| Greigite | |
|---|---|
 Greigite structure, SFe4 tetrahedra | |
| General | |
| Category | Sulfide mineral Thiospinel group Spinel structural group |
| Formula | Fe2+Fe3+2S4 |
| IMA symbol | Grg[1] |
| Strunz classification | 2.DA.05 |
| Crystal system | Cubic |
| Crystal class | Hexoctahedral (m3m) H-M symbol: (4/m3 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 anironsulfide mineral with the chemical formulaFe2+Fe3+2S4. It is the sulfur equivalent of the iron oxidemagnetite (Fe3O4). It was first described in 1964 for an occurrence inSan Bernardino County, California, and named after the mineralogist and physical chemistJoseph W. Greig (1895–1977).[4][5]
It occurs inlacustrine sediments withclays,silts andarkosic sand often invarvedsulfide rich clays. It is also found inhydrothermalveins. Greigite is formed bymagnetotactic bacteria and sulfate-reducing bacteria.[2] Greigite has also been identified in thesclerites ofscaly-foot gastropods.[6]
The mineral typically appears as microscopic (< 0.03 mm) isometric hexoctahedral crystals and as minute sooty masses. Association minerals includemontmorillonite,chlorite,calcite,colemanite,veatchite,sphalerite,pyrite,marcasite,galena anddolomite.[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 theorigin of life.[7] In particular, the cubic Fe4S4 unit of greigite is found in the Fe4S4 thiocubane units of proteins of relevance to theacetyl-CoA pathway.
Greigite has thespinel structure. The crystallographic unit cell is cubic, withspace group Fd3m. The S anions form a cubic close-packed lattice, and the Fe cations occupy both tetrahedral and octahedral sites.[2][8]
Like the related oxidemagnetite (Fe3O4), greigite isferrimagnetic, 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 amixed-valence compound, featuring both Fe(II) and Fe(III) centers in a 1:2 ratio. Both metal sites have highspin quantum numbers. The electronic structure of greigite is that of ahalf metal.[9][10]
A September 10, 2025 paper published inNature reported the "likely" detection of greigite andvivianite in theJezero crater onMars, by thePerseverance rover.[11] It is considered a potentialbiosignature.[12]