Microstructure-Compositional Variation in Iron Oxy-Hydroxide Minerals Formed with Manganese Mineralization, Eastern Ghats Supergroup, Orissa

Large volume of iron oxy-hydroxide minerals occur in association with manganese oxide phase in manganese ores of the Eastern Ghats Supergroup, Koraput district, Orissa. On the basis of OH content, the iron mineral can be classified into hydrohematite, goethite, and hydrogoethite. These minerals exhibit eight types of microstructures such as ooloidal, caterpillar, disseminated, reniform, worm, mosaic, globular and spherulitic. Chemical composition of such micro-structures obtained through EPMA, distinguishes them into three domains, developed under different environment. Hydrohematite, having 8-10 % H₂O, characteristically contains higher pc of manganese (>5%) and phosphorous (>0.6%) but have low silica and alumina. These are formed syngenetically with Mn-oxide minerals in a manganese rich paragenesis. Goethite containing 11 to 12% water, has relatively low level of manganese and phosphorous, and formed in a later period, as secondary open space filling. Hydrogoethite shows very high water content (>16%), almost devoid of manganese, and impoverished in phosphorous but having higher percentage of alumina, silica and appreciable copper and nickel. This was latest to form in the zone of oxidation under supergene condition.