Pyrite

The gold are sometimes found in pyrite. In fact, such auriferous pyrite is a valuable ore of gold.

Pyrite is the most common of the metamorphic rock, as well as in coal beds, and as the replacement mineral in fossils.

The name pyrite is from the Greek word πυρά (pura) meaning "fire". This is likely due to the sparks that result when pyrite is struck against flint. This capacity made it popular for use in early firearms such as the wheellock.

Weathering and release of sulfuric acid

Pyrite exposed to the environment during mining and excavation reacts with 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 can continue until the pyrite is exhausted.

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, though such applications are declining in importance. It is also used for costume jewelry.

Pyrites can show radio detectors [2].

Pyrite and marcasite

    Pyrite is often confused with the mineral graphite pair. Appearance is slightly more silver.

Marcasite is metastable relative to pyrite and will slowly be changed to pyrite if heated or given enough time. Marcasite is relatively rare, but may be locally abundant in some types of ore deposits, such as Mississippi Valley-type Pb-Zn deposits. Marcasite appears to form only from aqueous solutions.

Pyrite is often used in jewellery such as necklaces and bracelets, but although the two are similar, marcasite cannot be used in jewellery 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 jewellery trade name. The term is applied to small polished and sterling silver, but even though they are called marcasite, they are actually pyrite.

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, semi-conducting behavior of this family of inorganic solids.

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)[3].

References

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