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. thumb|manganese nodule thumb|nodules on the seabed nodules vary in size from tiny particles visible only under a microscope to large pellets more than  across. however, most nodules are between  in diameter, about the size of potatoes. their surface is generally smooth, sometimes rough, mammilated (knobby) or otherwise irregular. the bottom, buried in sediment, is generally rougher than the top. ==growth and composition== nodule growth is one of the slowest of all known geological phenomena, on the order of a centimeter over several million years. several processes are involved in the formation of nodules, including the precipitation of metals from seawater (hydrogenous), the remobilization of manganese in the water column (diagenetic), the derivation of metals from hot springs associated with volcanic activity (hydrothermal), the decomposition of basaltic debris by seawater (halmyrolitic) and the precipitation of metal hydroxides through the activity of microorganisms (biogenic). several of these processes may operate concurrently or they may follow one another during the formation of a nodule.{ thumb|polymetallic 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 %). other constituents include iron (6%), silicon (5%) and aluminium (3%), with lesser amounts of calcium, sodium, magnesium, potassium, titanium and barium, along with hydrogen and oxygen. == occurrence == nodules lie on the seabed sediment, often partly or completely buried. they vary greatly in abundance, in some cases touching one another and covering more than 70% of the sea floor. the total amount of polymetallic nodules on the sea floor was estimated at 500 billion tons by alan a. archer of the london geological museum in 1981. they can occur at any depth, even in lakes, but the highest concentrations have been found on vast abyssal plains in the deep ocean between {{convert|40
Manganese_minerals Underwater_mining