Manganese nodule

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. As nodules can be found in vast quantities, and contain valuable metals, deposits were identified as having economic interest in the 1960s by John Mero.

Nodules vary in size from tiny particles visible only under a microscope to large pellets more than 20 centimetres (8 in) across. However, most nodules are between 3 and 10 cm (1 and 4 in) in diameter, about the size of hens eggs or potatoes. Their surface textures vary from smooth to rough. They frequently have botryoidal (mammilated or knobby) texture and vary from spherical in shape to typically oblate, sometimes prolate, or are otherwise irregular. The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth.

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.

Polymetallic nodules are found in both shallow (e.g. the Baltic Sea) and deeper waters (e.g. the central Pacific), even in lakes, and are thought to have been a feature of the seas and oceans at least since the deep oceans oxidised in the Ediacaran period over 540 million years ago.

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