XRD and TEM studies on nanophase manganese oxides in freshwater ferromanganese nodules from Green Bay, Lake Michigan

Freshwater ferromanganese nodules (FFN) from Green Bay, Lake Michigan have been investigated by X-ray powder diffraction (XRD), micro X-ray fluorescence (XRF), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM). The samples can be divided into three types: Mn-rich nodules, Fe-Mn nodules, and Fe-rich nodules. The manganese-bearing phases are todorokite, birnessite, and buserite. The iron-bearing phases are feroxyhyte, goethite, 2-line ferrihydrite, and proto-goethite (intermediate phase between feroxyhyte and goethite). The XRD patterns from nodule cross sections suggest the transformation of birnessite to todorokite. The TEM-EDS spectra show that todorokite is associated with Ba, Co, Ni, and Zn; birnessite is associated with Ca and Na; and buserite is associated with Ca. The todorokite has an average chemical formula of Ba2+ 0.28(Zn+2 0.14Co3+ 0.05Ni2+ 0.02)(Mn4+ 4.99Mn3+_0.82Fe3+ 0.12Co3+ 0.05Ni2+ 0.02)O12·nH2O. Barium is the main cation in the structural tunnels of the todorokite. The average chemical formula of birnessite and Ca-buserite are: Na+ 0.14Ca2+ 0.19(Mn4+ 1.48Mn3+ 0.52)O4·nH2O and Ca2+ 0.27(Mn4+ 1.46Mn3+ 0.54)O4·nH2O, respectively. Most nodules have a concretionary structure of alternating Fe/Mn layers, commonly with a core of reddish feldspar containing phyllosilicates and hematite micro-crystals. Other cores consist of goethite, cristobalite, tridymite, hercynite, hematite-bearing quartz, coal, and chlorite-bearing rock fragments. The hexagonal or hexagonal-like structures of hematite micro-crystals and clay minerals in the cores may serve as heterogeneous nucleation sites for the Mn-oxides and Fe-(oxyhydr)oxides. The alternating Fe/Mn layers in FFNs might be caused by oscillatory redox condition fluctuations at the sediment-water interface due to changes in water level.