SIEGENITE
Siegenite – CoNi2S4 – is a very rare sulfide. It is typical of hydrothermal parageneses containing Co-Ni-Cu sulfides and, due to its rarity, is of no importance as an ore.
It is classified within the Linnaeite Subgroup, which includes grimmite (NiCo2S4) as well as the pure end-members of the series: linnaeite (the Co-dominant member) and polydymite (the Ni-dominant member). It was formerly defined as having a variable composition ranging between CoNi2S4 and NiCo2S4. Siegenite forms a solid-solution series with violarite and another with linnaeite. It may contain Cu, Fe, and Se (up to 11% Se).
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Light gray to steel-gray. Tarnishes to a gray-violet or the red of copper.
Habit: Compact, massive, granular. Octahedral crystals and other forms up to 1 cm.
Cleavage: {001} imperfect.
Tenacity: Brittle.
Twinning: In {111}, perhaps polysynthetic.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 4.5 – 5.5
Parting: No.
Streak: Grayish black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.5 – 4.8
2. Geology and Deposits
Siegenite, like other Co-Ni sulfides, is characteristic of hydrothermal veins, where it occurs in association with Cu-Ni-Fe sulfides.
3. Mineral Associations
Siegenite occurs associated with common gangue minerals such as carbonates (calcite, dolomite, siderite, ankerite).
It occurs alongside other common sulfides, including pyrite, marcasite, chalcopyrite, galena, and sphalerite.
It is found with copper sulfides such as bornite, digenite, covellite, tetrahedrite, and tennantite.
In its specific paragenetic assemblages, it associates with a range of cobalt and nickel minerals, such as millerite, gersdorffite, ullmannite, “bravoite” (Ni-bearing pyrite), carrollite, linnaeite, violarite, Co-bearing pentlandite, niccolite, vaesite, cattierite, parkerite, safflorite, maucherite, and langisite.
It also occurs with bitumen (petroleum), uraninite (pitchblende), native bismuth, and bismuthinite.
4. Transmitted Light Microscopy
Not applicable, as siegenite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Siegenite takes an excellent polish after careful grinding, as is the case with all Co-Ni sulfides. Its polishing hardness is significantly lower than that of arsenopyrite, gersdorffite, and cattierite; it is similar to the hardness of linnaeite and higher than that of pyrrhotite, chalcopyrite, and sphalerite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Cream-white, sometimes with a subtle pinkish tint.
Compared to the color of pyrite, the color of siegenite is brownish-pink.
Compared to the color of cattierite, the color of siegenite is less pinkish.
Pleochroism: No.
Reflectivity: 47.82%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: Other Co and Ni sulfides, as they are all very similar to one another.
Linnaeite is extremely similar; distinguishing it requires a test for Ni.
General Characteristics:
Grain shape: Siegenite occurs as euhedral to subhedral crystals, forming octahedrons, modified rhombic dodecahedrons, and other cubic forms. It also appears as aggregates of anhedral grains.
Cubic cleavage {100} may be visible, but can also be absent.
Twinning is not visible.
Zoning is not visible.
Replacements 1: Siegenite can be replaced by Cu-sulfides.
Replacements 2: Siegenite can replace pyrite, vaesite, bismuthinite, chalcopyrite, cattierite, and uraninite.
Intergrowths with (Co)-pentlandite may occur.
