Pyrite

"Fool's Gold" redirects here. For other uses, see Fool's Gold (disambiguation).

This article is about iron pyrite. For other pyrite minerals, see Pyrite group.

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	02.EB.05a
Dana classification	2.12.1.1
Crystal symmetry	Isometric diploidal Space group: Pa3 H-M symbol: 2/m3
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.
Crystal system	Isometric
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

The mineral pyrite, or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula FeS₂. This mineral'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.

Pyrite is the most common of the sulfide minerals. The name pyrite is derived from the Greek πυρίτης (pyritēs), "of fire" or "in fire", in turn from πύρ (pyr), "fire". 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. By Georgius Agricola's time, c. 1550, the term had become a generic term for all of the sulfide minerals.

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, pyrite is sometimes found in association with small quantities of gold. Gold and arsenic occur as a coupled substitution in the pyrite structure. In the Carlin–type gold deposits, arsenian pyrite contains up to 0.37 wt% gold.

Uses

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

Pyrite enjoyed brief popularity in the 16th and 17th centuries as a source of ignition in early firearms, most notably the wheellock, where the cock held a lump of pyrite against a circular file to strike the sparks needed to fire the gun.

Pyrite has been used since classical times to manufacture copperas, that is, 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, 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.

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; at around 700 °C p_S2 is about 1 atm.

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

Pyrite is a semiconductor material with a band gap of 0.95 eV.

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.

Pyrite has been proposed as an abundant, inexpensive material in low cost photovoltaic solar panels. Synthetic iron sulfide was used with copper sulfide to create the photovoltaic material.

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. 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.

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 disulfide, H₂S₂. Thus pyrite would be more descriptively called iron persulfide, not iron disulfide. In contrast, molybdenite, MoS₂, features isolated sulfide (S²⁻) 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⁻.

Crystallography

Crystal structure of pyrite. In the center of the cell a S₂²⁻ 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. 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. The unit cell is composed of a Fe face-centered cubic sublattice into which the S ions are embedded. 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.

In the first bonding sphere, the Fe atoms are surrounded by six S nearest neighbours, in a distorted octahedral arrangement. The material is a diamagnetic 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 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 (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 atomic parameter that takes different values in different pyrite-structure compounds (iron pyrite FeS₂: u(S) = 0.385 ). The shift from fluorite u = 0.25 to pyrite u = 0.385 is rather large and creates a 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 S₂²⁻ 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. 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 S²⁻ sulfide anions.

Crystal habit

Dodecahedron- shaped crystals from Italy.

Pyrite usually forms cuboid crystals, sometimes forming in close association to form raspberry-like framboids. However, under certain circumstances, it can form anastamozing filaments or T-shaped crystals. Pyrite can also form dodecahedral crystals and this suggests an explanation for the artificial geometrical models found in Europe as early as the 5th century BC.

Varieties

Cattierite (CoS₂) and vaesite (NiS₂) 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).

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. Chalcopyrite is brighter yellow with a greenish hue when wet and is softer (3.5–4 on Mohs' scale). 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 in the natural environment: in nature it is always being created or being destroyed. 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 and potentially acid rain. 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.

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. The presence of pyrite in the aggregate used to make concrete can lead to severe deterioration as the pyrite oxidizes. In early 2009, problems with Chinese drywall imported into the United States after Hurricane Katrina were attributed to oxidation of pyrite. In the United States, in Canada, and more recently in Ireland, where it was used as underfloor infill, pyrite contamination has caused major structural damage. Modern tests for aggregate materials certify such materials as free of pyrite.

Pyritised fossils

As a replacement mineral in an ammonite from France.

Pyrite and marcasite commonly occur as replacement pseudomorphs after fossils in black shale and other sedimentary rocks formed under reducing environmental conditions.

Disc or "pyrite dollar" from south of Tucson, United States; diameter 10 cm

However, pyrite dollars or pyrite suns which have an appearance similar to sand dollars are pseudofossils and lack the pentagonal symmetry of the animal.

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".