{{short description|Iron sulfide mineral of spinel structure}} {{Infobox mineral | name = Greigite | image = Greigite structure 110 SFe4 tetrahedra.png | alt = | caption = Greigite structure, SFe<sub>4</sub> tetrahedra | category = Sulfide mineral <br />Thiospinel group <br />Spinel structural group | formula = {{chem2|Fe^{2+}Fe^{3+}2S4}} | IMAsymbol = Grg<ref>{{Cite journal|last=Warr|first=L.N.|date=2021|title=IMA–CNMNC approved mineral symbols|journal=Mineralogical Magazine|volume=85|issue=3|pages=291–320|doi=10.1180/mgm.2021.43|bibcode=2021MinM...85..291W|s2cid=235729616|doi-access=free}}</ref> | molweight = | strunz = 2.DA.05 | dana = | system = Cubic | class = Hexoctahedral (m{{overline|3}}m) <br/>H-M symbol: (4/m {{overline|3}} 2/m) | symmetry = ''F''d{{overline|3}}m | unit cell = a = 9.876 Å; Z = 8 | color = Pale pink, tarnishes to metallic blue-black | colour = | habit = Spheres of intergrown octahedra and as disseminated microscopic grains | twinning = | cleavage = | fracture = | tenacity = | mohs = 4 to 4.5 | luster = Metallic to earthy | streak = | diaphaneity = Opaque | gravity = 4.049 | density = | polish = | opticalprop = | refractive = | birefringence = | pleochroism = | 2V = | dispersion = | extinction = | length fast/slow = | fluorescence = | absorption = | melt = | fusibility = | diagnostic = | solubility = | impurities = | alteration = | other = Strongly magnetic | prop1 = | prop1text = | references = <ref name=Handbook>{{cite book|editor1=Anthony, John W. |editor2=Bideaux, Richard A. |editor3=Bladh, Kenneth W. |editor4=Nichols, Monte C. |title= Handbook of Mineralogy|publisher= Mineralogical Society of America|place= Chantilly, VA, US|chapter-url=http://rruff.geo.arizona.edu/doclib/hom/greigite.pdf |chapter=Greigite |access-date=December 5, 2011|isbn=0-9622097-0-8 |volume=I (Elements, Sulfides, Sulfosalts)|year=1990}}</ref><ref name=Mindat>[http://www.mindat.org/min-1747.html Greigite]. Mindat.org</ref><ref name=Webmin>[http://webmineral.com/data/Greigite.shtml Greigite]. Webmineral</ref> }} '''Greigite''' is an iron sulfide mineral with the chemical formula {{chem2|Fe^{2+}Fe^{3+}2S4}}. It is the sulfur equivalent of the iron oxide magnetite (Fe<sub>3</sub>O<sub>4</sub>). It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogist and physical chemist Joseph W. Greig (1895–1977).<ref name=Webmin/><ref>{{cite journal | url = http://www.minsocam.org/ammin/AM49/AM49_543.pdf | title = Greigite, the thio-spinel of iron; a new mineral | journal = American Mineralogist |first1 = Brian J. | last1 = Skinner | first2 = Richard C. | last2= Erd | first3 = Frank S. | last3 = Grimaldi | pages = 543–55 | volume = 49 | year = 1964}}</ref>
==Natural occurrence and composition== {{See also|Iron-sulfur protein}} It occurs in lacustrine sediments with clays, silts and arkosic sand often in varved sulfide rich clays. It is also found in hydrothermal veins. Greigite is formed by magnetotactic bacteria and sulfate-reducing bacteria.<ref name=Handbook/> Greigite has also been identified in the sclerites of scaly-foot gastropods.<ref>{{Cite web|url=http://news.nationalgeographic.com/news/2003/11/1107_031107_snailarmor.html|archive-url=https://web.archive.org/web/20031110015525/http://news.nationalgeographic.com/news/2003/11/1107_031107_snailarmor.html|archive-date=November 10, 2003|title=Armor-Plated Snail Discovered in Deep Sea|website=news.nationalgeographic.com|access-date=2016-08-29}}</ref> The mineral typically appears as microscopic (< 0.03 mm) isometric hexoctahedral crystals and as minute sooty masses. Association minerals include montmorillonite, chlorite, calcite, colemanite, veatchite, sphalerite, pyrite, marcasite, galena and dolomite.<ref name=Handbook/><ref name=Mindat/>
Common impurities include Cu, Ni, Zn, Mn, Cr, Sb and As.<ref name=Mindat/> Ni impurities are of particular interest because the structural similarity between Ni-doped greigite and the {{chem2|(Fe,Ni)S}} clusters present in biological enzymes has led to suggestions that greigite or similar minerals could have acted as catalysts for the origin of life.<ref name="RussellMartin2004">{{cite journal|last1=Russell|first1=Michael J.|last2=Martin|first2=William|title=The rocky roots of the acetyl-CoA pathway|journal=Trends in Biochemical Sciences|volume=29|issue=7|year=2004|pages=358–363|issn=0968-0004|doi=10.1016/j.tibs.2004.05.007|pmid=15236743}}</ref> In particular, the cubic Fe<sub>4</sub>S<sub>4</sub> unit of greigite is found in the Fe<sub>4</sub>S<sub>4</sub> thiocubane units of proteins of relevance to the acetyl-CoA pathway.
