Formula: (Fe,Ni)1+xS (x = 0-0.07)
Specific gravity: 4.17 calculated
Common impurities: Co,Cu,Ni
Mackinawite is widespread in low-temperature aqueous systems and it is often one of the principal constituents
of acid volatile sulphides in marine sediments (Mindat). It is formed by hydrothermal activity in mineral
deposits, during serpentinisation of
peridotite, and in the reducing environment of river bottom muds;
it may be produced by magnetotactic and sulphate-reducing bacteria. It is also found rarely in iron and carbonaceous
chondrite meteorites (HOM) and lunar rocks (Dana).
Associated minerals include chalcopyrite,
maucherite and troilite
In the Outokumpu ore field, North Karelia, Finland, mackinawite typically occurs in serpentinite replacing nickel pentlandite and cobaltpentlandite and chalcopyrite, but grains which are probably of primary origin have also been found.
The most common modes of occurrence are:
(1) In serpentinite replacing nickel pentlandite, in grains to 1 mm across.
(2) As crystals in chalcopyrite - pyrrhotite - sphalerite ore at about 20 cm from the contact with quartzite.
(3) Some larger open cracks, coated with a crust of secondary quartz and serpentine minerals about 1.5 m from the contact have also been found to be rich in tiny cubanite crystals with some nickel pentlandite, chalcopyrite, galena, pyrrhotite and mawbyite.
(4) In chalcopyrite and cubanite in an assemblage of chalcopyrite, cubanite and pyrrhotite.
(5) Replacing cobaltpentlandite in an assemblage of pyrrhotite, cobaltpentlandite, chalcopyrite and cubanite.
(6) As independent grains in hisingerite in an assemblage of magnetite, hisingerite, pyrrhotite and cubanite.
(7) In chalcopyrite in an assemblage of chalcopyrite, cubanite, pyrrhotite and cobaltpentlandite.
(8) As independent grains in magnetite with pyrrhotite, and in massive chalcopyrite with magnetite, pyrrhotite and native silver.
9) Some sulfide droplets found in boulders of hornblende gabbro are composed of chalcopyrite, pyrrhotite and nickel pentlandite; the mackinawite also normally appears here in chalcopyrite or between chalcopyrite and pyrrhotite. (AM 48.511-524 as an unidentified iron sulphide, but identified as mackinawite in AM 49.1497).
At the Vlakfontein nickel pipe in the South African Transvaal mackinawite has been identified in polished sections. Large pyrrhotite crystals with intergrown coarse pentlandite occur here in basic rocks. The mackinawite occurs within the pentlandite as a replacement product (AM 54.1190-1193).
In the Udachnaya kimberlite, Siberia, Russia, a single gem lithospheric diamond with five sulphide inclusions has been found. The inclusions mainly consist of a polycrystalline aggregate of pentlandite and pyrrhotite, and data also suggest the presence of mackinawite. The total absence of any oxides in the sulphide assemblage indicates that the mackinawite is not an alteration of pyrrhotite and pentlandite due to secondary oxidising fluids entering diamond fractures after the diamond was transported to the surface. Instead, it is likely formed as a low-temperature phase that grew in a closed system within the diamond host (AM 102.2235-2243).
The type locality is the Mackinaw mine, Monte Cristo, Snohomish county, Washington, USA.
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