Pyrite
Background Information
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| Pyrite | |
|---|---|
 A mass of intergrown, striated pyrite crystals |
|
| General | |
| Category | Sulfide mineral |
| Formula (repeating unit) |
iron disulfide (FeS2) |
| Identification | |
| Colour | Pale brass yellow, 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 |
| Twinning | Penetration twinning |
| Cleavage | Poor |
| Fracture | Very uneven, sometimes conchoidal |
| Mohs scale hardness | 6–6.8 |
| Luster | Metallic, glistening |
| Streak | Greenish-black to brownish-black; smells of sulfur |
| Specific gravity | 4.95–5.10 |
| Refractive index | Opaque |
| Fusibility | 2.5–3 to a magnetic globule |
| Solubility | insoluble in water |
| Other characteristics | paramagnetic |
| References | |
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 πυρίτης (puritēs), “of fire” or "in fire”, from πύρ (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
This mineral occurs as 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 and can be identified in the field by the distinctive odour released when samples are pulverized.
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. In fact, such 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.
Pyrite and marcasite
Pyrite is often confused with the mineral marcasite, a mineral whose name is derived from the Arabic word for pyrite, due to their similar characteristics. Marcasite is a polymorph of pyrite, which means it has the same formula as pyrite but a different structure and, therefore, different symmetry and crystal shapes. The formal oxidation states are, however, the same as in pyrite because again the sulfur atoms occur in persulfide-like pairs. Marcasite/pyrite is probably the most famous polymorph pair next to the diamond/ 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 tends 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 faceted stones that are inlaid in sterling silver, but even though they are called marcasite, they actually contain 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+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. In contrast, molybdenite, MoS2, features isolated sulfide (S2-) 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+AsS3-.
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).
