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Greigite - Wikipedia

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Greigite
Greigite structure, SFe₄ tetrahedra
General
Category	Sulfide mineral Thiospinel group Spinel structural group
Formula (repeating unit)	Fe²⁺Fe³⁺₂S₄
IMA symbol	Grg^[1]
Strunz classification	2.DA.05
Crystal system	Cubic
Crystal class	Hexoctahedral (m3m) H-M symbol: (4/m 3 2/m)
Space group	Fd3m
Unit cell	a = 9.876 Å; Z = 8
Identification
Color	Pale pink, tarnishes to metallic blue-black
Crystal habit	Spheres of intergrown octahedra and as disseminated microscopic grains
Mohs scale hardness	4 to 4.5
Luster	Metallic to earthy
Diaphaneity	Opaque
Specific gravity	4.049
Other characteristics	Strongly magnetic
References	^[2]^[3]^[4]

Greigite is an iron sulfide mineral with the chemical formula Fe²⁺Fe³⁺₂S₄. It is the sulfur equivalent of the iron oxide magnetite (Fe₃O₄). 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).^[4]^[5]

Natural occurrence and composition

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.^[2] Greigite has also been identified in the sclerites of scaly-foot gastropods.^[6]

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.^[2]^[3]

Common impurities include Cu, Ni, Zn, Mn, Cr, Sb and As.^[3] Ni impurities are of particular interest because the structural similarity between Ni-doped greigite and the (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.^[7] In particular, the cubic Fe₄S₄ unit of greigite is found in the Fe₄S₄ 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.^[2]^[8]

Magnetic and electronic properties

Like the related oxide magnetite (Fe₃O₄), 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.^[9]^[10]

References

^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
^ ^a ^b ^c ^d Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C., eds. (1990). "Greigite" (PDF). Handbook of Mineralogy. Vol. I (Elements, Sulfides, Sulfosalts). Chantilly, VA, US: Mineralogical Society of America. ISBN 0962209708. Retrieved December 5, 2011.
^ ^a ^b ^c Greigite. Mindat.org
^ ^a ^b Greigite. Webmineral
^ Skinner, Brian J.; Erd, Richard C.; Grimaldi, Frank S. (1964). "Greigite, the thio-spinel of iron; a new mineral" (PDF). American Mineralogist. 49: 543–55.
^ "Armor-Plated Snail Discovered in Deep Sea". news.nationalgeographic.com. Archived from the original on November 10, 2003. Retrieved 2016-08-29.
^ Russell, Michael J.; Martin, William (2004). "The rocky roots of the acetyl-CoA pathway". Trends in Biochemical Sciences. 29 (7): 358–363. doi:10.1016/j.tibs.2004.05.007. ISSN 0968-0004. PMID 15236743.
^ Vaughan, D. J.; Craig, J. R. “Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0-521-21489-0.
^ Devey, A.J.; Grau-Crespo, R.; Leeuw, N.H. (2009). "Electronic and magnetic structure of Fe₃S₄: GGA+U investigation". Physical Review B. 79 (19): 195126. Bibcode:2009PhRvB..79s5126D. doi:10.1103/PhysRevB.79.195126.
^ Wang, Jun; Cao, Shi-He; Wu, Wei; Zhao, Guo-Meng (2011). "The Curie temperature and magnetic exchange energy in half-metallic greigite Fe₃S₄". Physica Scripta. 83 (4): 045702. Bibcode:2011PhyS...83d5702W. doi:10.1088/0031-8949/83/04/045702. S2CID 121375069.

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