Class : Sulfides and sulfosalts
    Subclass : Sulfides
    Crystal System : Cubic
    Chemistry : Co₂CuS₄
    Rarity : Rare

Carrollite belongs to the linnaeite group, the name of the most common mineral. This group of general formula X₃S₄ (with X=Co, Ni, Fe, or [Co + Cu]), can be put in parallel with that of spinels, sulfur replacing oxygen ; octahedral facies is also most common in both groups. The linnaeite group is composed of three solid solutions : Co (linnaeite) - Ni (polydimite), the intermediate term being siegenite ; Ni (polydimite) - Fe (greigite), the intermediate term being violarite and Co (linnaeite) - Cu. This last solid solution being the only one to be incomplete since stopping at the carrollite. Carrollite is a rare cobalt and copper sulphide from cobalt, nickel and copper hydrothermal deposits. It owes its name to its locality of discovery : Finksburg, Carroll County, Maryland, USA. From a luminous gray hue to a slightly pinkish steel gray, the crystals deteriorate under surface conditions to give black-colored asbolans. It is sought after by mineral collectors although little known, it is also a very accessory cobalt ore.

The carrollite often twinned on {111}, usually it is polysynthetic twinning not necessarily visible to the naked eye and more rarely there's also spinel twin law crystals (left photo).

Some crystals of carrollite can be glued on their calcite matrix.