Terrestrial Ferro-Manganese Nodules Formed in Limestone Areas of the Ryukyu Islands

Geochemical characteristics of terrestrial ferro-manganese nodules collected from various locations have been discussed through the study on major and minor chemical components for various sizes of the nodules. Manganese dioxide content of terrestrial ferro-manganese nodules varies from several to more than 20% with size from micronodule level of less than 0.1 cm and ordinary size of about 1 cm in radius. Minor element Co which is considered to be taken into nodules as tri-valent ion is very much concentrated as compared with bi-valent ions such as Ni, Zn, Cd, Pb and Cu which may be said to be captured on almost the same order of magnitude. Micro-nodules are depleted in manganese but enriched in iron. The chemical composition of the nodules changes very much with their size up to 0.2-0.3 cm in radius, while it remains almost similar to one another for larger size than 0.3 cm. The trend of minor element content with respect to size clearly shows that iron plays the most important role in the process of minor element concentration at the early stage of formation of micro-nodules, and that manganese dioxide taken into micro-nodules along with the growth of nodules becomes a predominant component for the concentration of minor elements such as Co, Ni, Cd, Pb and Cu in the later stage. Their contents then increase together with the growth of the nodules.