COLUSITE
Colusite – Cu13VAs3S16 – is a very rare sulfosalt, though likely far more common than it appears. When it does occur, it is always present in small quantities. It does not constitute an ore and can be classified as one of the “fahlore” minerals (complexly composed, dark-colored ores).
It belongs to the Germanite group and forms a solid-solution series with stibiocolusite. It may contain Fe, Zn, and Te as impurities.
Macroscopically, it closely resembles pyrite in terms of color, habit, and tarnishing. Its tetrahedral crystals can be mistaken for chalcopyrite crystals. Since it occurs in association with pyrite and chalcopyrite, there is a high probability of it going unnoticed.
1. Characteristics
Crystal system: Cubic hexatetrahedral.
Color: Bronze to rosy bronze. May range from hazy to iridescent.
Habit: Rare crystals up to 5 mm: tetrahedrons, pentagonal dodecahedrons, complex combined forms. Granular, massive.
Cleavage: No.
Tenacity: Extremely brittle.
Twinning: Rare, on {111}
Fracture: Irregular, splintery.
Mohs Hardness: 3 – 4
Parting: No.
Streak: Black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.2 – 4.5
2. Geology and Deposits
Colusite is characteristic of hydrothermal veins containing Cu minerals and of disseminated Cu deposits, where it is associated with other sulfides and sulfosalts.
It occurs in marbles, such as the marbles of Carrara (Italy).
3. Mineral Associations
It occur associated with common gangue minerals such as quartz and calcite and with common sulfides such as pyrite, galena, and sphalerite.
With Cu minerals such as chalcopyrite, chalcocite, enargite, bornite, digenite, covellite, sulvanite, vinciennite, tetrahedrite-tennantite, luzonite, stannoidite, goldfieldite, germanite, and renierite.
More rarely with gypsum, greenockite, wurtzite, bohanowiczite, and hinsdalite.
4. Transmitted Light Microscopy
Not applicable, as colusite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: The polishing hardness is significantly higher than that of bornite, slightly higher than that of tennantite, and lower than that of enargite.
PLANE POLARIZED LIGHT – PPL
Reflection color: Creamy khaki.
Compared to the color of pyrite, the color of colusite is much more brown to chestnut—a creamy bronze color.
Compared to the color of tennantite, the color of colusite is brass-yellow.
Compared to the color of enargite, the color of colusite is cream.
Compared to the color of chalcopyrite, the color of colusite is bronze.
Pleochroism: No.
Reflectivity: 31.74%
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic.
Internal reflections: No.
May be confused with: pyrite, which is very similar.
General Characteristics:
Grain shape: occurs as idiomorphic crystals or aggregates.
Zoning is very common and can be spectacularly developed, sometimes becoming apparent only after chemical etching. It is sometimes visible through differences in color tones and polishing hardness. These growth zones correspond to variations in chemical composition.
Relief may be exhibited, depending on the hardness of neighboring minerals.
Cleavage is not visible.
Replacements: colusite can be replaced by tetrahedrite, tennantite, enargite, chalcopyrite, chalcocite, and bornite.
Rims of stannite-group varieties—orange, brown, and yellow in color—may occur; these are members of the stannite and mawsonite series.
