Pyrite

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.

Being found worldwide, 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.

The paper industry often uses pyrite for the production of sulphur dioxide and in the manufacture of sulphuric acid, though not as much as it used to. The name pyrite is from the Greek word meaning fire, 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.

Table of contents

1 Pyrite and Marcasite
2 Varieties of Pyrite
3 See also

Pyrite and Marcasite

Pyrite is often confused with the mineral marcasite, a name derived from the 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 inlayed in sterling silver, but even though they are called marcasite, they are actually pyrite.

Varieties of Pyrite

Iron Pyrite (FeS), described above, is the most common form of pyrite.
Arsenopyrite (FeAsS) is known as a major ore of arsenic. Unlike the cubical crystals of regular pyrite, A typical arsenopyrite crystal contains a diamond shaped dome atop a prismatic crystal. The crystals have sharp acute angles that are a marked contrast from other sulfides that generally have only obtuse angles. In the crystals, twinning is common, often bending the crystal and sometimes forming crosses, x's or stars. Crystals are also typically striated.

Arsenopyrite was previously thought to be orthorhombic, but recent analysis proves it as monoclinic.

Chalcopyrite (CuFeS₂) is also called "copper iron sulfide." When it is first mined, it tends to be a gold-yellow color, but when it is exposed to air, it can tarnish to bright, irridescent colors. Chalcopyrite is sometimes treated with an acid to bring out the vivid colors. Especially colorful specimines are sometimes referred to as "peacock copper". The occurrance of crystals in chalcopyrite is very rare, and it is usually found in a solid, masive form. It is found worldwide.