THE MINERALOGY, CRYSTAL CHEMISTRY, AND ORIGIN OF FRESHWATER FERROMANGANESE NODULES FROM GREEN BAY, LAKE MICHIGAN

The mineralogy, incorporation mechanism of As, Ba, Co, Ni, and Zn, and origin of ferromanganese nodules from Green Bay were characterized by in-situ XRD, SEM, HRTEM, Z-contrast imaging, and ab initio calculations using DFT method. Green Bay nodules can be divided into three types: Mn-rich nodules, Fe-Mn nodules, and Fe-rich nodules. Mn-rich nodules are composed of todorokite, birnessite, and buserite. Fe-rich nodules consist of feroxyhyte, goethite, 2-line ferrihydrite, and an FeOOH nano-phase with guyanaite structure. Most of the nodules appear to be concretionary structure of alternating Mn- and Fe-rich bands with a nucleus. The nuclei commonly have a nucleus of reddish feldspar containing hematite micro-crystals. The hexagonal structure of cations in core minerals can serve as nucleation sites for Fe/Mn hydroxides. For Mn-oxides, XRD patterns indicate transformation from birnessite to todorokite after precipitation. TEM-EDS reveal that Ba, Co, Ni, and Zn are associated with todorokite, whereas Ca is associated with the interlayer of buserite. Todorkite has an average chemical formula of Ba²⁺_0.28(Zn⁺²_0.13, Co³⁺_0.05, Ni²⁺_0.02)(Mn⁴⁺_5.01, Mn³⁺_0.54, Fe³⁺_0.38 Co³⁺_0.05, Ni²⁺_0.02)O₁₂•nH₂O. Ba²⁺ is the main cation in the structure tunnels of todorokite. The chemical formula of Ca-buserite is Ca²⁺_0.19(Mn⁴⁺_1.62, Mn³⁺_0.38)O₄ •nH₂O. For Fe-hydroxides, Z-contrast images identified FeOOH nano-domains with guyanaite structure intergrown with goethite. DFT calculation revealed goethite is more stable phase than proto-goethite. Overall, it is proposed that ordering between Fe and vacancies in octahedral sites resulted in transformation from feroxyhyte to goethite through intermediate structure of proto-goethite. Combining Z-contrast images and TEM-EDS reveal that the arsenate AsO₄ tetrahedron may be preferentially retained at the proto-goethite surface through tridentate attachment. Alternating Fe/Mn banded FFN are formed by slow oscillating redox conditions at the sediment-water interface. Physical characteristic and mineralogy of Green Bay FFN may be an analog for the formation of Mars “blueberries” in early stage formation via groundwater diagenesis. The amorphous component of Mars soils at Gale Crater might have similar Fe nano-phases with Green Bay FFN according to CheMin’s XRD and APXS.