ແຮ່ໄພໄຣ

ແຮ່ ໄພໄຣ (pyrite), ຫຼື ແຮ່ເຫຼັກໄພໄຣ, ຮູ້ກັນໃນອີກຊື່ໜຶ່ງວ່າ ຄຳຄົນໂງ່ (fool's gold), ແມ່ນທາດ ເຫຼັກຊຸນຟິດ ທີ່ມີ ສູດທາງເຄມີ ວ່າ FeS2. ຄວາມມັນເງົາ ແລະ ສີເຫຼືອງເສົ້າຂອງແຮ່ທາດຊະນິດນີ້ ເຮັດໃຫ້ມັນມີລັກສະນະພາຍນອກຄ້າຍຄືກັບຄຳ, ດັ່ງນັ້ນມັນຈຶ່ງມີຊື່ເອີ້ນອີກຊື່ໜຶ່ງວ່າ ຄຳຄົນໂງ່. ສີຂອງມັນຍັງນຳໄປສູ່ຊື່ເອີ້ນແບບອື່ນໆເຊັ່ນ: brass, brazzle ແລະ Brazil, ໂດຍໃນເບື້ອງຕົ້ນຖືກໃຊ້ເພື່ອກ່າວເຖິງແຮ່ໄພໄຣທີ່ພົບໃນ ຖ່ານຫີນ.

ແຮ່ໄພໄຣແມ່ນແຮ່ຊຸນຟິດທີ່ພົບໄດ້ໂດຍທົ່ວໄປ. ຄຳວ່າໄພໄຣ (pyrite) ແມ່ນໄດ້ຮັບມາຈາກພາສາເກຣັກ ຄຳວ່າ πυρίτης (pyritēs), ເຊິ່ງແປວ່າ "ແຫ່ງໄຟ" ຫຼື "ໃນໄຟ", ເຊິ່ງຄຳເຄົ້າແມ່ນ πύρ (pyr), ແປວ່າ "ໄຟ". ໃນສະໄໝໂຣມັນບູຮານ, ຊື່ນີ້ໄດ້ໃຊ້ເອີ້ນຫີນຫຼາຍຊະນິດທີ່ສ້າງໝັດໄຟເມື່ອນຳໄປສຽດສີກັບເຫຼັກ; Pliny the Elder ໄດ້ອະທິບາຍເຖິງລັກສະນະຂອງແຮ່ຊະນິດນີ້ວ່າມີສີຄ້າຍຄືກັບທອງເຫຼືອງ (brassy), ເຊິ່ງເປັນສິ່ງອ້າງອີງເຖິງສິ່ງທີ່ເຮົາເອີ້ນວ່າໄພໄຣ (pyrite) ໃນປັດຈຸບັນ.
ຈົນກະທັງຮອດສະໄໝຂອງ ຈໍຈຽສ ອະກຣິໂກລາ (Georgius Agricola), ຄ.ສ. 1550, ຄຳສັບດັ່ງກ່າວໄດ້ກາຍມາເປັນຊື່ເອີ້ນທົ່ວໄປຂອງແຮ່ຊຸນຟິດທັງໝົດ.

ໂດຍທົ່ວໄປຈະພົບວ່າແຮ່ໄພໄຣມີຄວາມກ່ຽວຂ້ອງກັບທາດຊຸນຟິດ ຫຼື ອົດຊິດອື່ນໆໃນສາຍແຮ່ຄວອດສ໌ (quartz), ຫີນຕະກອນ, ແລະ ຫີນແປ (metamorphic rock), ເຊັ່ນດຽວກັບໃນຊັ້ນຫີນຂອງຖ່ານຫີນ ແລະ ໃນແຮ່ທາດທີ່ເຂົ້າແທນທີ່ກະດູກໃນຟອສຊິວ. ເຖິງວ່າຈະຖືກເອີ້ນວ່າຄຳຂອງຄົນໂງ່, ບາງຄັ້ງກໍພົບວ່າມີແຮ່ຄຳປະລິມານເລັກໜ້ອຍປະສົມຢູ່ໃນແຮ່ໄພໄຣ. ຄຳ ແລະ ອາເຊນິກ ມີການຈັບຕົວກັນເຂົ້າມາແທນທີ່ຢູ່ໃນໂຄງສ້າງຂອງແຮ່ໄພໄຣ. ໃນແຫຼ່ງແຮ່ຄຳທີ່ເກີດໃນຫີນຕະກອນລະດັບຕື້ນ, ແຮ່ໄພໄຣທີ່ມີສ່ວນປະກອບຂອງອາເຊນິກຈະມີຄຳປະສົມຢູ່ໃນປະລິມານສູງເຖິງ 0.37% ຂອງນ້ຳໜັກທັງໝົດ.

