Mackinawite

Mackinawite
Mackinawite
General
Category	Sulfide mineral
Formula (repeating unit)	(Fe,Ni)_{1 + x}S (where x = 0 to 0.11)
Strunz classification	2.CC.25
Crystal system	Tetragonal
Crystal class	Ditetragonal dipyramidal (4/mmm) H-M symbol: (4/m 2/m 2/m)
Space group	P4/nmm
Unit cell	a = 3.67 Å, c = 5.03 Å; Z = 2
Identification
Formula mass	85.42 g/mol
Color	Bronze to white grey
Crystal habit	As well-formed thin tabular crystals; massive, fine-feathery
Cleavage	Perfect on {001}
Mohs scale hardness	2.5
Luster	Metallic
Streak	Black
Diaphaneity	Opaque
Specific gravity	4.17
References

Mackinawite is an iron nickel sulfide mineral with formula (Fe,Ni)_{1 + x}S (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. 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.

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 Fe²⁺ 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 Fe²⁺, and electrochemically.

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