Pyrite
Pyrite cubic crystals on marl from Navajún, La Rioja, Spain (size: 95 by 78 millimetres [3.7 by 3.1 in], 512 grams [18.1 oz]; main crystal: 31 millimetres [1.2 in] on edge)
General
Category	Sulfide mineral
Formula (repeating unit)	FeS2
Strunz classification	2.EB.05a
Dana classification	2.12.1.1
Crystal system	Isometric
Crystal class	Diploidal (m3) H-M symbol: (2/m 3)
Space group	Pa3
Unit cell	a = 5.417 Å, Z = 4
Identification
Formula mass	119.98 g/mol
Color	Pale brass-yellow reflective; tarnishes darker and iridescent
Crystal habit	Cubic, faces may be striated, but also frequently octahedral and pyritohedron. Often inter-grown, massive, radiated, granular, globular, and stalactitic.
Twinning	Penetration and contact twinning
Cleavage	Indistinct on {001}; partings on {011} and {111}
Fracture	Very uneven, sometimes conchoidal
Tenacity	Brittle
Mohs scale hardness	6-6.5
Luster	Metallic, glistening
Streak	Greenish-black to brownish-black
Diaphaneity	Opaque
Specific gravity	4.95-5.10
Density	4.8-5 g/cm3
Fusibility	2.5-3 to a magnetic globule
Solubility	Insoluble in water
Other characteristics	paramagnetic
References	[1][2][3][4]

The mineral pyrite ^[5], or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS₂ (iron(II) disulfide). Pyrite is considered the most common form of sulfide minerals.

Pyrite's metallic luster and pale brass-yellow hue give it a superficial resemblance to gold, hence the well-known nickname of fool's gold. The color has also led to the nicknames brass, brazzle, and Brazil, primarily used to refer to pyrite found in coal.^[6][7]

The name pyrite is derived from the Greek ? (pyrit?s), "of fire" or "in fire",^[8] in turn from (pyr), "fire".^[9] In ancient Roman times, this name was applied to several types of stone that would create sparks when struck against steel; Pliny the Elder described one of them as being brassy, almost certainly a reference to what we now call pyrite.^[10]

By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulfide minerals.^[11]

Pyrite under normal and polarized light

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, but has also been identified in the sclerites of scaly-foot gastropods.^[12] Despite being nicknamed fool's gold, pyrite is sometimes found in association with small quantities of gold. A substantial proportion of the gold is "invisible gold" incorporated into the pyrite (see Carlin-type gold deposit). It has been suggested that the presence of both gold and arsenic is a case of coupled substitution but as of 1997 the chemical state of the gold remained controversial.^[13]

Uses

An abandoned pyrite mine near Pernek in Slovakia

Pyrite enjoyed brief popularity in the 16th and 17th centuries as a source of ignition in early firearms, most notably the wheellock, where a sample of pyrite was placed against a circular file to strike the sparks needed to fire the gun.^[]

Pyrite has been used since classical times to manufacture copperas (iron(II) sulfate). Iron pyrite was heaped up and allowed to weather (an example of an early form of heap leaching). The acidic runoff from the heap was then boiled with iron to produce iron sulfate. In the 15th century, new methods of such leaching began to replace the burning of sulfur as a source of sulfuric acid. By the 19th century, it had become the dominant method.^[14]

Pyrite remains in commercial use for the production of sulfur dioxide, for use in such applications as the paper industry, and in the manufacture of sulfuric acid. Thermal decomposition of pyrite into FeS (iron(II) sulfide) and elemental sulfur starts at 540 °C (1,004 °F); at around 700 °C (1,292 °F), p_S2 is about .^[15]

A newer commercial use for pyrite is as the cathode material in Energizer brand non-rechargeable lithium batteries.^[16]

Pyrite is a semiconductor material with a band gap of 0.95 eV.^[17] Pure pyrite is naturally n-type, in both crystal and thin-film forms, potentially due to sulfur vacancies in the pyrite crystal structure acting as n-dopants.^[18]

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. Pyrite detectors occupied a midway point between galena detectors and the more mechanically complicated perikon mineral pairs. Pyrite detectors can be as sensitive as a modern 1N34A germanium diode detector.^[19][20]

