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Environmentally important, poorly crystalline Fe/Mn hydrous oxides: Ferrihydrite and a possibly new vernadite-like mineral from the Clark Fork River Superfund Complex

Michael F. Hochella, Jr.^1,*, Takeshi Kasama^2,, Andrew Putnis², Christine V. Putnis² and Johnnie N. Moore³

¹ Nanogeoscience and Technology Lab, Department of Geosciences, Virginia Tech, Blacksburg, Virginia 24061-0420, U.S.A.
² Institüt für Mineralogie and Interdisciplinary Centre for Electron Microscopy and Microanalysis, Universität Münster, Corrensstrasse 24, D-48149 Münster, Germany
³ Department of Geology, University of Montana, Missoula, Montana 59812, U.S.A.

Correspondence: ^* E-mail: hochella{at}vt.edu

Ferrihydrite and a vernadite-like mineral, in samples collected from the riverbeds and floodplains of the river draining the largest mining-contaminated site in the United States (the Clark Fork River Superfund Complex), have been studied with transmission electron microscopy (TEM) and energy dispersive X-ray (EDX) analysis. These poorly crystalline minerals are environmentally important in this system because contaminant heavy metals (As, Cu, Pb, and/or Zn) are always associated with them. Both two- and six-line ferrihydrite have been identified with selected-area electron diffraction. For the vernadite-like mineral, the two d values observed are approximately between 0.1 and 0.2 Å larger than those reported for vernadite, the Mn hydrous oxide that is thought to have a birnessite-like structure, but which is disordered in the layer stacking direction. In several field specimens, the ferrihydrite and vernadite-like minerals are intimately mixed on the nanoscale, but they also occur separately. It is suggested that the vernadite-like mineral, found separately, is produced biogenically by Mn-oxidizing bacteria, whereas the same mineral associated with ferrihydrite is produced abiotically via the heterogeneous oxidation of Mn²⁺ _aq initially on ferrihydrite surfaces. Evidence from this study demonstrates that the vernadite-like mineral sorbs considerably more toxic metals than does ferrihydrite, demonstrating that it may be a good candidate for application to heavy-metal sorption in permeable reactive barriers.

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