CARROLLITE
Carrollite – CuCo2S4 – is a very rare sulfide that occurs in cobalt ores. It usually contains Ni.
It is classified in the Carrollite Subgroup, in the Thiospinel Group, of the Spinel Supergroup, being isostructural with linnaeite.
1. Characteristics
Crystal system: Cubic hexaoctahedral.
Color: Bright silver color. Rarely tarnishes to copper red or violet gray.
Habit: Massive, compact, granular. Octahedral and cubic crystals, euhedral, subhedral and anhedral.
Cleavage: No.
Tenacity: Very brittle.
Twinning: {111} polysynthetic.
Fracture: Irregular, conchoidal, subconchoidal.
Mohs Hardness: 4.5 – 5.5.
Parting: On {001}, poor.
Streak: Gray black.
Lustre: Metallic.
Diaphaneity: Opaque.
Density (g/cm³): 4.5 – 4.8
2. Geology and Deposits
Carrollite is typical of hydrothermal vein deposits.
May be found in layered igneous intrusions with related minerals of the Platinum Group Elements.
3. Mineral Associations
Carrollite associates with common gangue minerals like quartz and carbonates (calcite, dolomite).
Also with Cu minerals (chalcopyrite, bornite, chalcocite, djurleite, malachite, covellite) and with pyrite.
With Co minerals like siegenite and spherocobaltite.
4. Transmitted Light Microscopy
This does not apply, as carrollite is completely opaque.
5. Reflected Light Microscopy
Sample preparation: Carrollite achieve a great polish after a good preliminar grinding. The polishing hardness is:
– higher than that of chalcopyrite, pyrrhotite and sphalerite and
– much lower than that of aresenopyrite, gersdorffite and pyrite.
PLANE POLARIZED LIGHT – PPL
Reflection color: White, with a slight pinkish or creamy tint.
Compared to the color of cobaltite, the color of carrollite is often very similar.
Compared to linnaeite, siegenite and violarite, the color of carrollite is very similar, but somewhat ligther. Some specimens have a pinkish tint.
Compared to the color of gersdorffite, the color of carrollite has a red tint.
Pleochroism: No.
Reflectivity: 41.99% (45.4% if with Ni).
Bireflectance: No.
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: Isotropic, rarely slight anisotropy.
Internal reflections: No.
May be confused with: other Co minerals.
Linnaeite, siegenite and violarite are very similar in polished section.
General Characteristics:
Exsolution lamellae with slightly higher Fe composition than the matrix may occur.
Cleavage: (100) often visible, (111) may be visible.
Alteration of carrollite may be replaced by covellite.
Twins were not observed.
Substitutions: Carrollite may be replaced by Cu-minerals like chalcocite, bornite, chalcopyrite, digenite, covellite. Also by pyrite and pyrrhotite.
Inclusions OF carrollite may occur in chalcopyrite.
Inclusions IN carrollite may be of sphalerite, chalcopyrite and other copper sulfides.
