Manganese is a silvery-gray transition metal element identified by the symbol Mn and atomic number 25. It is a ubiquitous element, ranking as the 12th most abundant element in the Earth’s crust, constituting approximately 0.1% of its mass. It is widely distributed across the planet, found in rocks, soils, sediments, and various water bodies. The metal does not naturally occur in its pure form but is found in combination with other elements within mineral compounds.
Primary Geological Reservoirs
The vast majority of manganese on Earth is locked within the solid structure of the planet’s crust and ocean floor. On land, manganese exists in numerous mineral forms, which serve as the primary source for the element. Some of the most significant manganese minerals include pyrolusite (manganese dioxide, \(\text{MnO}_2\)) and rhodochrosite (manganese carbonate, \(\text{MnCO}_3\)). These concentrated mineral deposits are the focus of global mining operations.
Beyond terrestrial deposits, enormous concentrations of manganese are found in the deep ocean in the form of ferromanganese nodules. These polymetallic concretions are lumps composed mainly of iron and manganese oxides, often lying loosely on the abyssal plains of the ocean floor at depths between 3,000 and 6,000 meters. The largest deposits of these nodules are concentrated in the Clarion-Clipperton Zone in the Pacific Ocean.
Environmental Transport and Bioavailability
Once released from its geological reservoirs through weathering and erosion, manganese enters the environmental cycle, where its mobility is governed by chemical reactions. Manganese can exist in several oxidation states, with the most common being the oxidized, less soluble Manganese(IV), and the reduced, more soluble Manganese(II) (\(\text{Mn}^{2+}\)). This shift between oxidation states, driven by redox chemistry, determines how easily the element moves through water and soil.
In soil, only the divalent form (\(\text{Mn}^{2+}\)) is readily absorbed by plant roots. The availability of this soluble form is highly dependent on the surrounding soil conditions. High soil pH, common in alkaline environments, causes the soluble \(\text{Mn}^{2+}\) to rapidly oxidize and precipitate into less available manganese oxides. This is why manganese deficiency in crops is a common problem in calcareous or over-limed soils.
Manganese also enters the aquatic system through the weathering of rocks and the decomposition of organic matter. In surface water and groundwater, the element’s solubility increases under low oxygen (anaerobic) conditions and in acidic environments. Microorganisms play an active role, influencing manganese cycling by altering the local pH or directly oxidizing the soluble \(\text{Mn}^{2+}\) form.
Essential Role and Dietary Sources
Manganese is a trace mineral required by all living organisms, including humans, to support various biological processes. Within the human body, it functions primarily as a cofactor, necessary for the activation of important enzymes. One of its most significant roles is as a component of the antioxidant enzyme superoxide dismutase (Mn-SOD), which helps neutralize harmful free radicals.
The mineral is also involved in the metabolism of carbohydrates, amino acids, and cholesterol, as well as playing a part in bone formation. Since the body cannot produce manganese, humans must acquire it through diet. The primary natural sources of manganese in the human diet are plant-based foods.
Foods rich in manganese include:
- Whole grains, such as brown rice and oats.
- Nuts and seeds, including hazelnuts and pecans.
- Legumes.
- Leafy green vegetables.
- Certain beverages like tea.
The element is stored in the body, mostly concentrated in the bones, liver, kidneys, and pancreas, where it supports metabolic and structural functions.