Pyrites, a name employed by mineralogists to designate a large class of minerals, which are compounds of metals with sulphur, or with arsenic, or with both. They are crystalline, hard, generally brittle, and frequently yellow. The name pyrites originally belonged to the disulphide of iron, known as iron-pyrites or simply pyrite, and was given to it in consequence of its striking fire with steel (Gr. pyr, 'fire'), so that it was used for kindling powder in the pans of muskets before gun-flints were introduced. Pyrite (iron-pyrites) is commonly of a bright brass-yellow colour; it is often found crystallised in cubes, in which form small crystals of it are abundantly disseminated in some roofing-slates, and very large ones occur in some of the mines of Cornwall; it is also found crystallised in dodecahedrons and other forms, more rarely in oblique four-sided prisms; and it often occurs massive, globular, stalactitic, capillary, or investing other minerals as an incrustation. Beautiful specimens of globular pyrite are found in the chalk of England. It is a very widely diffused and plentiful mineral, occurring in many different kinds of rock. It is too abundant in many coal-seams, the action of water and air changing it into sulphate of iron (vitriol), during which change so much heat is evolved that the coal is frequently kindled by it, mines become unworkable, and the progress of the fire can only be stopped, if at all, by building up portions of them to cut off the access of air, or by the admission of a plentiful supply of water. Sandstones containing pyrite ought not to be employed for building purposes, as it is prone to oxidation. Sometimes it is changed into sulphate of iron, but when other bases are present in the rock the sulphuric acid often unites with these in preference, leaving the iron of the original sulphide free. The iron then becomes oxidised, and appears as dark brown blotches. The presence of pyrite thus leads to corrosion and unsightly staining. The colour of pyrite has often caused it to be mistaken for gold, a mistake which its hardness and comparative lightness should prevent, or its ready solubility in nitric acid, and its burning before the blowpipe on charcoal with bluish flame and smell of sulphur. But it sometimes does contain a small proportion of gold, occasionally even in visible grains. This auriferous pyrite is found in Siberia and in South America. Pyrite is never used as an ore of iron, but it is much employed in the manufacture of sulphuric acid, and sulphur is obtained from it by sublimation. It is also used for the manufacture of alum. A rather unstable variety of iron disulphide of a very pale colour is called Marcasite; it crystallises in orthorhombic forms. Another sulphide of iron known as Pyrrhotine () is magnetic.
Copper Pyrites, also called Yellow Copper and Chalcopyrite, is the most abundant of all the ores of copper, and yields a large proportion (perhaps a third) of the copper used in the world. It is brass-yellow, the colour varying with the amount of copper which it contains, a rich colour indicating much copper, and a pale colour the presence of a comparatively large amount of iron; for this ore is not a sulphide of copper alone, but of copper and iron. It occurs massive and disseminated in rocks of almost every class, and is often found crystallised in octahedrons and tetrahedrons, but generally in very small crystals. It may at once be distinguished from iron-pyrites by its comparative softness, yielding readily to the knife, and by the green colour of its solution in nitric acid. Before the blowpipe, with borax and soda, it yields a bead of copper.—Cobaltite, an arsenio-sulphide of copper, is a principal ore of cobalt. It is generally of a silver-white colour, and occurs massive, disseminated, or crystallised in cubes, octahedrons, dodecahedrons, and polyhedrons, in schistose rocks.—Nickelite, used as an ore of nickel, is a compound of nickel and arsenic. It is generally found massive, and is of a copper-red colour; hence it is called by the German miners Kupfer-nickel, because they mistook it for an ore of copper.