TRANSMISSION ELECTRON MICROSCOPY OF GRAIN AND BIOCOATINGS FROM LEAD AND ZINC CONTAMINATED SEDIMENTS IN THE LOWER COEUR D'ALENE RIVER VALLEY

Significant lead and zinc contamination occurs downstream from the Coeur d'Alene mining district. Prior sequential extraction and SEM examination of materials collected from different environments have demonstrated complex mineralogical relationships dependent on physical dispersion, local redox conditions, and biologic controls. A substantial amount of the total metals in the lower Coeur d'Alene valley are held as sub-micron grain or bio-coatings that defy adequate characterization using SEM and XRD analysis. Effective geochemical modeling and eventual remediation will require improved phase characterization. Analytical electron microscopy using a 200KV transmission electron microscope (TEM) capable of nano-beam electron diffraction and high resolution imaging has proven very useful in determining particulate metal speciation in these problematic materials. A new technique for examining heterogeneous grain mounts has been applied to four unconsolidated samples from different fluvial subenvironments in the lower Coeur d'Alene river valley. TEM analysis has resulted in identification of several new phases and better constraints on the chemistry of many other phases. In oxidized samples from river levees, Zn is strongly partitioned into siliceous ferri-hydrite with typical zinc concentrations of 0.5-3 oxide wt% in this common grain coating. Zinc is a minor component in more complex manganese oxy-hydroxides. Lead occurs in a wide variety of ferro-manganese/oxy-hydroxide phases that are typically non-stoichiometric and nano-crystalline (domains on the order of 5-10nm). In samples from transitional and reduced redox environments, a wide variety of lead bearing phases have been identified including microcrystalline galena and amorphous biocoatings with highly variable metal sulfide compositions (complex Pb-Zn-Cu-S stoichiometries). Siliceous, ferro-phosphatic spherules occur in one transitional redox sample and contain up to 30 wt% PbO. Zinc is largely constrained to microcrystalline sphalerite or mixed nano-crystalline sulfides in the transitional and reducing fluvial subenvironments. TEM analysis has demonstrated that particulate speciation is far more complex than expected from conventional SEM/XRD analysis of the same samples.