The Role of Clay Minerals in the Formation of Ferromanganese Nodules (FMNs) in Subtropical Monsoonal China

This study focuses on the developmental characteristics of ferromanganese nodules (FMNs) in the lower horizons of a subtropical soil profile, aiming to elucidate their formation mechanisms and mineralogical evolution. Large FMNs were analyzed using a suite of techniques, including X-ray diffraction (XRD), scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, optical microscopy, major element analysis, electron probe microanalysis (EPMA) and high-resolution transmission electron microscopy (HRTEM). These methods enabled detailed characterization of the metal oxide assemblages, elemental distributions, and structural features of the FMNs. Analysis of the soils surrounding the nodules reveals that the region has undergone intense chemical weathering, with a chemical index of alteration (CIA) exceeding 90, indicative of terminal-stage weathering. Key features include near-complete leaching of silicon, extensive Fe-Mn oxidation leading to abundant metal oxide assemblages, and the widespread formation of secondary phases such as gibbsite. The weathering intensity in the study area not only surpasses that of typical subtropical regions but also approaches the levels characteristic of tropical environments. The FMNs exhibit irregular morphologies and are composed of well-defined Fe-rich and Mn-rich regions, which contain abundant clay minerals and aluminum oxides such as kaolinite, illite, and gibbsite. Both regions host iron (hydr)oxides (FeOx) and manganese oxides (MnOx); however, the Fe-rich zones are dominated by well-crystallized FeOx (e.g., hematite and goethite), whereas the Mn-rich zones are enriched in MnOx, poorly ordered FeOx (e.g., ferrihydrite), and clay minerals, forming relatively dense microstructures. Valence analysis reveals that Fe occurs exclusively as Fe(III) in Fe-rich zones, while both Fe(II) and Fe(III) coexist with Mn(II), Mn(III), and Mn(IV) in Mn-rich zones, reflecting repeated redox fluctuations during nodule formation. Micro-scale observations further reveal the widespread presence of concentric, fine-grained Mn-rich sub-nodules within the Mn-rich zones, consisting of MnOx cores coated with clay minerals that exhibit Mn-Al interlayering. Repeated leaching cycles have significantly altered the redox conditions of the soil profile, diluting Mn2⁺ concentrations while promoting clay mineral weathering and transformation. Variations in oxidation environments, coupled with prolonged nodule growth, are likely to have contributed to the zonal differentiation within the FMNs. The results indicate that these nodules formed under extreme weathering conditions, where the progressive weathering of cemented clay minerals led to the formation of gibbsite and concentric Mn-rich sub-nodules, together shaping the irregular Mn-enriched domains within depositional-type FMNs. This study highlights the critical role of clay minerals in the genesis of giant FMNs in subtropical soils and provides new insights into their formation and evolutionary pathways.