Pyrite

Pyrite, also iron pyrite or fool’s gold, mineral composed of iron sulfide, FeS₂, the most common sulfide mineral. It crystallizes in the isometric system (see Crystal) and frequently occurs as well-defined crystals as well as in massive formations. The mineral is brass yellow, is opaque, and has a metallic luster. The resemblance of pyrite to gold caused many prospectors to mistake it for gold, and it became known as fool’s gold. It is distinguished from gold by its brittleness and by its hardness, which ranges between 6 and 6.5; the specific gravity is 4.95 to 5.1.

Pyrite is a common mineral in sedimentary rocks and also occurs in igneous and metamorphic rocks. It is often associated with coal formations and sometimes occurs associated with gold or copper. Large deposits are found throughout the world. Pyrite is not mined as an iron ore, except in countries where iron-oxide ores are not available, because of the difficulty of removing the sulfur. It is used mainly in the commercial production of sulfuric acid and of copperas, or ferrous sulfate (see Sulfur).

Marcasite, a mineral of the same composition as pyrite, is called white iron pyrite. It is opaque, with a metallic luster, and is pale—bronze yellow or almost white when freshly fractured. The hardness is the same as that of pyrite. The specific gravity varies from 4.85 to 4.90. Marcasite is distinguished from pyrite by the difference in color, crystal habit, and by chemical tests. It is more easily decomposed than pyrite and is much less common in occurrence. Marcasite is used, to a much lesser extent than pyrite, in making sulfuric acid.