major sewage treatment plant. Vivianitc particles are concentrated in a surface layer of coarse-sand dredge spoil, which overlies much finer sediment similar to that found in adjacent areas. Although saturation indices indicate that the pore waters of virtually all the sediments in the study area are supersaturated with respect to vivianite, it is found only in association with dredge spoil. Pore-water profiles of Fe, P, and Eh, as well as the size and morphology of individual grains, indicate that the vivianite is authigenic and was not introduced with he dredge spoil. It is suggested that the major control on the occurrence of the mineral is the presence or absence of amorphous ferric oxy-hydroxides. which apparently react with pore-water phosphorus to form stable ferric hydroxy-phosphates preferentially to the formation of vivianite.