Mackinawite is an iron nickel sulfide mineral with the chemical formula (Fe,Ni)1+xS (where x = 0 to 0.11).

[6] Mackinawite occurs as opaque bronze to grey-white tabular crystals and anhedral masses.

[5] Mackinawite occurs in serpentinized peridotites as a hydrothermal alteration product, in meteorites, and in association with chalcopyrite, cubanite, pentlandite, pyrrhotite, greigite, maucherite, and troilite.

[3] Mackinawite also occurs in reducing environments such as freshwater and marine sediments as a result of the metabolism of iron and sulfate-reducing bacteria.

Depending on the redox conditions mackinawite can form more stable phases such as greigite[14] and ultimately pyrite,[15] an important mineral in anoxic aqueous settings that is preserved in sedimentary deposits, especially black shale.