Pyrite

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Pyrite
Pyrite foolsgold.jpg
A mass of intergrown, striated pyrite crystals
General
Category Mineral
Chemical formula iron persulfide (FeS2)
Identification
Color Pale, dull gold
Crystal habit Cubic, faces may be striated, but also frequently octahedral and pyritohedron. Often inter-grown, massive, radiated, granular, globular and stalactitic.
Crystal system Isometric; bar 3 2/m
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; smells of sulfur
Specific gravity 4.95 - 5.10
Melting point 1,177-1,188°C [1]
Fusibility 2.5-3
Solubility insoluble in water
Other Characteristics paramagnetic

Pyrite (or iron pyrite) is the most common of the sulfide minerals. Chemically, it is known as iron disulfide (FeS2). Its metallic luster and brassy hue have earned it the nickname fool's gold. Ironically, small quantities of actual gold are occasionally associated with pyrite. Its polymorphic form, called marcasite, has the same chemical formula but is structurally different.

Pyrite is often used to make necklaces, bracelets, and other types of jewelry. In addition, it is used for the manufacture of sulfuric acid and sulfur dioxide, such as for the paper industry. It has also been used experimentally for radio detectors in oscillator circuits.

Contents

Etymology

The name pyrite is from the Greek word pyr, meaning "fire." This is likely a reference to the sparks produced when pyrite is struck against steel. This capacity made it popular for use in early firearms, such as the wheellock.

Occurrence and acid mine drainage

Crystals of cubic pyrite with sharp, well-formed faces.

It is usually found associated with other sulfides or oxides in quartz veins, sedimentary rock and metamorphic rock, as well as in coal beds. It is often the replacement mineral in fossils.

As noted above, small quantities of gold are sometimes found in pyrite. Such auriferous pyrite is a valuable ore of gold.

Pyrite exposed to the environment during mining and excavation reacts with oxygen and water to form sulfuric acid. This process, known as acid mine drainage, involves the action of Thiobacillus bacteria, which generate their energy by using oxygen to oxidize ferrous iron (Fe2+) to ferric iron (Fe3+). The ferric iron in turn reacts with pyrite to produce ferrous iron and sulfuric acid. The ferrous iron is then available for oxidation by the bacteria. This cycle may continue until the pyrite is exhausted.

Characteristics

Pyrite is brittle and has a slightly uneven, conchoidal fracture. It has a hardness of 6–6.5 on the Mohs scale, and a specific gravity of 4.95–5.10. The powdered mineral smells of sulfur—a property that is useful for identifying it in the field.

This mineral has isometric crystals that usually appear as cubes. It also frequently occurs as octahedral crystals and as pyritohedra.[1]

The sulfur atoms in pyrite occur in pairs, with clear S-S bonds. These S-S pairs are known as "persulfide units," which can be viewed as derived from hydrogen persulfide, H2S2. Thus pyrite would be more appropriately called iron persulfide, not iron disulfide.

Polymorphs: pyrite and marcasite

Pyrite is often confused with the mineral marcasite (named from the Arabic word for pyrite), due to their similar characteristics. Marcasite is a polymorph of pyrite, which means they have the same chemical formula but different structures. Their crystal shapes and symmetry therefore differ. The pyrite/marcasite pair is probably the second most well-known pair of polymorphs, after the diamond/graphite pair.

Marcasite is metastable relative to pyrite and slowly changes to pyrite over time, or if heated. Marcasite is relatively rare, but it may be locally abundant in some types of ore deposits, such as the Mississippi Valley-type lead-zinc deposits. Marcasite appears to form only from aqueous solutions.

Formal oxidation states

From the perspective of classical inorganic chemistry, which assigns formal oxidation states to each atom, pyrite is probably best described as Fe2+S22-. In other words, the formal oxidation state of iron is Fe2+. This formalism recognizes that the sulfur atoms in pyrite occur in persulfide-like pairs. The sulfur atoms in marcasite occur in similar pairs, and the formal oxidation states of the atoms in marcasite are the same as for those in pyrite.

By contrast, molybdenite (MoS2) features isolated sulfide (S2-) centers. Consequently, the oxidation state of molybdenum is Mo4+, or Mo(IV).

Uses

Pyrite is often used in jewelry such as necklaces and bracelets. Its polymorph marcasite, although similar, cannot be used in jewelry as it has a tendency to crumble into powder. Adding to the confusion between marcasite and pyrite is the use of the word "marcasite" as a jewelry trade name. Small, polished, faceted stones of pyrite may be inlaid in sterling silver, but they are erroneously called marcasite.

In addition, pyrite is used for the production of sulfur dioxide, such as for the paper industry, and in the manufacture of sulfuric acid. These applications, however, are declining in importance.

Pyrites can show negative resistance and have been used experimentally in oscillator circuits as radio detectors.[2]

Related minerals

Bravoite is a variety of pyrite enriched in nickel, with more than 50 percent substitution of Ni2+ for Fe2+. It is named after Peruvian scientist Jose J. Bravo (1874-1928), but it is not a formally recognized mineral.[3]

Arsenopyrite is a mineral with the chemical formula FeAsS. Whereas pyrite has S2 subunits, arsenopyrite has AsS units.

See also

Notes

  1. A pyritohedron is an irregular dodecahedron. Like the regular dodecahedron, it has 12 identical pentagonal faces, with three meeting in each of the 20 corners. The pentagons, however, are not regular, and the structure has no five-fold symmetry axes; instead, it has tetrahedral symmetry.
  2. Steiner, Nyle. 2001. Iron Pyrites Negative Resistance Oscillator Retrieved October 9, 2007.
  3. Mindat.org. 2007. Bravoite Mindat.org. Retrieved October 9, 2007.

References

  • Farndon, John. 2006. The Practical Encyclopedia of Rocks & Minerals: How to Find, Identify, Collect and Maintain the World's best Specimens, with over 1000 Photographs and Artworks. London: Lorenz Books. ISBN 0754815412
  • Klein, Cornelis, and Barbara Dutrow. 2007. Manual of Mineral Science. 23rd ed. New York: John Wiley. ISBN 978-0471721574
  • Mineral Gallery. 2006. The Mineral Pyrite Amethyst Galleries. Retrieved October 9, 2007.
  • Pellant, Chris. 2002. Rocks and Minerals. Smithsonian Handbooks. New York: Dorling Kindersley. ISBN 0789491060
  • Shaffer, Paul R., Herbert S. Zim, and Raymond Perlman. 2001. Rocks, Gems and Minerals. Rev. ed. New York: St. Martin's Press. ISBN 1582381321

External links

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