What is "greigite"

n. (context mineral English) A magnetic form of iron sulfide, iron²⁺Fe³⁺₂sulfur₄, analogous to magnetite, produced by microorganisms and found in lake sediments; Note: half the iron ions are ferrous and half ferric.

Greigite is an iron sulfide mineral with formula FeS ( Iron(II,III) sulfide). It is the sulfur equivalent of the iron oxide magnetite (FeO). 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).