Small black to brown concretionary
nodules made mostly of manganese salts and manganese
oxides are common on the floors of many parts of the
world’s oceans and great lakes, especially in areas of slow
pelagic sedimentation. They generally contain about 15–30
percent manganese (Mn) and so represent a significant manganese
resource, though technically difficult to mine because
of their depth beneath the ocean. They also contain smaller
amounts of copper, cobalt, nickel, and other minerals. Manganese
nodules were first discovered during the 1872–76 voyage
of the research vessel H.M.S. Challenger.
The nodules consist of a number of different minerals,
including compounds of manganese and iron. The nodules
are typically concentrically layered like an onion and have a
nucleus of foreign material where growth started. The nucleii
are highly varied between different nodules, with examples
including radiolaria, diatoms, clays, basalts, and even shark’s
teeth. Growth rates are thought to be about a few millimeters
per million years. Manganese occurs primarily as the oxide
MnO2, but also as the mineral todorokite and birnessite. Iron
occurs as goethite and as ferric hydroxide. The cobalt, copper,
and nickel typically occur in the manganese minerals and
absorbed on the surface of manganese dioxide.
The mechanism of growth of manganese nodules and the
origin of the Mn and other metals has remained elusive.
Manganese may be derived from erosion of the continents or
from seafloor volcanism. Once in the oceans, the metals may
accumulate in the sedimentary pile that forms on the seafloor
and gradually move upward through the sediments where it
becomes oxidized and accumulates in nodules on the seafloor.
There has been speculation that microorganisms might be
involved in the process, but no evidence of biologic processes
has yet been documented.
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