Greigite is an iron sulfide mineral with the chemical formula . 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).

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. 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 Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

, colemanite, veatchite,

sphalerite Sphalerite is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in Sedimentary exhalative deposits, sedimentary exhalative, Carbonate-hoste ...

, 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 clusters present in biological enzymes has led to suggestions that greigite or similar minerals could have acted as catalysts for the

origin of life Abiogenesis is the natural process by which life arises from abiotic component, non-living matter, such as simple organic compounds. The prevailing scientific hypothesis is that the transition from non-living to organism, living entities on ...

. 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.Vaughan, D. J.; Craig, J. R. “Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. .

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.

References

{{Reflist Thiospinel group Iron(II,III) minerals Ferromagnetic materials Magnetic minerals Cubic minerals Minerals in space group 227