==Crystal structure== Greigite has the spinel structure. The crystallographic unit cell is cubic, with space group Fd3m. The S anions form a cubic close-packed lattice, and the Fe cations occupy both tetrahedral and octahedral sites.<ref name=Handbook/><ref name=Vaughan>Vaughan, D. J.; Craig, J. R. "Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. {{ISBN|0-521-21489-0}}.</ref>
==Magnetic and electronic properties== Like the related oxide magnetite (Fe<sub>3</sub>O<sub>4</sub>), greigite is ferrimagnetic, with the spin magnetic moments of the Fe cations in the tetrahedral sites oriented in the opposite direction as those in the octahedral sites, and a net magnetization. It is a mixed-valence compound, featuring both Fe(II) and Fe(III) centers in a 1:2 ratio. Both metal sites have high spin quantum numbers. The electronic structure of greigite is that of a half metal.<ref name=devey09>{{cite journal|last1=Devey|first1=A.J.|last2=Grau-Crespo|first2=R.|last3=Leeuw|first3=N.H.|title=Electronic and magnetic structure of Fe<sub>3</sub>S<sub>4</sub>: GGA+U investigation|journal=Physical Review B|year=2009|volume=79|article-number=195126| doi=10.1103/PhysRevB.79.195126|issue=19|bibcode = 2009PhRvB..79s5126D }}</ref><ref>{{cite journal| title=The Curie temperature and magnetic exchange energy in half-metallic greigite Fe<sub>3</sub>S<sub>4</sub>|doi=10.1088/0031-8949/83/04/045702|year=2011|volume=83|article-number=045702| last1=Wang| first1=Jun| last2=Cao| first2=Shi-He| last3=Wu| first3=Wei| last4=Zhao| first4=Guo-Meng| journal=Physica Scripta| issue=4|bibcode = 2011PhyS...83d5702W |s2cid=121375069 }}</ref>
== On Mars == A September 10, 2025 paper published in ''Nature'' reported the "likely" detection of greigite and vivianite in the Jezero crater on Mars, by the ''Perseverance'' rover.<ref name="Nature2025"/> It is considered a potential biosignature.<ref name="NASA2025"/>
==References== {{Reflist| refs=
<ref name="Nature2025">{{cite journal |last1=Hurowitz |first1=J. A. |last2=Tice |first2=M. M. |last3=Allwood |first3=A. C. |title=Redox-driven mineral and organic associations in Jezero Crater, Mars |journal=Nature |date=11 September 2025 |volume=645 |pages=332–340 |doi=10.1038/s41586-025-09413-0 |doi-access=free }}</ref>
<ref name="NASA2025">{{cite web |last1=Taveau |first1=Jessica |title=NASA Says Mars Rover Discovered Potential Biosignature Last Year |url=https://www.nasa.gov/news-release/nasa-says-mars-rover-discovered-potential-biosignature-last-year/ |website=nasa.gov |publisher=NASA|date=2025-09-10}}</ref>
}}
Category:Thiospinel group Category:Iron(II,III) minerals Category:Ferromagnetic materials Category:Magnetic minerals Category:Cubic minerals Category:Minerals in space group 227