Mackinawite is an

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow to ...

sulfide mineral The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide (S22−) as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the ...

with the chemical formula (where x = 0 to 0.11). The mineral crystallizes in the

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the cube becomes a rectangular prism with a square ...

crystal system In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices. Space groups are classified into crystal systems according to their po ...

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 The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

of 2.5 and a

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...

of 4.17. It was first described in 1962 for an occurrence in the Mackinaw mine,

Snohomish County, Washington Snohomish County () is a county located in the U.S. state of Washington. With a population of 827,957 as of the 2020 census, it is the third-most populous county in Washington, after nearby King and Pierce counties, and the 75th-most populous ...

for which it was named.

Occurrence

Mackinawite occurs in serpentinized

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...

s as a

hydrothermal alteration Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical co ...

product, in

meteorite A meteorite is a solid piece of debris from an object, such as a comet, asteroid, or meteoroid, that originates in outer space and survives its passage through the atmosphere to reach the surface of a planet or Natural satellite, moon. When the ...

s, and in association with

chalcopyrite Chalcopyrite ( ) is a copper iron sulfide mineral and the most abundant copper ore mineral. It has the chemical formula CuFeS2 and crystallizes in the tetragonal system. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mo ...

cubanite Cubanite is a copper iron sulfide mineral that commonly occurs as a minor alteration mineral in magmatic sulfide deposits. It has the chemical formula CuFe2S3 and when found, it has a bronze to brass-yellow appearance. On the Mohs hardness scale, ...

pentlandite Pentlandite is an iron–nickel sulfide with the chemical formula . Pentlandite has a narrow variation range in Ni:Fe but it is usually described as having a Ni:Fe of 1:1. It also contains minor cobalt, usually at low levels as a fraction of wei ...

pyrrhotite Pyrrhotite is an iron sulfide mineral with the formula Fe(1-x)S (x = 0 to 0.2). It is a nonstoichiometric variant of FeS, the mineral known as troilite. Pyrrhotite is also called magnetic pyrite, because the color is similar to pyrite and it is ...

greigite Greigite is an iron sulfide mineral with the chemical formula . It is the sulfur equivalent of the iron oxide magnetite (Fe3O4). It was first described in 1964 for an occurrence in San Bernardino County, California, and named after the mineralogis ...

maucherite Maucherite is a grey to reddish silver white nickel arsenide mineral. It crystallizes in the tetragonal crystal system. It occurs in hydrothermal veins alongside other nickel arsenide and sulfide minerals. It is metallic and opaque with a hardnes ...

, and

troilite Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1-x)S (x = 0 to 0.2) which is iron deficient. As troilite lacks the iron deficiency whic ...

. Mackinawite also occurs in reducing environments such as freshwater and marine sediments as a result of the metabolism of iron and

sulfate-reducing bacteria Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate () as termina ...

. 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

Transformations in the environment

Depending on the redox conditions mackinawite can form more stable phases such as

Csákberényi-Malasics, D., Rodriguez-Blanco, J.D., Kovács Kis, V., Rečnik, A., Benning, L.G., and Pósfai, M. (2012) Structural properties and transformations of precipitated FeS. Chemical Geology, 294-295, 249-258. doi: 10.1016/j.chemgeo.2011.12.009. and ultimately

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

, an important mineral in anoxic aqueous settings that is preserved in sedimentary deposits, especially black shale. While it has been determined that mackinawite is a necessary precursor to pyrite, the pathway of iron sulfide mineral formation from aqueous species to solid mineral is still nebulous. Many iron sulfide minerals may exist in the transition between poorly ordered mackinawite and crystalline

such as

, smithite, and

; however, studies have also indicated that pyrite formation from mackinawite can occur where oxidation has commenced and the sulfur present is in intermediate oxidation states (-1 to +6) and intermediate sulfur species such as elemental sulfur or

polysulfide Polysulfides are a class of chemical compounds containing chains of sulfur atoms. There are two main classes of polysulfides: inorganic and organic. Among the inorganic polysulfides, there are ones which contain anions, which have the general formu ...

s and surface oxidized

monosulfide Monosulfide may refer to: * Carbon monosulfide, chemical compound with the formula CS * Copper monosulfide, chemical compound with the formula CuS * Mononitrogen monosulfide, inorganic compound with the formula SN * Silicon monosulfide, chemical c ...

species, such as oxidized mackinawite or

are present.

References

{{Meteorites Sulfide minerals Meteorite minerals Tetragonal minerals Minerals in space group 129

Occurrence

Transformations in the environment

See also

References