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Formula: (Fe,Ni)1+xS (x = 0-0.07)
Sulphide, nickel-bearing mineral
Crystal System: Tetragonal
Specific gravity: 4.17 calculated
Hardness: 2½
Streak: Black
Colour: Bronze
Common impurities: Co,Cu,Ni
Environments
Hydrothermal environments
Extraterrestrial environments
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,
cubanite, pentlandite,
pyrrhotite, greigite,
maucherite and troilite
(HOM).
Localities
The Nymagee Copper Mine, Nymagee, Mouramba county, New South Wales, Australia, closed in 1907, then re-opened
from 1913 to 1917 then closed again. In order of abundance,
primary minerals are
pyrrhotite, chalcopyrite,
sphalerite, galena,
magnetite, arsenopyrite,
cubanite, valleriite,
tetrahedrite, mackinawite,
bismuth and argentite
(AJM 11.2.119).
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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