This article is about iron pyrite. For other pyrite minerals, seePyrite group.
Pyrite
Intergrowth of lustrous, cubic crystals of pyrite, with some surfaces showing characteristic striations, from Huanzala mine, Ancash, Peru. Specimen size: 7.0 × 5.0 × 2.5 cm
Cubic, faces may be striated, but also frequently octahedral and pyritohedral. Often inter-grown, massive, radiated, granular, globular, and stalactitic.
Pyrite cubic crystals onmarl fromNavajún,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)
Pyrite's metallicluster and pale brass-yellowhue give it a superficial resemblance togold, hence the well-known nickname offool's gold. The color has also led to the nicknamesbrass,brazzle, andbrazil, primarily used to refer to pyrite found incoal.[8][9]
The namepyrite is derived from theGreekπυρίτης λίθος (pyritēs lithos), 'stone or mineral which strikes fire',[10] in turn fromπῦρ (pŷr), 'fire'.[11] In ancient Roman times, this name was applied to several types of stone that would create sparks when struck againststeel;Pliny the Elder described one of them as being brassy, almost certainly a reference to what is now called pyrite.[12]
Pyrite is usually found associated with other sulfides oroxides inquartzveins,sedimentary rock, andmetamorphic rock, as well as in coal beds and as a replacement mineral infossils, but has also been identified in thesclerites ofscaly-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. It has been suggested that the presence of both gold andarsenic is a case ofcoupled substitution but as of 1997 the chemical state of the gold remained controversial.[15]
Pyrite gained a brief popularity in the 16th and 17th centuries as a source ofignition in earlyfirearms, most notably thewheellock, 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 manufacturecopperas (ferrous sulfate). Iron pyrite was heaped up and allowed to weather (an example of an early form ofheap 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 ofsulfuric acid. By the 19th century, it had become the dominant method.[18]
Pyrite remains in commercial use for the production ofsulfur dioxide, for use in such applications as thepaper 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),pS2 is about1 atm.[19]
Pyrite is asemiconductor material with aband gap of 0.95eV.[21] Pure pyrite is naturallyn-type, in both crystal and thin-film forms, potentially due to sulfur vacancies in the pyrite crystal structure acting as n-dopants.[22]
During the early years of the 20th century, pyrite was used as amineral detector inradio receivers, and is still used bycrystal radio hobbyists. Until thevacuum tube matured, the crystal detector was the most sensitive and dependabledetector available—with considerable variation between mineral types and even individual samples within a particular type of mineral. Pyrite detectors occupied a midway point betweengalena detectors and the more mechanically complicatedperikon mineral pairs. Pyrite detectors can be as sensitive as a modern 1N34Agermaniumdiode detector.[23][24]
Pyrite has been proposed as an abundant, non-toxic, inexpensive material in low-costphotovoltaic solar panels.[25] Synthetic iron sulfide was used withcopper 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 makemarcasite jewelry. Marcasite jewelry, using small faceted pieces of pyrite, often set insilver, has been made since ancient times and was popular in theVictorian 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 mineralmarcasite 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 areSoria andLa Rioja provinces (Spain).[28]
In value terms,China ($47 million) constitutes the largest market for imported unroasted iron pyrites worldwide, making up 65% of global imports. China is also the fastest growing in terms of the unroasted iron pyrites imports, with aCAGR of +27.8% from 2007 to 2016.[29]
In July 2020 scientists reported that they have observed a voltage-induced transformation of normallydiamagnetic pyrite into aferromagnetic material, which may lead to applications in devices such as solar cells or magnetic data storage.[30][31]
Researchers atTrinity 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]
Formal oxidation states for pyrite, marcasite, molybdenite and arsenopyrite
From the perspective of classicalinorganic chemistry, which assigns formal oxidation states to each atom, pyrite and marcasite are probably best described as Fe2+[S2]2−. This formalism recognizes that the sulfur atoms in pyrite occur in pairs with clear S–S bonds. Thesepersulfide [−S–S−] units can be viewed as derived fromhydrogen 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 mineralarsenopyrite has the formula FeAsS. Whereas pyrite has [S2]2− units, arsenopyrite has [AsS]3− units, formally derived fromdeprotonation of arsenothiol (H2AsSH). Analysis of classical oxidation states would recommend the description of arsenopyrite as Fe3+[AsS]3−.[34]
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 thecrystallographic pyrite structure. The structure iscubic and was among the firstcrystal structures solved byX-ray diffraction.[35] It belongs to the crystallographicspace groupPa3 and is denoted by theStrukturbericht notation C2. Under thermodynamic standard conditions thelattice constant of stoichiometric iron pyrite FeS2 amounts to541.87 pm.[36] Theunit cell is composed of a Feface-centered cubic sublattice into which theS 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 otherMX2 compounds oftransition metalsM andchalcogensX =O,S,Se andTe. Certaindipnictides withX standing forP,As andSb 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 asemiconductor. The Fe ions are usually considered to below spindivalent state (as shown byMössbauer spectroscopy as well as XPS). The material as a whole behaves as a Van Vleckparamagnet, despite its low-spin divalency.[38]
The sulfur centers occur in pairs, described asS2− 2.[39] Reduction of pyrite with potassium givespotassium dithioferrate, KFeS2. This material features ferric ions and isolated sulfide (S2−) centers.
