how do manganese nodules form
Polymetallic nodules, also called manganese nodules, are rock concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core.The core may be microscopically small and is sometimes completely transformed into manganese minerals by crystallization.When visible to the naked eye, it can be a small test (shell) of a microfossil (radiolarian or foraminifer), a phosphatized shark tooth, basalt debris or even fragments of earlier nodules.olymetallic nodules. The chemical composition of nodules varies according to the kind of manganese minerals and the size and characteristics of the core. Those of greatest economic interest contain manganese (27-30%), nickel (1.25-1.5 %), copper (1-1.4 %) and cobalt (0.2-0.25 %).