Pyrite

Pyrite
A mass of intergrown pyrite crystals
General
Category	Sulfide mineral
Chemical formula	iron disulfide (FeS2)
Identification
Color	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; Pa-3
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	[1][2][3]

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.

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.^[4] 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 from Ampliación a Victoria Mine, Navajún, La Rioja, Spain

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.

During the early years of the 20th century, pyrite was used as a mineral detector in radio receivers, and is still used by 'crystal radio' hobbyists. Until the vacuum tube matured, the crystal detector was the most sensitive and dependable detector available- with considerable variation between mineral types and even individual samples within a particular type of mineral. The most sensitive mineral was galena, which was very sensitive also to mechanical vibration, and easily knocked off the sensitive point; the most stable were perikon mineral pairs; and midway between was the pyrites detector, which is approximately as sensitive as a modern 1N34A diode detector.

Pyrite has been proposed as an abundant inexpensive material in low cost photovoltaic solar panels.^{[5] [6]}

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-.^[7]

Crystallography

Crystal structure of pyrite

Iron-pyrite FeS₂ represents the prototype compound of the crystallographic pyrite structure. The structure is simple cubic and was among the first crystal structures solved by x-ray diffraction ^[8]. It belongs to the crystallographic space group Pa3 and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant a of stoichiometric iron pyrite FeS₂ amounts to 541.87 pm ^[9]. The unit cell is composed of a Fe face-centered cubic sublattice into which the S ions are embedded. The pyrite structure is also taken by other compounds MX₂ of transition metals M and chalcogens X = O, S, Se and Te. Also certain dipnictides with X standing for P, As and Sb etc. are known to adopt the pyrite structure ^[10].

The positions of X ions in the pyrite structure may be derived from the fluorite structure: whereas F ions in CaF₂ occupy the centre positions of the eight subcubes of the cubic unit cell (¼ ¼ ¼) etc., the S ions in FeS₂ are shifted from these high symmetry positions along <111> axes to reside on (uuu) and symmetry-equivalent positions. Here, the parameter u should be regarded as a free lattice parameter that takes different values in different pyrite-structure compounds (iron pyrite FeS₂: u(S) = 0.385 ^[11]). In the first bonding sphere, Fe ions are surrounded by six S nearest neighbours, while S ions have bonds with three Fe and one S ion. The site symmetry at Fe and S positions is accounted for by point symmetry groups C_3i and C₃, respectively. The missing center of inversion at S lattice sites has important consequences for the crystallographic and physical properties of iron pyrite. These consequences derive from the crystal electric field active at the sulphur lattice site, which causes a polarisation of S ions in the pyrite lattice ^[12]. The polarisation can be calculated on the basis of higher-order Madelung constants and has to be included in the calculation of the lattice energy by using a generalised Born-Haber cycle.

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).^[13]

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://webmineral.com/data/Pyrite.shtml Webmineral
3. ^ http://www.mindat.org/min-3314.html Pyrite on Mindat.org
4. ^ 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
5. ^ Wadia, Kammen and Alivisatos, http://pubs.acs.org, Environmental Science & Technology. Environ. Sci. Technol., 2009, 43 (6), pp 2072–2077 DOI: 10.1021/es8019534 Publication Date (Web): February 13, 2009
6. ^ Wadia, Kammen, Alivisatos article announcement http://www.berkeley.edu/news/media/releases/2009/02/17_solar.shtml
7. ^ Vaughan, D. J.; Craig, J. R. “Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0521214890.
8. ^ W. L. Bragg (1913). "The structure of some crystals as indicated by their diffraction of X-rays". Proc. Roy. Soc. Lond., Ser. A 89: 248-277. 
9. ^ M. Birkholz, S. Fiechter, A. Hartmann, and H. Tributsch (1991). "Sulfur deficiency in iron pyrite (FeS_2-x) and its consequences for band structure models". Phys. Rev. B 43: 11926. doi:10.1103/PhysRevB.43.11926. 
10. ^ N. E. Brese, and H. G. von Schnering (1994). "Bonding Trends in Pyrites and a Reinvestigation of the Structure of PdAs₂, PdSb₂, PtSb₂ and PtBi₂". Z. anorg. allg. Chem. 620: 393. doi:10.1002/zaac.19946200302. 
11. ^ E. D. Stevens, M. L. de Lucia, and P. Coppens (1980). "Experimental observation of the Effect of Crystal Field Splitting on the Electron Density Distribution of Iron Pyrite". Inorg. Chem. 19: 813. doi:10.1021/ic50206a006. 
12. ^ M. Birkholz (1992). "The crystal energy of pyrite". J. Phys.: Condens. Matt. 4: 6227. doi:10.1088/0953-8984/4/29/007. 
13. ^ 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

Wikimedia Commons has media related to: Pyrite

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