Ocean-floor Manganese Nodule

Required Geological Setting

Ocean floor

Occurrence

Mn-Fe nodules formed on the ocean floor. Nodules are generally globules 10-40 mm in diameter. Concentric structture is observed in nodules on cores of rock fragments and shark teeth. Mn-Fe nodules grew a few mm to a few cm per a million years. Nodules are oberved on modern deep-sea basins of the Pacific ocean and the Atlantic ocean, and in Cretaceous to Jurassic sediments. Nodules crops out on the deep-sea ocean floor or in shallow zones of ocean-floor sediments less than a few cm. It is composed of Mn and Fe with minor Ni and Cu. Wet density: 1.9-2.0 g/cm3, dry density: 2.5-3.5 g/cm3, water content: 20-30 wt%, porosity: 50-60 %, specific surface area: 100-300 m2/g, Mohs' hardness: 1-4. It is considered that nodules were formed by deposition of metallic elements in seawater during diagenesis process of sediments in weakly reductive zone on the ocean floor. As growth speed of nodules is very slow, they are not observed in such circumstances as continental shelves where large amounts of sediments are supplied.

Mineral Assemblages

Todorokite, Birnessite, Buserite, Pyrolusite, Goethite