1976 Volume 26 Issue 140 Pages 371-384
Petrological, mineralogical and chemical investigations of marine manganese nodules from the West Pacific revealed the intimate relation between the chemical and mineral compositions and the remarkable preferential partitioning of metal elements in the ferromanganese minerals. The microscopic observations of textures of manganese nodules tell the growth history of manganese nodules and the formation conditions of ferromanganese minerals.
Chemical compositions of nodules from Komahashi-Daini Seamount are very similar to those of the nodules from marginal banks and seamounts. Compositional variations in the bulk composition of nodules collected from the same dredge haul are considerably small, suggesting the similarity of the growth history of individual nodules, although the contents of metal elements vary remarkably from layer to layer in a single nodule.
Such variations and inter-element relationships are generally controlled by the mineral compositions and the chemical properties of costituent ferromanganese minerals, especially of 10 Å manganite. Most of Cu and Ni are concentrated in 10 Å manganite and Mn/Ni ratios of 10 Å manganite are fairly constant. High Fe contents in the nodules analysed in the present work are attributed to δ-MnO2, which is composed of cryptocrystalline aggregates of Mn oxide and amorphous Fe hydroxide.
Microscopic observations and electron microprobe analyses of ferromanganese minerals revealed that the formation mechanisms of the two major constituent ferromanganese minerals are evidently different from each other. Optical and chemical homogeneity and characteristic textures, such as veinlets and clastic structures of 10 Å manganite suggest that it was rapidly formed in a short period as compared to δ-MnO2, probably due to the supply of metal elements from interstitial waters of associated sediments. On the other hand, it is inferred that δ-MnO2 was formed through slow accumulation of colloidal Mn- and Fe-hydroxide, incorporating many detrital materials. Therefore it is presumed from the present results that the growth of manganese nodules was not uniform during their formation but variable and intermittent in accordance with the environmental variation of the sea bottom.
