Mackinawite

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 either Fe ions or 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 Fe, and electrochemically.