Mackinawite is an iron nickel sulfide mineral with formula (Fe,Ni)1 + xS (where x = 0 to 0.11). It crystallizes in the tetragonal crystal system and occurs as opaque bronze to grey-white tabular crystals and anhedral masses. It has a Mohs hardness of 2.5 and a specific gravity of 4.17.
It occurs in serpentinized peridotites as a hydrothermal alteration product. It also is found in the reducing environment of fine grained river sediments, possibly produced by the action of magnetotactic and sulfate-reducing bacteria. It has also been reported from meteorite occurrences. It occurs in association with chalcopyrite, cubanite, pentlandite, pyrrhotite, greigite, maucherite and troilite.
It was first described in 1962 for an occurrence in the Mackinaw mine, Snohomish County, Washington for which it was named.
1. ^ a b Handbook of Mineralogy