pyrite [key], pale brass-yellow mineral, the bisulfide of iron, FeS2. It occurs most commonly in crystals (belonging to the isometric system and usually in the form of cubes and pyritohedrons) but is also found in massive, granular, and stalactite form. In spite of its nickname, “fool's gold,” it often is associated with true gold; auriferous pyrite is a commercially important source of gold. Other metals that sometimes replace a part of the iron are cobalt, nickel, arsenic, and copper. The most common sulfide mineral, pyrite is widely distributed in rocks of all ages and types. Its chief use is as a source of sulfur in the manufacture of sulfuric acid. The term pyrites is applied to any of a number of metallic sulfides that strike fire with steel. Some minerals resembling pyrite in appearance or composition are arsenopyrite, chalcopyrite (copper pyrites), cobaltite, marcasite (white iron pyrites or spear pyrites), and pyrrhotite (magnetic pyrites).
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