LINNAEITE
Linnaeite – Co2+Co3+2S4 – is a rare sulfide that occurs in Co and Ni ores.
It is classified in the Linnaeite Subgroup. It forms a series with polydymite and another series with siegenite.
Common impurities are Ni, Cu, Fe, and Se. It is possible that some Ni-rich reports of this mineral refers to the mineral grimmite. There is a gold-bearing variety.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Pale gray to steel gray (bluish), violet.
Habit: It forms octahedral crystals up to 5 mm in size, but is generally massive, granular, and compact.
Cleavage: {001} imperfect.
Tenacity: no information available.
Twinning: According to {111}, by the Spinel Law.
Fracture: Irregular to subconchoidal.
Mohs Hardness: 4.5 – 5.5
Parting: No.
Streak: Grayish black.
Lustre: Metallic, but it tarnishes easily.
Diaphaneity: Opaque.
Density (g/cm³): 4.5 – 4.8
2. Geology and Deposits
Linnaeite occurs almost exclusively in Co-Ni-Ag-As type hydrothermal veins.
3. Mineral Associations
It occurs associated with common gangue minerals such as quartz and carbonates (siderite).
It occurs with common sulfides such as pyrite, pyrrhotite, marcasite, galena, and sphalerite.
Also with common silicates (actinolite, allanite) and common oxides (magnetite, hematite).
It is associated with various Cu sulfides such as chalcopyrite, bornite, chalcocite, digenite, cubanite, wittichenite, and villamannite.
In the specific paragenesis, linnaeite can be found together with millerite, gersdorffite, nickeline, pentlandite, safflorite, siegenite, tetrahedrite, pyrite-(Ni) (bravoite), carrollite, cattierite, ullmannite, skutterudite, rammelsbergite, cubanite, cobaltite, bismuthinite, native bismuth, and breithauptite.
Secondary minerals that are associated with it are erythrite and covellite.
4. Transmitted Light Microscopy
This does not apply, as linnaeite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Linnaeite acquires an excellent polish after careful grinding. Its hardness on polish is:
– greater than the hardness of chalcopyrite, sphalerite and pyrrhotite,
– approximately the same as magnetite and skutterudite,
– less than gersdorffite and
– much less than arsenopyrite and pyrite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Light pink or white with a red or cream tint.
Compared to the color of eskutterudite, the color of linnaeite is a grayish white.
Compared to the color of ullmannite, the color of linnaeite is cream or yellowish, as ullmannite is white.
Compared to the color of gersdorffite, the color of linnaeite is cream or yellowish, as gersdorffite is white.
Compared to the color of millerite, the color of linnaeite is much darker, a very light pinkish brown, while millerite is light yellow.
Compared to the color of chalcopyrite, the color of linnaeite is distinctly pinkish.
Pleochroism: No.
Reflectivity: 44.93%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: Several other minerals from the paragenesis. Mistakes are very likely in sections without relief (of recent and very good polishing).
Cobaltite has a very similar color, but cobaltite is not perfectly isotropic.
Ullmannite, gersdorffite, and skutterudite are lighter and less yellow than linnaeite and polydymite.
Arsenopyrite and pyrrhotite are anisotropic.
General Characteristics:
Grain shape: there is a clear tendency towards idiomorphy, forming small octahedrons (up to 5 mm) that are filled with inclusions (sieve texture) probably incorporated during idioblastic growth. Typically the inclusions are chalcopyrite. It also occurs massively, compactly or granularly in aggregates of subhedral to anhedral grains.
Cleavage or parting is usually visible.
Twinning does not occur.
Zoning is not observed.
Substitutions of linnaeite by chalcopyrite and pyrite occur.
Intergrowths of linnaeite occur with millerite, chalcopyrite, pentlandite and pentlandite-(Co), with these minerals forming a network included in the linnaeite. On the other hand, linnaeite occurs as exsolved lamellae in pyrrhotite. It also occurs as intergrowths, sometimes oriented, with bornite, native bismuth and pyrite-(Ni) (bravoite).
Exsolutions of millerite in linnaeite can occur, forming a network in 3 directions.
Inclusions of linnaeite occur in chalcopyrite and pyrrhotite.
Substitutions 1: linnaeite replaces eskutterudite.
Substitutions 2: linnaeite is replaced by covellite, chalcocite, chalcopyrite, pyrite, native bismuth, and wittichenite.
