pyrite (py'-rite). (a) A common, pale-bronze or brass-yellow, cubic mineral: FeS2 . It is dimorphous with marcasite, and often contains small amounts of other metals. Pyrite has a brilliant metallic luster and an absence of cleavage, and has been mistaken for gold (which is softer and heavier). It commonly crystallizes in cubes (whose faces are usually striated), octahedrons, or pyritohedrons, and it also occurs in shapeless grains and masses. Pyrite is the most widespread and abundant of the sulfide minerals and occurs in all kinds of rocks, such as in nodules in sedimentary rocks and coal seams or as a common vein material associated with many different minerals. Pyrite is an important ore of sulfur, less so of iron, and is burned in making sulfur dioxide and sulfuric acid; it is sometimes mined for the associated gold and copper. Cf: pyrites. Syn: iron pyrites; fool's gold; mundic; common pyrites. (b) A group name for minerals isomorphous with pyrite, with the general formula AX2 , where A = Fe, Co, Ni, Mn, Cu, Au, Ru, Os, Ir or Pt, and X = S and, more rarely, Se, Te, Sb or As.