Pyrites (Gr. , from , fire), a name given to yellow sulphuret of iron because it struck fire with steel. The German name Kies is similar to that for flint, Kiesel, and in the earliest firearms the powder was ignited by a piece of pyrites, the use of flints being later. It is now extended to sulphurets of other metals, and also to certain arsenides and double compounds of metals with sulphur. There are three kinds of iron pyrites: cubic or yellow, marcite or white, and magnetic pyrites. The first two are isomeric, having the formula FeS2, but are not isomorphous. Cubic pyrites crystallizes in several monometric forms, of which the cube, octahedron, and dodecahedron are the chief; while marcite belongs to the trimetric or rhombic system. Magnetic pyrites when pure has the formula Fe7S8, and crystallizes in the hexagonal system. Cubic or yellow pyrites, or mundic as it is called in Wales, is found in all geological formations, from the most ancient crystalline to recent alluvial.

Very large cubes have been found in some of the Cornish mines, dodecahedrons 6 in. in diameter in the island of Elba, and large octahedral crystals at Persberg in Sweden; in Connecticut, at Lane's mine in octahedrons, and at Orange and Milford in cubes in chlorite state; and in Pennsylvania, at Cornwall, Lebanon co., in cubo-octahedrons an inch in diameter. Cubic pyrites is largely used in the manufacture of copperas and sulphuric acid, and in Sweden for obtaining sublimed sulphur; and enormous quantities are exported from Spain to Great Britain. Yellow pyrites, from its resemblance to the precious metal, by which many have been deceived, is sometimes called "fool's gold." In the chemical works of Yorkshire "coal brasses," as pyrites is called, are exposed in their beds, where by the action of air and moisture they are converted into copperas; heat is developed during the process. In the coal fields subterranean fires are sometimes kindled by the conversion of masses of pyrites into copperas. At Quarrel-town in Renfrewshire, Scotland, is a deep hollow where about 100 years ago the ground fell from a subterranean fire thus kindled.

The conversion of pyrites into copperas is more conveniently conducted by roasting. (See Sulphur, and Sulphuric Acid.) Copper pyrites (calcopyrite of Dana) is the common copper ore of Cornwall, where from 10,000 to 12,000 tons of copper are smelted from 150,000 to 160,000 tons of ore. It is a double sulphuret of copper and iron, containing sulphur 34.9, copper 34.6, iron 30.5. It crystallizes in the dimetric system, often in tetrahedrons. Copper pyrites in massive crystals occurs at Ellen-ville, Ulster co., N. Y., composed of sulphur 36.65, copper 32.43, and iron 31.25. Fire pyrites is found in the Cornish mines having the following composition: sulphur 30.0, tin 27.2, copper 29.7, iron 13.1. Leucopyrite (Dana) is an arsenide of iron, and mispickle is a sul-phuret of arsenic.