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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.[8][9]
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.[14] 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.[15]
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.[16]
Pyrite has been used since classical times to manufacture copperas (ferrous 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.[18]
Pyrite is a semiconductor material with a band gap of 0.95 eV.[21] 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.[22]
Pyrite has been proposed as an abundant, non-toxic, inexpensive material in low-cost photovoltaic solar panels.[25] Synthetic iron sulfide was used with copper sulfide to create the photovoltaic material.[26] More recent efforts are working toward thin-film solar cells made entirely of pyrite.[22]
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.[27] 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 are Soria and La Rioja provinces (Spain).[28]
In July 2020 scientists reported that they have observed a voltage-induced transformation of normally diamagnetic pyrite into a ferromagnetic material, which may lead to applications in devices such as solar cells or magnetic data storage.[30][31]
Researchers at Trinity College Dublin, Ireland have demonstrated that FeS2 can be exfoliated into few-layers just like other two-dimensional layered materials such as graphene by a simple liquid-phase exfoliation route. This is the first study to demonstrate the production of non-layered 2D-platelets from 3D bulk FeS2. Furthermore, they have used these 2D-platelets with 20% single walled carbon-nanotube as an anode material in lithium-ion batteries, reaching a capacity of 1000 mAh/g close to the theoretical capacity of FeS2.[32]
In 2021, a natural pyrite stone has been crushed and pre-treated followed by liquid-phase exfoliation into two-dimensional nanosheets, which has shown capacities of 1200 mAh/g as an anode in lithium-ion batteries.[33]
Iron-pyrite FeS2 represents the prototype compound of the crystallographic pyrite structure. The structure is cubic and was among the first crystal structures solved by X-ray diffraction.[35] It belongs to the crystallographic space group Pa3 and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant a \displaystyle a of stoichiometric iron pyrite FeS2 amounts to 541.87 pm.[36] The unit cell is composed of a Fe face-centered cubic sublattice into which the S
2 ions are embedded. (Note though that the iron atoms in the faces are not equivalent by translation alone to the iron atoms at the corners.) The pyrite structure is also seen in other MX2 compounds of transition metals M and chalcogens X = O, S, Se and Te. Certain dipnictides with X standing for P, As and Sb etc. are also known to adopt the pyrite structure.[37]
The Fe atoms are bonded to six S atoms, giving a distorted octahedron. The material is a semiconductor. The Fe ions is usually considered to be low spin divalent state (as shown by Mssbauer spectroscopy as well as XPS). The material as a whole behaves as a Van Vleck paramagnet, despite its low-spin divalency.[38]
Pyrite usually forms cuboid crystals, sometimes forming in close association to form raspberry-shaped masses called framboids. However, under certain circumstances, it can form anastomosing filaments or T-shaped crystals.[42]Pyrite can also form shapes almost the same as a regular dodecahedron, known as pyritohedra, and this suggests an explanation for the artificial geometrical models found in Europe as early as the 5th century BC.[43][clarification needed]
Pyrite is distinguishable from native gold by its hardness, brittleness and crystal form. Pyrite fractures are very uneven, sometimes conchoidal because it does not cleave along a preferential plane. Native gold nuggets, or glitters, do not break but deform in a ductile way. Pyrite is brittle, gold is malleable.
Pyrite oxidation by atmospheric O2 in the presence of moisture (H2O) initially produces ferrous ions (Fe2+
) and sulfuric acid which dissociates into sulfate ions and protons, leading to acid mine drainage (AMD). An example of acid rock drainage caused by pyrite is the 2015 Gold King Mine waste water spill.
Pyrite oxidation is sufficiently exothermic that underground coal mines in high-sulfur coal seams have occasionally had serious problems with spontaneous combustion.[47] The solution is the use of buffer blasting and the use of various sealing or cladding agents to hermetically seal the mined-out areas to exclude oxygen.[48]
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.[49]
Building stone containing pyrite tends to stain brown as pyrite oxidizes. This problem appears to be significantly worse if any marcasite is present.[50] The presence of pyrite in the aggregate used to make concrete can lead to severe deterioration as pyrite oxidizes.[51] In early 2009, problems with Chinese drywall imported into the United States after Hurricane Katrina were attributed to pyrite oxidation, followed by microbial sulfate reduction which released hydrogen sulfide gas (H2S). These problems included a foul odor and corrosion of copper wiring.[52] In the United States, in Canada,[53] and more recently in Ireland,[54][55][56] where it was used as underfloor infill, pyrite contamination has caused major structural damage. Concrete exposed to sulfate ions, or sulfuric acid, degrades by sulfate attack: the formation of expansive mineral phases, such as ettringite (small needle crystals exerting a huge crystallization pressure inside the concrete pores) and gypsum creates inner tensile forces in the concrete matrix which destroy the hardened cement paste, form cracks and fissures in concrete, and can lead to the ultimate ruin of the structure. Normalized tests for construction aggregate[57] certify such materials as free of pyrite or marcasite.
Pyrite is the most common of sulfide minerals and is widespread in igneous, metamorphic, and sedimentary rocks. It is a common accessory mineral in igneous rocks, where it also occasionally occurs as larger masses arising from an immiscible sulfide phase in the original magma. It is found in metamorphic rocks as a product of contact metamorphism. It also forms as a high-temperature hydrothermal mineral, though it occasionally forms at lower temperatures.[2]
Pyrite occurs both as a primary mineral, present in the original sediments, and as a secondary mineral, deposited during diagenesis.[2] Pyrite and marcasite commonly occur as replacement pseudomorphs after fossils in black shale and other sedimentary rocks formed under reducing environmental conditions.[58] Pyrite is common as an accessory mineral in shale, where it is formed by precipitation from anoxic seawater, and coal beds often contain significant pyrite.[59]
Notable deposits are found as lenticular masses in Virginia, U.S., and in smaller quantities in many other locations. Large deposits are mined at Rio Tinto in Spain and elsewhere in the Iberian Peninsula.[60]
In addition to pyrite, common sulfides are chalcopyrite (copper iron sulfide), pentlandite (nickel iron sulfide), and galena (lead sulfide). The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, and the sulfosalts. Many sulfides are economically important as metal ores.
Pyrite is so named from the Greek word for fire (pyr) because it can create sparks for starting a fire when struck against metal or stone. This property made it useful for firearms at one time but this application is now obsolete. Pyrite was once a source of sulfur and sulfuric acid, but today most sulfur is obtained as a byproduct of natural gas and crude oil processing.
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