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Greigite
The sulfide analogue of magnetite — a magnetic iron sulfide described in 1964 from California.
Intro adapted from Wikipedia (CC BY-SA).
Greigite is an IMA-recognized mineral species in the sulfide class, with the chemical formula Fe²⁺Fe³⁺₂S₄.
It crystallizes in the cubic crystal system (space group Fd3m). It is a member of the Spinel structural group.
Key properties: Mohs hardness 4–4.5; specific gravity 4.049; Metallic. R: (400) 30.5, (420) 31.1, (440) 31.7, (460) 32.2, (480) 32.7, (500) 33.2, (520) 33.6, (540) 34.2 luster.
IMA symbol Grg, IMA status: approved, named in 1963.
Formed in lacustrine beds consisting of interbedded calcareous clays, silts, and fine- to medium-grained arkosic sands; in varvelike laminae of grayish black sulfide-bearing clays and fine silts.
It is found associated with Montmorillonite, chlorite, calcite, colemanite, veatchite (Kramer-Four Corners area, California, USA); sphalerite, pyrite, marcasite, galena, calcite.
The mineral was named to honor Dr. Joseph Wilson Greig (1895–1977), mineralogist and physical chemist, Pennsylvania State University, State College, Pennsylvania, USA.