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mackinawite
Mackinawite is an iron nickel sulfide mineral with formula (Fe,Ni)1 + xS (where x = 0 to 0.11). The mineral crystallizes in the tetragonal crystal system and has been described as a distorted, close packed, cubic array of S atoms with some of the gaps filled with Fe.〔Taylor, L.A.; Finger, L.W. (1970) Structural refinement and composition of mackinawite. ''Carnegie Institute of Washington Geophysical Laboratory Annual Report'' 69: 318–322.〕 Mackinawite 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 was first described in 1962 for an occurrence in the Mackinaw mine, Snohomish County, Washington for which it was named.〔
==Occurrence==
Mackinawite occurs in serpentinized peridotites as a hydrothermal alteration product, in meteorites, and in association with Chalcopyrite, Cubanite, Pentlandite, Pyrrhotite, Greigite, Maucherite, and Troilite.〔 Mackinawite also occurs in reducing environments such as freshwater and marine sediments as a result of the metabolism of iron and sulfate-reducing bacteria.
In anoxic environments, mackinawite is formed by the reaction of HS− with Fe2+ ions with Fe metal. Mackinawite is a metastable mineral that occurs predominantly as a poorly crystalline precipitate. After the initiation of precipitation, mackinawite can take up to 2 years to form at 25 °C. It has been reported that mackinawite can be stable for up to 16 weeks at temperatures up to 100 °C at pH values from 3-12. Laboratories have also produced synthetic mackinawite to study its formation using several different methods such as reacting sulfide with metallic iron or a solution of ferrous iron, growing sulfide reducing bacteria using Fe2+, and
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