Pyrite has been proposed as an abundant, non-toxic, inexpensive material in low-cost photovoltaic solar panels.^[21] Synthetic iron sulfide was used with copper sulfide to create the photovoltaic material.^[22] More recent efforts are working toward thin-film solar cells made entirely of pyrite.^[23]

Pyrite is used to make marcasite jewelry. Marcasite jewelry, made from small faceted pieces of pyrite, often set in silver, was known since ancient times and was popular in the Victorian era.^[24] At the time when the term became common in jewelry making, "marcasite" referred to all iron sulfides including pyrite, and not to the orthorhombic FeS₂ mineral marcasite which is lighter in color, brittle and chemically unstable, and thus not suitable for jewelry making. Marcasite jewelry does not actually contain the mineral marcasite. The specimens of pyrite, when it appears as good quality crystals, are used in decoration. They are also very popular in mineral collecting. Among the sites that provide the best specimens, highlights the exploited in Navajún, La Rioja (Spain).^[25]

China represents the main importing country with an import of around 376,000 tonnes, which resulted at 45% of total global imports. China is also the fastest growing in terms of the unroasted iron pyrites imports, with a CAGR of +27.8% from 2007 to 2016. In value terms, China ($47 million) constitutes the largest market for imported unroasted iron pyrites worldwide, making up 65% of global imports.^[26]

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 disulfide, H₂S₂. Thus pyrite would be more descriptively called iron persulfide, not iron disulfide. In contrast, molybdenite, MoS₂, features isolated sulfide (S^2-) centers and 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-.^[27]

Crystallography

Crystal structure of pyrite. In the center of the cell a S₂^2- pair is seen in yellow

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.^[28] It belongs to the crystallographic space group Pa3 and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant ${\displaystyle a}$ of stoichiometric iron pyrite FeS₂ amounts to .^[29] The unit cell is composed of a Fe face-centered cubic sublattice into which the ions are embedded. (Note though that the structure is simple cubic with four iron atoms per cubic unit cell, not face-centered cubic. The iron atoms in the faces are not equivalent by translation alone to the iron atoms at the corners.) The pyrite structure is also used 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.^[30]

In the first bonding sphere, the Fe atoms are surrounded by six S nearest neighbours, towards six of the eight faces of a distorted octahedron. The material is a semiconductor and the Fe ions should be considered to be in a low spin divalent state (as shown by Mössbauer spectroscopy as well as XPS), rather than a tetravalent state as the stoichiometry would suggest. The material as a whole acts as a Van Vleck paramagnet, despite its low-spin divalency^[31].

The positions of X ions in the pyrite structure may be derived from the fluorite structure, starting from a hypothetical Fe²⁺(S^-)₂ structure. Whereas F^- ions in CaF₂ occupy the centre positions of the eight subcubes of the cubic unit cell (​ ​ ​) 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 atomic parameter that takes different values in different pyrite-structure compounds (iron pyrite FeS₂: u(S) = 0.385^[32]). The shift from fluorite u = 0.25 to pyrite u = 0.385 is rather large and creates an S-S distance that is clearly a binding one. This is not surprising as in contrast to F^- an ion S^- is not a closed shell species. It is isoelectronic with a chlorine atom, also undergoing pairing to form Cl₂ molecules. Both low spin Fe²⁺ and the disulfide S₂^2- moeties are closed shell entities, explaining the diamagnetic and semiconducting properties.

The S atoms have bonds with three Fe and one other S atom. 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 sulfur lattice site, which causes a polarisation of S ions in the pyrite lattice.^[33] 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. This reflects the fact that the covalent bond in the sulfur pair is inadequately accounted for by a strictly ionic treatment.

Arsenopyrite has a related structure with heteroatomic As-S pairs rather than homoatomic ones. Marcasite also possesses homoatomic anion pairs, but the arrangement of the metal and diatomic anions is different from that of pyrite. Despite its name a chalcopyrite does not contain dianion pairs, but single S^2- sulfide anions.

Crystal habit

Pyritohedron-shaped crystals from Italy

Pyrite usually forms cuboid crystals, sometimes forming in close association to form raspberry-shaped masses called framboids. However, under certain circumstances, it can form anastamozing filaments or T-shaped crystals.^[34] Pyrite can also form shapes almost the same as a regular dodecahedral, known as pyritohedra, and this suggests an explanation for the artificial geometrical models found in Europe as early as the 5th century BC.^{[35][clarification needed]}

Varieties

Cattierite (Co S₂) and vaesite (Ni S₂) are similar in their structure and belong also to the pyrite group.