ການນຳໃຊ້

ແຮ່ໄພໄຣເປັນທີ່ນິຍົມໃນໄລຍະສັ້ນໆ ໃນສະຕະວັດທີ 16 ແລະ 17 ເຊິ່ງໃຊ້ເປັນຕົວກຳເນີດການຈູດໄຟ ໃນອາວຸດປືນໃນໄລຍະຕົ້ນ, ເຊິ່ງໃຊ້ເປັນຫົວສັບຂອງປືນ ໂດຍມີກ້ອນແຮ່ໄພໄຣເພື່ອກໍໃຫ້ເກີດປະກາຍໄຟໃນຂະນະທີ່ຍິງປືນ.

ແຮ່ໄພໄຣໄດ້ຖືກນຳໃຊ້ມາຕັ້ງແຕ່ຍຸກໂຣມັນບູຮານເພື່ອຜະລິດ copperas, ເຊິ່ງກໍຄື ເຫຼັກ (II) ຊຸນຟັດ.  ແຮ່ເຫຼັກໄພໄຣຈະຖືກນຳໄປກອງສຸມກັນ ແລະ ປ່ອຍໄວ້ໃຫ້ຖືກກັບອາກາດ (ຕົວຢ່າງໜຶ່ງຂອງຮູບການລະລາຍແຮ່ໃນໄລຍະຕົ້ນ).  ຫຼັງຈາກນັ້ນຂອງແຫຼວທີ່ເປັນອາຊິດທີ່ໄຫຼອອກມາຈາກກອງແຮ່ຈະຖືກນຳໄປຕົ້ມລວມກັບເຫຼັກເພື່ອຜະລິດເຫຼັກຊຸນຟັດ.  ໃນສະຕະວັດທີ 15, ວິທີການແຍກແຮ່ແບບດັ່ງກ່າວເລີ່ມໄດ້ເຂົ້າມາແທນທີ່ການເຜົາຊຸນເຟີເພື່ອໃຊ້ເປັນແຫຼ່ງກຳເນີດອາຊິດຊຸນຟູຣິກ. ມາຮອດສະຕະວັດທີ 19, ວິທີການດັ່ງກ່າວໄດ້ກາຍເປັນຂະບວນການທີ່ນຳໃຊ້ກັນຢ່າງພົ້ນເດັ່ນ..

ແຮ່ໄພໄຣຍັງຄົງຖືກນຳໃຊ້ໃນທາງການຄ້າສຳລັບການຜະລິດຊຸນເຟີດີອົກຊິດ ເພື່ອໃຊ້ເຂົ້າໃນອຸດສາຫະກຳການຜະລິດເຈ້ຍ ແລະ ໃນການຜະລິດອາຊິດຊຸນຟູຣິກ. ການສະຫຼາຍຄວາມຮ້ອນຂອງແຮ່ໄພໄຣກາຍເປັນ FeS (ເຫຼັກ(II) ຊຸນຟິດ) ແລະ ທາດຊຸນເຟີເລີ່ມຕົ້ນທີ່ ; ແລະ ທີ່ປະມານ  pS2 ແມ່ນມີປະມານ .

ການນຳໃຊ້ແຮ່ໄພໄຣໃນທາງການຄ້າສະໄໝໃໝ່ແມ່ນໃຊ້ເປັນວັດຖຸກາໂຕດ (cathode) ໃນແບັດເຕີຣີລິທຽມແບບສາກໄຟບໍ່ໄດ້ຂອງຍີ່ຫໍ້Energizer.

ແຮ່ໄພໄຣແມ່ນວັດຖຸເຄິ່ງໂຕນຳ (semiconductor material) ທີ່ມີແຖບພະລັງງານຂະໜາດ 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

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 Pa and is denoted by the Strukturbericht notation C2. Under thermodynamic standard conditions the lattice constant  of stoichiometric iron pyrite FeS2 amounts to . 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 C3i 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 

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 quasicrystals 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

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. 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

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

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

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".
 

Pyrite group
Sulfide minerals
Iron minerals
Alchemical substances
Firelighting
Semiconductor materials
Cubic minerals
Transition metal dichalcogenides