English: Pelagite (deep seafloor manganese nodule) from the Pacific Ocean seafloor. (8-9 mm thick)

Some portions of the deepest seafloors of the world have common to abundant, dark-colored, manganese-rich nodules. These are called pelagites. They were first discovered in the 1860s in the Kara Sea. They are composed of various Mn, Fe, Cu, Ni, and Co minerals. Some materials commonly identified in these nodules include todorokite ((Mn,Ca,Mg)Mn3O7·H2O), birnessite (~Na4Mn14O27·9H2O), amorphous hydrous Fe-hydroxyoxides, goethite, detrital volcanic aluminosilicates, zeolites, etc. Pelagites are surprisingly lightweight for their size, and do not have the look & feel of “normal rocks”.

Manganese nodules grow extremely slowly, estimated at ~5 mm per million years. Cross-sections show irregular concentric layering. They appear to form intermittently, rather than by continuous accretion of layers.

Manganese nodule formation is not well understood. Unsolved problems include: 1) explaining how they stay at the seafloor surface, even where sedimentation rates exceed nodule growth rates; and 2) larger nodules appear to grow more quickly. More than one growth mechanism may be involved in the genesis of manganese nodules around the world.

The pelagite specimen shown above is broken so that it shows a cross-section through its center. Irregularly concentric banding is visible, apparently nucleated around an orangish-brown rock fragment.