Bacterially produced manganese oxide and todorokite: UV Raman spectroscopic comparison

UV Raman spectroscopic data measured at two distinct conditions, low and high laser power, establish that the manganese oxide (SP6-MnO _x) produced by the freshwater bacterium (Leptothrix discophora SP-6) closely resembles the 3 × 3-tunnel todorokite among the MnO ₂ materials studied. Under the two conditions, the effect of hydration/cation and the phase transition of todorokite or SP6-MnO _x to Mn ₃O ₄ or birnessite will be described. A higher concentration of Mg ²⁺ incorporated in the framework of the SP6-MnO _x than in todorokite is probably responsible for the formation of a new Raman band obtained at high laser powers, matching the most intense UV Raman band of synthetic birnessite. Also, we present the assignment of Raman bands for todorokite mineral and discuss the mutual exclusion principle that should hold for all the MnO ₂ materials studied. These experiments provide characterization of hydrous, poorly crystalline, or nanocrystalline metal oxides, which are frequently difficult to identify.