Polymetallic nodules are mineral-rich rock-like accretions, roughly the size of potatoes, that grow slowly on the deep seafloor of the abyssal plains, typically between 4,000 and 6,000 meters water depth. Their composition includes several metals deemed critical to modern technologies.
Metals found in polymetallic nodules
Nodules are mostly made up of manganese (around 29%) and iron (around 6%), bound together in layers of metal oxides. These form around a small nucleus – often a fossil shell, shark tooth fragment, or piece of basalt – and grow outward over millions of years. The nucleus provides a surface on which metals can accumulate. Metals slowly precipitate from seawater and from sediment pore-waters. These metals form concentric layers of oxides and hydroxides around the nucleus.
Polymetallic nodules lie unattached on the abyssal plains. A cross section of a polymetallic nodule. Image sources left to right: Geomar, ISA
In addition to manganese and iron, nodules can also contain economically important amounts of nickel (~1.4%), copper (~1.3%), and cobalt (~0.25%). These are sometimes referred to as battery metals because of their use in electric vehicles and renewable energy technologies.
Other elements include silicon, aluminum, calcium, magnesium, sodium, potassium, titanium, barium, and rare earth elements (REEs). The mix varies depending on the geological and chemical conditions where the nodules formed.
| Metal | Typical Range (%) | Average content by weight percent (% of total dry mass) | Typical role |
| Manganese (Mn) | 20 to 35 | ~29 | Steel alloys, batteries |
| Iron (Fe) | 5 to 15 | ~6 | Construction, magnets |
| Nickel (Ni) | 0.9 to 1.5 | ~1.4 | Stainless steel, Electric vehicle batteries |
| Copper (Cu) | 0.8 to 1.3 | ~1.3 | Electrical wiring |
| Cobalt (Co) | 0.2 to 0.5 | ~0.25 | Energy storage, catalysts, superalloys, aerospace |
| Silicon (Si) | 5 to 30 | ~5 | Glass, electronics |
| Aluminium (Al) | 2 to 5 | ~3 | Lightweight alloys |
| Other trace elements (Mo, Ti, Rare Earth Elements [REEs]) | < 1 total | <1 | Multiple uses including aerospace components, lenses, lasers, magnets, etc. |
Table showing the typical range and average content of various minerals and other trace elements in polymetallic nodules.
The value of a nodule field depends on how many nodules lie on the seafloor and their composition. Nodules with higher concentrations of nickel, copper, and cobalt are generally more sought after, because these metals are essential for low-carbon technologies.
Understanding the composition of polymetallic nodules is an important part of discussions about their future, from scientific research to environmental management and resource policy.