MACKINAWITE
Mackinawite – (Fe,Ni)9S8 – is a relatively common sulfide, also described as (Fe,Ni)1+xS (x = 0 – 0.11). It is part of polymetallic ores of Cu, Ni, and Co as a product of unmixing or alteration.
It is much more common than valleriite, with which it was confused for decades. Under a microscope, it is very similar to valleriite.
It is generally composed of very small crystals that cannot be identified macroscopically. It may contain Co, Cu, Ni, and up to 9% by weight of Cr.
1. Characteristics
Crystal system: Tetragonal, ditetragonal bipyramidal.
Color: Bronze
Habit: Massive, feathery, fine tabular. Idiomorphic crystals up to 1 mm.
Cleavage: {001} perfect.
Tenacity: No information available.
Twinning: Lamellar.
Fracture: No information available.
Mohs Hardness: 2.5
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.17 (calculated).
2. Geology and Deposits
Mackinawite forms in low-temperature aqueous systems, often being one of the main constituents of volatile acid sulfides in marine sediments.
It also forms:
– during hydrothermal activity in mineral deposits,
– during serpentinization of ultrabasic rocks (peridotites),
– in the reducing environment of riverbed muds,
– by magnetotactic and sulfate-reducing bacteria.
It rarely occurs in iron meteorites and carbonaceous chondrites.
3. Mineral Associations
It occurs associated with some well-known sulfides such as arsenopyrite, sphalerite, chalcopyrite, chalcocite, cubanite, pentlandite, pyrrhotite, and cobaltite.
It also occurs with less common sulfides such as greigite, valleriite, wurtzite, and maucherite.
It occurs with oxides such as magnetite and chromite.
With awaruite (Fe,Ni).
In meteorites, it is associated with troilite and native iron.
4. Transmitted Light Microscopy
This does not apply, as makinawite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Mackinawite polishes well. Its hardness upon polishing is medium, similar to that of pyrrhotite, but Co and Ni contents cause the hardness to vary. Basal sections of larger grains do not polish well and may appear blurred.
PLANE POLARIZED LIGHT – PPL
Reflection color: Reddish-gray to pinkish, can be cream in various shades. The color varies greatly, even in well-polished sections. Mackinawites with a high Cr content show a distinctly brown hue that does not occur in “normal” makinawites.
Compared to the color of valleriite, the color of makinawite does not show bronze and brown colors.
Compared to the color of pyrrhotite, the color of makinawite is very similar.
Pleochroism: Moderate to strong, between pinkish gray and gray, may be cream.
Reflectivity: 22 – 46%
Bireflectance: Distinct.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Extreme anisotropy between grayish-white, bluish, and brownish. The intensity of the anisotropy is reminiscent of graphite and molybdenite.
With exactly crossed nicols: grayish-white to dark gray or black.
With not exactly crossed nicols: bluish-white to Sienna brown. No bronze polarization colors as in valleriite.
Over the weeks, the anisotropy of the sections decreases significantly, but polishing brings back the original effects.
Internal reflections: No.
May be confused with: few other minerals.
Valleriite is very similar, but is much rarer and does not show white as one of the anisotropy colors. Some literature insists that it is not possible to distinguish valleriite from mackinawite under the microscope, only paragenesis can suggest an identification. Other analytical methods are needed to differentiate valleriite and mackinawite.
The association of mackinawite with chalcopyrite, pentlandite and cubanite is characteristic.
General Characteristics:
Grain shape:
a) Mackinawite generally occurs as small dissolution bodies in chalcopyrite, pentlandite, pyrrhotite, and cubanite. These forms can be worm-like, lamellar, or occur as “flames” on small inclusions in the host mineral. It can form fusiform dissolutions in pentlandite arranged in (100), along the cube faces of the pentlandite (in two perpendicular directions). It can also form “herringbone” textures. Rounded or extremely fine grains are rare, and may occur in chalcopyrite included in basalts. In cubanite, mackinawite is much rarer, but exceptionally large “flames” can form in the vicinity of cubanite.
b) More rarely, it forms longer tabular crystals of constant thickness, straight or folded, with perfect basal cleavage, resembling graphite.
c) When altered, mackinawite forms anhedral to subhedral, irregular flakes and microveins in pentlandite, cobalt-pentlandite, magnetite, chalcopyrite, pyrrhotite, galena, schapbachite, and native silver.
d) It can form reaction boundaries between chalcopyrite and pyrrhotite.
Cleavage (001) may be visible, in open spaces it may open (exfoliation).
Lamellar twinning may be visible.
Substitutions: mackinawite substitutes pentlandite, cobalt-pentlandite, chalcopyrite, cubanite and pyrrhotite.