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).^[36]

Distinguishing similar minerals

It is distinguishable from native gold by its hardness, brittleness and crystal form. Natural gold tends to be anhedral (irregularly shaped), whereas pyrite comes as either cubes or multifaceted crystals. Pyrite can often be distinguished by the striations which, in many cases, can be seen on its surface. Chalcopyrite is brighter yellow with a greenish hue when wet and is softer (3.5-4 on Mohs' scale).^[37]Arsenopyrite is silver white and does not become more yellow when wet.

Hazards

A pyrite cube (center) has dissolved away from a host rock, leaving behind trace gold

Iron pyrite is unstable at Earth's surface: iron pyrite exposed to air and water decomposes into iron oxides and sulfate. This process is hastened by the action of Acidithiobacillus bacteria which oxidize the pyrite to produce ferrous iron and sulfate. These reactions occur more rapidly when the pyrite is in fine crystals and dust, which is the form it takes in most mining operations.

Acid drainage

Sulfate released from decomposing pyrite combines with water, producing sulfuric acid, leading to acid rock drainage. An example of acid rock drainage caused by pyrite is the 2015 Gold King Mine waste water spill.

Dust explosions

Pyrite oxidation is sufficiently exothermic that underground coal mines in high-sulfur coal seams have occasionally had serious problems with spontaneous combustion in the mined-out areas of the mine. The solution is to hermetically seal the mined-out areas to exclude oxygen.^[]

In modern coal mines, limestone dust is sprayed onto the exposed coal surfaces to reduce the hazard of dust explosions. This has the secondary benefit of neutralizing the acid released by pyrite oxidation and therefore slowing the oxidation cycle described above, thus reducing the likelihood of spontaneous combustion. In the long term, however, oxidation continues, and the hydrated sulfates formed may exert crystallization pressure that can expand cracks in the rock and lead eventually to roof fall.^[39]

Weakened building materials

Building stone containing pyrite tends to stain brown as the pyrite oxidizes. This problem appears to be significantly worse if any marcasite is present.^[40] The presence of pyrite in the aggregate used to make concrete can lead to severe deterioration as the pyrite oxidizes.^[41] In early 2009, problems with Chinese drywall imported into the United States after Hurricane Katrina were attributed to oxidation of pyrite, which releases hydrogen sulfide gas. These problems included a foul odor and corrosion of copper wiring.^[42] In the United States, in Canada,^{[43][dead link]} and more recently in Ireland,^[44][45][46] where it was used as underfloor infill, pyrite contamination has caused major structural damage. Modern tests for aggregate materials^[47] certify such materials as free of pyrite.

Pyritised fossils

Pyrite and marcasite commonly occur as replacement pseudomorphs after fossils in black shale and other sedimentary rocks formed under reducing environmental conditions.^[48] However, pyrite dollars or pyrite suns which have an appearance similar to sand dollars are pseudofossils and lack the pentagonal symmetry of the animal.

Images

• 

  As a replacement mineral in an ammonite from France

• 

  Pyrite from Ampliación a Victoria Mine, Navajún, La Rioja, Spain

• 

  Pyrite from the Sweet Home Mine, with golden striated cubes intergrown with minor tetrahedrite, on a bed of transparent quartz needles

• 

  Radiating form of pyrite

• 

  Paraspirifer bownockeri in pyrite

• 

  Pink fluorite perched between pyrite on one side and metallic galena on the other side

References

Further reading

• American Geological Institute, 2003, Dictionary of Mining, Mineral, and Related Terms, 2nd ed., Springer, New York, ISBN 978-3-540-01271-9.
• David Rickard, Pyrite: A Natural History of Fool's Gold, Oxford, New York, 2015, ISBN 978-0-19-020367-2.

External links

• Educational article about the famous pyrite crystals from the Navajun Mine
• How Minerals Form and Change "Pyrite oxidation under room conditions".
• Poliakoff, Martyn (2009). "Fool's Gold". The Periodic Table of Videos. University of Nottingham.