Difference between revisions of "Pyrite" - New World Encyclopedia
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The name pyrite is from the [[Greek language|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 [[firearm]]s, such as the [[wheellock]]. | The name pyrite is from the [[Greek language|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 [[firearm]]s, such as the [[wheellock]]. | ||
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| + | == Occurrence == | ||
| + | |||
| + | It is usually found associated with other sulfides or [[oxide]]s in [[quartz]] veins, [[sedimentary rock]] and [[metamorphic rock]], as well as in [[coal]] beds, and as the replacement mineral in [[fossil]]s. | ||
| + | [[Image:Cubicpyrite.jpg|thumb|left|150px|[[Euhedral]] cubic pyrite crystals]] | ||
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| + | Pyrite exposed to the environment during [[mining]] and excavation reacts with [[oxygen]] and [[water]] to form [[sulfuric acid]], resulting in [[acid mine drainage]]. This <!--what is "this"—> results from the action of ''[[Thiobacillus]]'' bacteria, which generate their energy by using oxygen to oxidize ferrous iron (Fe<sup>2+</sup>) to ferric iron (Fe<sup>3+</sup>). 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. | ||
== Characteristics == | == Characteristics == | ||
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Pyrite has [[Cubic (crystal system)|isometric]] [[crystal]]s that usually appear as cubes. The cube faces may be striated (parallel lines on crystal surface or cleavage face) as a result of alternation of the cube and [[pyritohedron]] faces. Pyrite also frequently occurs as [[octahedra|octahedral]] crystals and as pyritohedra (a dodecahedron with pentagonal faces). It has a slightly uneven and conchoidal fracture, a [[Mohs scale of mineral hardness|hardness]] of 6–6.5, and a [[specific gravity]] of 4.95–5.10. It is brittle. A major identifier, useful in the field, is the streak, as the powdered mineral smells of sulfur. Its metallic [[luster]] and [[brass]]-yellow hue have earned it the nickname '''fool's gold'''. Ironically, small quantities of actual [[gold]] are sometimes found in pyrite. In fact, such auriferous pyrite is a valuable [[ore]] of gold. | Pyrite has [[Cubic (crystal system)|isometric]] [[crystal]]s that usually appear as cubes. The cube faces may be striated (parallel lines on crystal surface or cleavage face) as a result of alternation of the cube and [[pyritohedron]] faces. Pyrite also frequently occurs as [[octahedra|octahedral]] crystals and as pyritohedra (a dodecahedron with pentagonal faces). It has a slightly uneven and conchoidal fracture, a [[Mohs scale of mineral hardness|hardness]] of 6–6.5, and a [[specific gravity]] of 4.95–5.10. It is brittle. A major identifier, useful in the field, is the streak, as the powdered mineral smells of sulfur. Its metallic [[luster]] and [[brass]]-yellow hue have earned it the nickname '''fool's gold'''. Ironically, small quantities of actual [[gold]] are sometimes found in pyrite. In fact, such auriferous pyrite is a valuable [[ore]] of gold. | ||
| + | === Polymorphs: pyrite and marcasite === | ||
| + | |||
| + | Pyrite is often confused with the mineral [[marcasite]] (named from the [[Arabic language|Arabic]] word for pyrite), due to their similar characteristics. Marcasite is a [[polymorphism (materials science)|polymorph]] of pyrite, which means it has the same chemical formula as pyrite but a different structure. Their crystal shapes and symmetry therefore differ. The formal oxidation states are, however, the same for both polymorphs, because the sulfur atoms occur in persulfide-like pairs. Marcasite/pyrite is probably the second most famous polymorph pair, 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 Fe<sup>2+</sup>S<sub>2</sub><sup>2-</sup>. 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, H<sub>2</sub>S<sub>2</sub>. Thus pyrite would be more descriptively called iron persulfide, not iron disulfide. By contrast, [[molybdenite]], MoS<sub>2</sub>, features isolated sulfide (S<sup>2-</sup>) centers. Consequently, the oxidation state of molybdenum is Mo<sup>4+</sup>, or Mo(IV). | |
| − | + | The mineral arsenopyrite has the formula FeAsS. Whereas pyrite has S<sub>2</sub> subunits, arsenopyrite has AsS units, formally derived from [[deprotonation]] of H<sub>2</sub>AsSH. | |
| − | |||
| − | + | == Uses == | |
Pyrite is used for the production of [[sulfur dioxide]], such as for the paper industry, and in the manufacture of sulfuric acid, though such applications are declining in importance. | Pyrite is used for the production of [[sulfur dioxide]], such as for the paper industry, and in the manufacture of sulfuric acid, though such applications are declining in importance. | ||
