XRD and TEM Studies on Nanophase Manganese Oxides in Freshwater Ferromanganese Nodules from Green Bay, Lake Michigan

Abstract

AbstractFreshwater 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 Ba0.28p2+(Zn0.14p+2Co0.05p3+Ni0.02p2+) (Mn4.99p4+Mn0.82p3+Fe0.12p3+Co0.05p3+Ni0.02p2+) O12·nH2O. Barium is the main cation in the structural tunnels of the todorokite. The average chemical formula of birnessite and Ca-buserite are: Na0.14p+Ca0.19p2+(Mn1.48p4+Mn0.52p3+0.52) O4·nH2O and Ca0.27p2+(Mn1.46p4+Mn0.54p3+)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.