Manganese-Iron Nodules from the Indian Ocean

239-253Manganese nOdules in the Indian Ocean cover a large area (over 10 million kml) which is next only to the Pacific Ocean. Nodules occur far from the major sources of terrigenous material in the Indian Ocean, the Bay of Bengal and the Arabian Sea, i.e. the deeper parts of the ocean where rates of sedimentation are low. Thin and polished sections of nodules Show a nucleus surrounded by wavy concentric rhythmic layers exhibiting colloform texture. Differential thermal analyses (DTA) of nodules indicate a prominent endothermic peak between 122 and 132°C,the curves resemble those of manganous manganite and vernadite with possibly admixture of gels. The amplitude and the area of this peak increases towards the Outer layers. Heating; at 100°Cfor more than 5 hr and at 600°Cfor 6 hr affects DTA curves and the reaction is irreversible presumably because of phase chang;es. X-ray a.nalyses of some samples indicate that these consist 10 and 7 A mang;anites/todorokite and birnessite. The resemblance of DTA curves to those of vernadite may be due to higher interlayer water in the Indian Ocean manganites/todorokite or larger admixture of gels. More than 120 chemical analyses of nodules (including; 20 new reported in this paper) indicate that Mn ranges from 0,5 to 37, Fe, 1•2 to 37'3, Ni 0•07 to 1,55,Cu 0•009to 1•36,Co 0•014to 0•998,Mo0•0045to 0•091,Pb 0'005 to 0•256. Cr 0•001 to 0•201 and Zn 0•009 to 0•085%. Concentration of Mn varies inversely to that of Fe. Mn increases with depth while Ni and Cu increase with Mn. Concentration of elements within a nodule also varies from the inner to the outer layers. Distribution maps prepared for Mn. Fe, Ni, Cu, Co and Mo indicate that large areas in the basins to the east of the Central Indian Ridge (Central Indian, West Australian and South Australian basins) contain nodules with a hiah percentag;e of Mn (> 15), Ni and Cu (> 0•25) and these appear to be more promisina basins for detailed exploration for 'ore grade nodules