Английская Википедия:Greigite

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Шаблон:Short description Шаблон:Infobox mineral Greigite is an iron sulfide mineral with the chemical formula Шаблон:Chem2. 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

Шаблон:See also 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 [[Iron–nickel clusters|Шаблон:Chem2 clusters]] present in biological enzymes has led to suggestions that greigite or similar 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.

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.

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.