Pyrite
An alluring mass of relatively worthless foolsgold
General
Category	Mineral
Chemical formula	iron disulfide (FeS2)
Identification
Colour	Pale, dull gold
Crystal habit	Cubical, pyritohedrons. Often intergrown, massive, radiated, granular, globular and stalactitic.
Crystal system	Isometric
Cleavage	Poor
Fracture	Very uneven, sometimes conchoidal
Mohs Scale hardness	6 - 6.5
Luster	Metellic, glistening
Refractive index	Opaque
Pleochroism	N/A
Streak	Greenish-black to brownish-black
Specific gravity	4.95 - 4.97
Fusibility	?
Solubility	?
Major varieties

The mineral pyrite, or iron pyrite, is iron disulfide, FeS₂. It has isometric crystals that usually appear as cubes or pyritohedrons. It has a slightly uneven and conchoidal fracture, a hardness of 6 - 6.5, and a specific gravity of 4.95 - 5.10. Its metallic luster and pale-to-normal brass-yellow color have earned it the nickname fool's gold, but ironically enough, small quantities of actual gold can sometimes be found in pyrite. In fact, some auriferous pyrite is a very valuable ore of gold, and others are excellent ores of arsenic, nickel, cobalt and copper.

Pyrite is the most common of the sulfide minerals. It is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock and metamorphic rock, as well as in coal beds, and as the replacement mineral in fossils. Pyrites, interestingly, can show negative resistance, acting as radio detectors and have been used in oscillator circuits. (http://home.earthlink.net/~lenyr/iposc.htm)

The paper industry often uses pyrite for the production of sulfur dioxide and in the manufacture of sulfuric acid, though not as much as it used to. The name pyrite is from the Greek word meaning "fire". This is likely due to the sparks that result when pyrite is struck against steel. This capacity made it popular for use in early firearms such as the wheellock.

Pyrite and Marcasite

Pyrite is often confused with the mineral marcasite, a name derived from the Arabic word for pyrite, due to their similar characteristics. Marcasite is a polymorph of pyrite, which means it has the same chemistry as pyrite but a different structure and, therefore, different symmetry and crystal shapes. The marcasite/pyrite polymorph pair is probably the most famous polymorph pair next to the diamond/graphite pair. Pyrite is often used in jewelry such as necklaces and bracelets, but although the two are similar, marcasite cannot be used in jewelry as it has a tendency to crumble and turn into powder. Adding to the confusion between marcasite and pyrite is the use of the word marcasite as a jewelry trade name. The term is applied to small polished and faceted stones that are inlaid in sterling silver, but even though they are called marcasite, they are actually pyrite.

See also

• List of minerals