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Greigite

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Greigite

Greigite
Greigite structure, SFe4 tetrahedra
General
Category Sulfide mineral
Thiospinel group
Spinel structural group
Formula
(repeating unit)
Fe2+Fe3+2S4
Strunz classification 02.DA.05
Crystal symmetry Isometric hexoctahedral
H-M symbol: (4/m32/m)
Space group: F d3m
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
Crystal system Cubic
Mohs scale hardness 4 to 4.5
Luster Metallic to earthy
Diaphaneity Opaque
Specific gravity 4.049
Other characteristics Strongly magnetic
References [1][2][3]

Greigite is an iron sulfide mineral with formula Fe3S4 (Iron(II,III) sulfide). 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).[3][4]

Contents

  • Natural occurrence and composition 1
  • Crystal structure 2
  • Magnetic and electronic properties 3
  • References 4

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.[1]

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.[1][2]

Common impurities include Cu, Ni, Zn, Mn, Cr, Sb and As.[2] 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 minerals could have acted as catalysts for the origin of life.[5] 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.[1][6]

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.[7][8]

References

  1. ^ a b c d Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W. and Nichols, Monte C., ed. (1990). "Greigite". Handbook of Mineralogy (PDF). I (Elements, Sulfides, Sulfosalts). Chantilly, VA, US: Mineralogical Society of America.  
  2. ^ a b c Greigite. Mindat.org
  3. ^ a b Greigite. Webmineral
  4. ^ 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. 
  5. ^ Russell, Michael J.; Martin, William (2004). "The rocky roots of the acetyl-CoA pathway". Trends in Biochemical Sciences 29 (7): 358–363.  
  6. ^ Vaughan, D. J.; Craig, J. R. “Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0-521-21489-0.
  7. ^ Devey, Anthony; R. Grau-Crespo and N.H. de Leeuw (2009). "Electronic and magnetic structure of Fe3S4: GGA+U investigation". Physical Review B 79 (19): 195126.  
  8. ^ 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.  
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