The S atoms are tetrahedral, being bonded to three Fe centers and one other S atom. The site symmetry at Fe and S positions is accounted for bypoint symmetry groupsC3i andC3, respectively. The missingcenter 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 apolarization of S ions in the pyrite lattice.[40] The polarisation can be calculated on the basis of higher-orderMadelung constants and has to be included in the calculation of thelattice energy by using a generalisedBorn–Haber cycle. This reflects the fact that the covalent bond in the sulfur pair is inadequately accounted for by a strictly ionic treatment.[41]
Arsenopyrite has a related structure with heteroatomic As–S pairs rather than S-S pairs. Marcasite also possesses homoatomic anion pairs, but the arrangement of the metal and diatomic anions differs from that of pyrite. Despite its name, chalcopyrite (CuFeS 2) does not contain dianion pairs, but single S2− sulfide anions.
Pyrite usually forms cuboid crystals, sometimes forming in close association to form raspberry-shaped masses calledframboids. However, under certain circumstances, it can formanastomosing filaments or T-shaped crystals.[42]Pyrite can also form shapes almost the same as a regulardodecahedron, 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]
Bravoite is a nickel-cobalt bearing variety of pyrite, with > 50% substitution ofNi2+ for Fe2+ within pyrite. Bravoite is not a formally recognised mineral, and is named after the Peruvian scientist Jose J. Bravo (1874–1928).[44]
Pyrite is distinguishable fromnative gold by its hardness, brittleness and crystal form. Pyrite fractures are veryuneven, sometimesconchoidal because it does not cleave along a preferential plane. Nativegold nuggets, or glitters, do not break but deform in aductile way. Pyrite is brittle, gold is malleable. Natural gold tends to beanhedral (irregularly shaped without well defined faces), whereas pyrite comes as either cubes or multifaceted crystals with well developed and sharp faces easy to recognise. Well crystallised pyrite crystals areeuhedral (i.e., with nice faces). Pyrite can often be distinguished by the striations which, in many cases, can be seen on its surface.
Chalcopyrite (CuFeS2) is brighter yellow with a greenish hue when wet and is softer (3.5–4 on Mohs' scale).[45]Arsenopyrite (FeAsS) is silver white and does not become more yellow when wet.
A pyrite cube (center) has dissolved away from a host rock, leaving behind trace gold
Iron pyrite is unstable when exposed to theoxidizing conditions prevailing at the Earth's surface: iron pyrite in contact with atmosphericoxygen and water, or damp, ultimately decomposes intoiron oxyhydroxides (ferrihydrite, FeO(OH)) andsulfuric acid (H 2SO 4). This process is accelerated by the action ofAcidithiobacillus bacteria which oxidize pyrite to first produceferrous ions (Fe2+ ),sulfate ions (SO2− 4), and release protons (H+, orH3O+). In a second step, the ferrous ions (Fe2+ ) are oxidized byO2 intoferric ions (Fe3+ ) whichhydrolyze also releasingH+ ions and producing FeO(OH). These oxidation reactions occur more rapidly when pyrite is finely dispersed (framboidal crystals initially formed bysulfate reducing bacteria (SRB) in argillaceous sediments or dust from mining operations).
Pyrite oxidation is sufficientlyexothermic that undergroundcoal mines in high-sulfur coal seams have occasionally had serious problems withspontaneous combustion.[47] The solution is the use of buffer blasting and the use of various sealing or cladding agents tohermetically 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 ofdust 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 thehydratedsulfates formed may exert crystallization pressure that can expand cracks in the rock and lead eventually toroof fall.[49]
Building stone containing pyrite tends to stain brown as pyrite oxidizes. This problem appears to be significantly worse if anymarcasite is present.[50] The presence of pyrite in theaggregate used to makeconcrete can lead to severe deterioration as pyrite oxidizes.[51] In early 2009, problems withChinese drywall imported into theUnited States afterHurricane Katrina were attributed to pyrite oxidation, followed by microbial sulfate reduction which releasedhydrogen sulfide gas (H2S). These problems included a foul odor andcorrosion ofcopper 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 bysulfate attack: the formation of expansive mineral phases, such asettringite (small needle crystals exerting a huge crystallization pressure inside the concrete pores) andgypsum creates innertensile forces in the concrete matrix which destroy the hardenedcement paste, form cracks and fissures in concrete, and can lead to the ultimate ruin of the structure. Normalized tests forconstruction 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 animmiscible sulfide phase in the original magma. It is found in metamorphic rocks as a product ofcontact metamorphism. It also forms as a high-temperaturehydrothermal 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 duringdiagenesis.[2] Pyrite andmarcasite commonly occur as replacementpseudomorphs afterfossils inblack shale and othersedimentary rocks formed underreducing 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 the beliefs of the Thai people (especially those in the south), pyrite is known asKhao tokPhra Ruang,Khao khon bat Phra Ruang (ข้าวตอกพระร่วง, ข้าวก้นบาตรพระร่วง) orPhet na tang,Hin na tang (เพชรหน้าทั่ง, หินหน้าทั่ง)[clarification needed]. It is believed to be asacred item that has the power to prevent evil,black magic or demons.[61][62]
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