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Cubes of fully or partially oxidized and hydrated pseudomorphs of pyrite are known colloquially as ''devil's dice''. | Cubes of fully or partially oxidized and hydrated pseudomorphs of pyrite are known colloquially as ''devil's dice''. | ||
| − | + | Pyrite is often used in [[jewelry]] such as necklaces and bracelets. Its polymorph marcasite, although similar, cannot be used in jewelery 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. The term is applied to small, polished, [[facet]]ed stones that are inlaid in [[sterling silver]], but although they are called marcasite, they are actually pyrite. | |
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| − | Pyrite is often used in [[ | ||
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== Related minerals and varieties == | == Related minerals and varieties == | ||
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* [[Iron]] | * [[Iron]] | ||
* [[Mineral]] | * [[Mineral]] | ||
| + | * [[Nickel]] | ||
* [[Sulfide]] | * [[Sulfide]] | ||
Revision as of 18:57, 20 April 2007
| Pyrite | |
|---|---|
 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 corresponds to iron sulfide (FeS2).
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
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.
Pyrite exposed to the environment during mining and excavation reacts with oxygen and water to form sulfuric acid, resulting in acid mine drainage. This results from 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 can continue until the pyrite is exhausted.
Characteristics
Pyrite has isometric crystals that usually appear as cubes. The cube faces may be striated (parallel lines on crystal surface or cleavage face) as a result of alternation of the cube and pyritohedron faces. Pyrite also frequently occurs as octahedral crystals and as pyritohedra (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. It is brittle. A major identifier, useful in the field, is the streak, as the powdered mineral smells of sulfur. Its metallic luster and brass-yellow hue have earned it the nickname fool's gold. Ironically, small quantities of actual gold are sometimes found in pyrite. In fact, such auriferous pyrite is a valuable ore of gold.
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 it has the same chemical formula as pyrite but a different structure. Their crystal shapes and symmetry therefore differ. The formal oxidation states are, however, the same for both polymorphs, because the sulfur atoms occur in persulfide-like pairs. Marcasite/pyrite is probably the second most famous polymorph pair, 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-. 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. By contrast, molybdenite, MoS2, features isolated sulfide (S2-) centers. Consequently, the oxidation state of molybdenum is Mo4+, or Mo(IV).
The mineral arsenopyrite has the formula FeAsS. Whereas pyrite has S2 subunits, arsenopyrite has AsS units, formally derived from deprotonation of H2AsSH.
Uses
Pyrite is used for the production of sulfur dioxide, such as for the paper industry, and in the manufacture of sulfuric acid, though such applications are declining in importance.
Pyrites can show negative resistance and have experimentally been used in oscillator circuits as radio detectors [2].
Cubes of fully or partially oxidized and hydrated pseudomorphs of pyrite are known colloquially as devil's dice.
Pyrite is often used in jewelry such as necklaces and bracelets. Its polymorph marcasite, although similar, cannot be used in jewelery 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. The term is applied to small, polished, faceted stones that are inlaid in sterling silver, but although they are called marcasite, they are actually pyrite.
Related minerals and varieties
Bravoite is a nickeloan variety of pyrite, with more than 50% substitution of Ni2+ for Fe2+ within pyrite. Bravoite is not a formally recognized mineral. It is named after Peruvian scientist Jose J. Bravo (1874-1928)[3].
See also
ReferencesISBN links support NWE through referral fees
- 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.
- 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.
- Mineral Gallery. 2006. The Mineral Pyrite. Amethyst Galleries. Retrieved April 11, 2007.
External links
- Pyrite Mineral Data. Webmineral.com. Retrieved April 11, 2007.
- Pyrite. Mindat.org. Retrieved April 11, 2007.
- Bravoite. Mindat.org. Retrieved April 11, 2007.
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