Greigite

Greigite
Greigite structure, SFe4 tetrahedra
General
Category	Sulfide mineral Thiospinel group Spinel structural group
Formula (repeating unit)	Fe2+Fe3+2S4
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

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).

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. Greigite has also been identified in the sclerites of scaly-foot gastropods.

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.

Common impurities include Cu, Ni, Zn, Mn, Cr, Sb and As. 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. In particular, the cubic Fe₄S₄ unit of greigite is found in the Fe₄S₄ thiocubane units of proteins of relevance to the acetyl-CoA pathway.