Pyrite

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Pyrite
Missing image
Pyrite_foolsgold.jpg
An alluring mass of relatively worthless fool's gold

A mass of interwoven pyrite crystals

General
CategoryMineral
Chemical formula iron disulfide (FeS2)
Identification
Colour Pale, dull gold
Crystal habit Cubic, faces may be striated, but also frequently octahedral and pyritohedron. Often intergrown, massive, radiated, granular, globular and stalactitic.
Crystal system Isometric; bar 3 2/m
Melting Point 2140C [1] (http://www.mfa.org/_cameo/frontend/material_description.asp?name=pyrite&language=1)
Cleavage Poor
Fracture Very uneven, sometimes conchoidal
Mohs Scale hardness 6 - 6.5
Luster Metallic, glistening
Refractive index Opaque
Streak Greenish-black to brownish-black
Specific gravity 4.95 - 5.10
Fusibility 2.5-3
Solubility insoluble in water
Magnetism magnetic after heating

The mineral pyrite, or iron pyrite, is iron disulfide, FeS2. It has isometric crystals that usually appear as cubes, faces may be striated (parallel lines on crystal surface or cleavage face), but is also frequently octahedral and occurs as pyritohedrons (a dodecahedron with pentagonal faces). 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 hue 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.

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)

Pyrite exposed to the environment during mining and excavation can react with oxygen and water to form acid mine drainage in the form of sulfuric acid. Similarly, 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 jewellery (jewelry) such as necklaces and bracelets, but although the two are similar, marcasite cannot be used in jewellery 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 jewellery 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.

References

  • Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p 285-286, ISBN 0471805807
  • Mineral galleries (http://mineral.galleries.com/minerals/sulfides/pyrite/pyrite.htm)
  • Webmineral (http://webmineral.com/data/Pyrite.shtml)
  • Mindat (http://www.mindat.org/min-3314.html)de:Pyrit

fr:Pyrite nl:Pyriet ja:黄鉄鉱 pl:Piryt pt:Pirita sk:Pyrit zh:黄铁矿

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