Mackinawite
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| Mackinawite | |
|---|---|
 | |
| General | |
| Category | Sulfide mineral |
| Formula | (Fe,Ni) 1+xS (where x=0 to 0.11) |
| IMA symbol | Mkw[1] |
| 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 | [2][3][4][5] |
Mackinawite (/ˈmækɪnɔːaɪt/ MAK-i-naw-eyet[6]) is an iron sulfide mineral, which can contain nickel substituting for iron(II), with as chemical formula (Fe,Ni)
1+xS (where x = 0 to 0.11). As indicated by its chemical composition, Fe(1+×), Fe is present with an excess of × over S, and it is thus an iron-rich, or a sulfur-deficient, iron monosulfide mineral. 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.[7] 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.[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.
In anoxic environments, mackinawite is formed by the reaction of HS− with either Fe2+ ions or with Fe metal.[8] Mackinawite is a metastable mineral that occurs predominantly as a poorly crystalline precipitate.[9] After the initiation of precipitation, mackinawite can take up to 2 years to form at 25 °C.[10] 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.[11] 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.[12][8][13][11][14]