The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS₂. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold due to its resemblance to gold. Pyrite is the most common of the sulfide minerals. The name pyrite is derived from the Greek ??????? (purit?s), ?of fire? or "in fire?, from ??? (pur), ?fire?. This name is likely due to the sparks that result when pyrite is struck against steel or flint. This property made pyrite popular for use in early firearms such as the wheellock.

Mineralogy

Pyrite 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 a replacement mineral in fossils. Despite being nicknamed fool's gold, small quantities of gold are sometimes found associated with pyrite. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin, Nevada gold deposit, arsenian pyrite contains up to 0.37 wt% gold.^[2] Auriferous pyrite is a valuable ore of gold.

Weathering and release of sulfate

Pyrite exposed to the atmosphere during mining and excavation reacts with oxygen and water to form sulfate, resulting in acid mine drainage. This acidity results from the action of Acidithiobacillus bacteria, which generate their energy by oxidizing ferrous iron (Fe²⁺) to ferric iron (Fe³⁺) using oxygen. The ferric iron in turn attacks the pyrite to produce ferrous iron and sulfate. The ferrous iron is then available for oxidation by the bacterium; this cycle continues until the pyrite is depleted.

Uses

Pyrite is used commercially for the production of sulfur dioxide, for use in such applications as the paper industry, and in the manufacture of sulfuric acid, although such applications are declining in importance.

Formal oxidation states for pyrite, marcasite, and arsenopyrite

From the perspective of classical inorganic chemistry, which assigns formal oxidation states to each atom, pyrite is probably best described as Fe²⁺S₂^2-. This formalism recognizes that the sulfur atoms in pyrite occur in pairs with clear S-S bonds. These persulfide units can be viewed as derived from hydrogen persulfide, H₂S₂. Thus pyrite would be more descriptively called iron persulfide, not iron disulfide. In contrast, molybdenite, MoS₂, features isolated sulfide (S^2-) centers. Consequently, the oxidation state of molybdenum is Mo⁴⁺. The mineral arsenopyrite has the formula FeAsS. Whereas pyrite has S₂ subunits, arsenopyrite has AsS units, formally derived from deprotonation of H₂AsSH. Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe³⁺AsS^3-.^[3]

Varieties

Bravoite is a nickel-cobalt bearing variety of pyrite, with >50% substitution of Ni²⁺ for Fe²⁺ within pyrite. Bravoite is not a formally recognised mineral, and is named after Peruvian scientist Jose J. Bravo (1874-1928).^[4]

Cattierite (CoS₂) and Vaesite (NiS₂) are similar in their structure and belong also to the pyrite group.

References

1. ↑ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., John Wiley and Sons, New York, p 285-286, ISBN 0-471-80580-7
2. ↑ http://www.minsocam.org/msa/AmMin/toc/Articles_Free/1997/Fleet_p182-193_97.pdf MICHAEL E. FLEETl AND A. HAMID MUMIN, Gold-bearing arsenian pyrite and marcasite and arsenopyrite from Carlin Trend gold deposits and laboratory synthesis, American Mineralogist, Volume 82, pages 182-193, 1997
3. ↑ Vaughan, D. J.; Craig, J. R. ?Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0521214890.
4. ↑ http://www.mindat.org/min-759.html Mindat - bravoite

• American Geological Institute, 2003, Dictionary of Mining, Mineral, and Related Terms, 2nd ed., Springer, New York, ISBN 978-3540012719
• Mineral galleries

External links

• How Minerals Form and Change "Pyrite oxidation under room conditions".

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