A feature of wide areas of the deep ocean floor below about 2000 m is the presence of sizable lumps or nodules lying on the surface of the sediment. These are known as manganese nodules or ferromanganese concretions, and occur in many irregular shapes, sizes and forms ranging in size from particles a millimetre or so in diameter to occasional huge lumps a metre or more across. They are especially numerous in the Pacific, where parts of the bottom are almost covered with nodules, but they are also found beneath other areas of deep ocean.
In structure the nodules usually have a lamellated form surrounding a central nucleus which may be a core of silty material, or sometimes a fragment of rock, a fish tooth, or even a whale ear-ossicle. Nodules from different areas are very variable in composition (Tooms and Summerhayes, 1969), but are commonly rich in manganese and iron. Mero (1960), gives the average composition by weight of 30 samples of nodules from all oceans as manganese dioxide 32 per cent, iron oxides 22 per cent, silicon dioxide 19 per cent, water 14 per cent, with smaller quantities of aluminium oxides, calcium and magnesium carbonates, and other metals including nickel, copper, cobalt, zinc and molybdenum.
Nodule formation and the way in which they become enriched with metals, is not fully understood but is basically a process whereby solids are formed by chemical reactions in sea water. The process is thought to be initiated by the precipitation of colloidal particles, for example, manganese dioxide or ferric hydroxide, which tend to attract ions of other metals from the water. These particles are electrically charged, and may be attracted to electrically conductive objects on the bottom which become the nucleus of a developing nodule. The availability of suitable nuclei may be one of the causes of their patchy distribution. The nodules form very gradually over a period of several million years. Abyssal nodules, which have the highest nickel and copper concentrations, are calculated to grow at a rate of 3 to 8 mm per million years.
Manganese nodules were first discovered by the Challenger expedition. Since then, specialized mining systems and ships such as the Prospector have been developed to lift the nodules, which can be treated to extract manganese, nickel, copper and cobalt. At present, only one area, between the Clarion-Clipperton fracture zone near Hawaii, appears to be economically attractive and so far, no-one has attempted commercial exploitation. Nevertheless, some mining claims have been filed and international consortiums formed to develop and test recovery systems.
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