Manganese is also used in portland cement mixtures. Manganese is an important element for human health, essential for development, metabolism, and the antioxidant system. Nevertheless, excessive exposure or intake may lead to a condition known as manganism, a neurodegenerative disorder that causes dopaminergic neuronal death and symptoms similar to Parkinson's disease. The classes of enzymes that have manganese cofactors is large and includes oxidoreductases, transferases, hydrolases, lyases, isomerases, ligases, lectins, and integrins. The reverse transcriptases of many retroviruses (though not lentiviruses such as HIV) contain manganese. The best-known manganese-containing polypeptides may be arginase, the diphtheria toxin, and Mn-containing superoxide dismutase (Mn-SOD).
Manganese as rhodochrosite (MnCO). The most important manganese ore is pyrolusite (MnO). Other economically important manganese ores usually show a close spatial relation to the iron ores. Land-based resources are large but irregularly distributed. About 80% of the known world manganese resources are in South Africa; other important manganese deposits are in Ukraine, Australia, India, China, Gabon and Brazil. According to 1978 estimate, the ocean floor has 500 billion tons of manganese nodules. Attempts to find economically viable methods of harvesting manganese nodules were abandoned in the 1970s. The CIA once used mining manganese modules on the ocean floor as a