Pyrolusite

(Mn,O)

Pyrolusite, the most important ore mineral of MANGANESE, forms dark gray to black coatings, crusts, columnar or granular masses, and rare crystals (polianite) that have one perfect cleavage. Mohs hardness is 2 to 6 (masses) or 6 to 6.5 (crystals), luster is metallic to dull, streak is black or bluish black, and specific gravity is 4.75 (masses) or 5.01 (crystals). Pyrolusite results from alteration of MANGANITE and other manganese minerals. The Earth's crust contains 850 ppm manganese in chemically bonded form. By far the most important manganese mineral is PYROLUSITE, which consists largely of manganese dioxide. Pyrolusite is brown black in color and often somewhat magnetic; the name manganese is a corrupted form of the Latin word for a form of magnetic stone, magnesia. Although manganese ores are not scarce, extraction is economically feasible only with open-cast mining. In addition, extensive deposits of manganese nodules are found at many sites on the ocean floor. Trace amounts of manganese are essential for plant growth, because the element is involved in nitrogen and iron metabolism and in the function of some of the enzymes involved in photosynthesis and respiration. Higher animals also require trace amounts of the element for activating many of the metabolic enzymes, for good bone structure, and for central nervous system functions. Pure manganese is rarely used, as it is a moderately reactive and brittle metal. About 95% of the world's annual production of manganese is used by the iron and steel industry to purify iron and to make alloys. Manganese is added to iron because it reduces iron oxide to form manganese oxide, which dissolves well in molten slag and is easily separated from the iron. In alloys, manganese increases the durability and corrosion resistance of iron and steel and makes steel more malleable when forged. This nonmagnetic, tough, durable, and shockproof alloy is used in grinding machinery, wrecking equipment, caterpillar trucks, and mechanical pounding equipment used in heavy-duty construction. The iron manganese alloys, which are used for making other alloys, are ferromanganese (about 80% Mn) and spiegeleisen (15-30% Mn); they contain some carbon and silicon. The most important manganese compound, pyrolusite or manganese dioxide, is also an oxidizing agent. Pyrolusite is used extensively in the electrodes of dry batteries, where it absorbs liberated hydrogen gas and then chemically bonds it. It is also used as an oxygen source in fireworks and as a chemical catalyst. All other manganese compounds are made from pyrolusite.

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