Formation of diverse Mn oxides: a review of bio/geochemical processes of Mn oxidation

Mn (oxyhydr)oxides, widely distributed in most geological settings, are found in more than 30 different types with different atomic arrangements, a wide range of crystallinity, and mixed valences of structural Mn. Each type of Mn (oxyhydr)oxides is characterized by its crystal structure and the valence state of structural Mn, which are likely the primary factors controlling its distinct reactivity. It is well known that varying pathways of Mn(II) oxidation, including homogeneous, surface catalyzed, or microbial processes, are responsible for the diversity of Mn (oxyhydr)oxide formation. However, the investigation of Mn(II) oxidation products through various oxidation processes have not been fully understood yet partly because the chemical and mineralogical identification of nanosized and poorly crystalline Mn (oxyhydr)oxides has been difficult at times. In this work, the previous studies on the formation of diverse Mn (oxyhydr)oxides through varying geochemical or microbial oxidation processes are thoroughly reviewed. This work may provide general background information for understanding the complex bio/geochemical processes controlling the formation of Mn (oxyhydr)oxides under various conditions of the natural environment.