Manganese nodule
Polymetallic nodules, also called manganese nodules, are mineral concretions on the sea bottom formed of concentric layers of iron and manganese hydroxides around a core. As nodules can be found in vast quantities, and contain valuable metals, deposits have been identified as a potential economic interest. Depending on their composition and authorial choice, they may also be called ferromanganese nodules. Ferromanganese nodules are mineral concretions composed of silicates and insoluble iron and manganese oxides that form on the ocean seafloor and terrestrial soils. The formation mechanism involves a series of redox oscillations driven by both abiotic and biotic processes. As a byproduct of pedogenesis, the specific composition of a ferromanganese nodule depends on the composition of the surrounding soil. The formation mechanisms and composition of the nodules allow for couplings with biogeochemical cycles beyond iron and manganese. The high relative abundance of nickel, copper, manganese, and other rare metals in nodules has increased interest in their use as a mining resource.
Nodules vary in size from tiny particles visible only under a microscope to large pellets more than across. However, most nodules are between in diameter, about the size of hen's eggs. Their surface textures vary from smooth to rough. They frequently have botryoidal texture and vary from spherical in shape to typically oblate, sometimes prolate, or are otherwise irregular. The bottom surface, buried in sediment, is generally rougher than the top due to a different type of growth.
Occurrence
Nodules lie on the seabed sediment, often partly or completely buried. They vary greatly in abundance, in some cases touching one another and covering more than 70% of the sea floor surface. The total amount of polymetallic nodules on the sea floor was estimated at 500 billion tons by Alan A. Archer of the London Geological Museum in 1981.Polymetallic nodules are found in both shallow and deeper waters, even in lakes, and are thought to have been a feature of the seas and oceans at least since the deep oceans were oxygenated in the Ediacaran period over 540 million years ago.
Polymetallic nodules were discovered in 1868 in the Kara Sea, in the Arctic Ocean of Siberia. During the scientific expeditions of HMS Challenger, they were found to occur in most oceans of the world.
Their composition varies by location, and sizeable deposits have been found in the following areas:
- Penrhyn Basin within the Cook Islands.
- North central Pacific Ocean in a region called the Clarion–Clipperton zone roughly midway between Hawaii and Clipperton Islands.
- Peru Basin in the southeast Pacific, and
- Southern tropical Indian Ocean in a region termed the Indian Ocean Nodule Field roughly 500 km SE of Diego Garcia Island.
- In the Eastern Pacific, including the area around Juan Fernández Islands and the abyssal plain offshore Loa River.
All of these deposits are in international waters apart from the Penrhyn Basin, which lies within the exclusive economic zone of the Cook Islands.
Growth and composition
In both marine and terrestrial environments, ferromanganese nodules are composed primarily of iron and manganese oxide concretions supported by an aluminosilicate matrix and surrounding a nucleus. Typically terrestrial nodules are more enriched in iron, while marine nodules tend to have higher manganese to iron ratios, depending on the formation mechanism and surrounding sedimentary composition. Regardless of where they form, the nodules are characterized by enrichment in iron, manganese, heavy metals, and rare earth element content when compared to the Earth's crust and surrounding sediment. However, organically-bound elements in the surrounding environment are not readily incorporated into nodules.Marine nodules
On the seabed the abundance of nodules varies and is likely controlled by the thickness and stability of a geochemically active layer that forms at the seabed. Pelagic sediment type and seabed bathymetry likely influence the characteristics of the geochemically active layer.Nodule growth is one of the slowest of all known geological phenomena, on the order of a centimeter over several million years. Several processes are hypothesized to be involved in the formation of nodules, including the precipitation of metals from seawater, the remobilization of manganese in the water column, the derivation of metals from hot springs associated with volcanic activity, the decomposition of basaltic debris by seawater and the precipitation of metal hydroxides through the activity of microorganisms. The sorption of divalent cations such as Mn2+, Fe2+, Co2+, Ni2+, and Cu2+ at the surface of Mn- and Fe-oxyhydroxides, known to be strong sorbents, also plays a main role in the accumulation of these transition metals in the manganese nodules. These processes may operate concurrently or they may follow one another during the formation of a nodule.
Manganese nodules are essentially composed of hydrated phyllomanganates. These are layered Mn-oxide minerals with interlayers containing water molecules in variable quantities. They strongly interact with trace metals because of the octahedral vacancies present in their layers. The particular properties of phyllomanganates explain the role they play in many geochemical concentration processes. They incorporate traces of transition metals mainly via cation exchange in their interlayer like clay minerals and surface complexation by formation of inner sphere complexes at the oxide surface as it is also the case with hydrous ferric oxides, HFO. Slight variations in their crystallographic structure and mineralogical composition may result in considerable changes in their chemical reactivity.
The mineral composition of manganese-bearing minerals is dependent on how the nodules are formed; sedimentary nodules, which have a lower Mn2+ content than diagenetic, are dominated by Fe-vernadite, Mn-feroxyhyte, and asbolane-buserite while diagenetic nodules are dominated by buserite I, birnessite, todorokite, and asbolane-buserite. The growth types termed diagenetic and hydrogenetic reflect suboxic and oxic growth, which in turn could relate to periods of interglacial and glacial climate. It has been estimated that suboxic-diagenetic type 2 layers make up about 50–60% of the chemical inventory of the nodules from the Clarion–Clipperton zone whereas oxic-hydrogenetic type 1 layers comprise about 35–40%. The remaining part of the nodules consists of incorporated sediment particles occurring along cracks and pores.
The chemical composition of nodules varies according to the type of manganese minerals and the size and characteristics of their core. Those of greatest economic interest contain manganese, nickel, copper and cobalt. Other constituents include iron, silicon and aluminium, with lesser amounts of calcium, sodium, magnesium, potassium, titanium and barium, along with hydrogen and oxygen as well as water of crystallization and free water. In a given manganese nodule, there is one part of iron oxide for every two parts of manganese dioxide.
A wide range of trace elements and trace minerals are found in nodules with many of these incorporated from the seabed sediment, which itself includes particles carried as dust from all over the planet before settling to the seabed.
The size of marine ferromanganese nodules can range from a diameter of 1–15 cm, surrounding a nucleus. The nucleus itself can be made from a variety of small objects in the surrounding environment, including fragments from previously broken down nodules, rock fragments, or sunken biogenic matter. Total nodule composition varies based on the formation mechanism, broadly broken down into two major categories: hydrogenetic and diagenetic. Hydrogenetic nodules have a higher iron and cobalt enrichment with manganese to iron ratios less than 2.5, while diagenetic nodules are more enriched with manganese, nickel, and copper with manganese to iron ratios typically between 2.5 and 5 but upwards to 30+ in sub-oxic conditions. The parent mineral for hydrogenetic nodules is vernadite and buserite for diagenetic nodules. The majority of observed nodules are a mixture of hydrogenetic and diagenetic regions of growth, preserving the changes in formation mechanisms over time. Generally, diagenetic layers are found on the bottom where the nodule is either buried in or touching the sea floor sediment and hydrogenetic layers are found towards the top where it is exposed to the overlying water column. Nodule layers are discontinuous and vary in thickness on micro to nanometer scale with those composed of higher manganese content typically brighter and those with higher iron content dark and dull.