Biomineral nanostructures of manganese oxides in oceanic ferromanganese nodules

Abstract

Manganese oxides, which are widespread and of great practical importance, are formed and transformed by the active role of microorganisms. Manganese aggregates occur as both crystallized varieties and disordered fine-grained phases with significant ore grade and up to 50–60 vol % of X-ray amorphous components. X-ray amorphous nanosized Mn oxides in Fe-Mn nodules from the Pacific Ocean floor were examined from the standpoint of their biogenic origin. SEM examination showed abundant mineralized biofilms on the studied samples. The chemical composition of bacterial mass is as follows (wt %): 28.34 MnO, 17.14 Fe2O3, 7.11 SiO2, 2.41 CaO, 17.90 TiO2, 1.74 Na2O, 1.73 Al2O3, 1.30 MgO, 1.25 P2O5, 1.25 SO3, 0.68 CoO, 0.54 CuO, 0.53 NiO, and 0.50 K2O. The chemical composition of fossilized cyanobacterial mats within the interlayer space of nodulesis as follows (wt %): 48.35 MnO, 6.23 Fe2O3, 8.76 MgO, 5.05 Al2O3, 4.45 SiO2, 3.63 NiO, 2.30 Na2O, 2.19 CuO, 1.31 CaO, and 0.68 K2O is direct evidence for participation of bacteria in Mn oxide formation. This phase consists of mineralized glycocalix consisting of nanosized flakes of todorokite. Native metals (Cu, Fe, and Zn) as inclusions 10–20 μm in size were identified in ferromanganese nodules as well. The formation of nativemetals can be explained by their crystallization at highly reducing conditions maintained by organic matter.