Formula: CuCo2S4. The valence state of copper is Cu+ and the electronic formula is Cu1.2+(Co2.4+)2(Si1.4-)4 (CM 46.1317-1322).
Sulphide, carrollite subgroup of the thiospinel group, cobalt-bearing mineral
Crystal System: Isometric
Specific gravity: 4.5 to 4.8 measured, 4.83 calculated
Hardness: 4½ to 5½
Colour: Bright silver
Luminescence: Not fluorescent under UV
Common impurities: Ni
Carrollite occurs in hydrothermal vein deposits. Associated minerals include
ullmannite and cobalt-bearing
At the Kamoya South II Mine, Kamoya, Kambove District, Haut-Katanga, DR Congo, carrollite is frequently etched, due to weathering in the oxidised or transition zone. Only crystals from below the transition zone escape etching. Carrollite in the oxidised zones may be completely converted to asbolane, heterogenite or spherocobaltite. Vugs with lightly or partially etched carrollite sometimes have late chalcocite, cobalt-bearing calcite, or cobalt-bearing dolomite in addition to malachite and/or chrysocolla. Late carrollite deposition has been observed on chalcopyrite and some carrollite crystals (R&M 84.4.364).
At the type locality, the Patapsco Mine, Patapsco Mines, Finksburg, Carroll county, Maryland, USA, mineralised veins appear in quartz-chlorite-biotite schist, whereas the waste dumps at the mines consist of hornblende-biotite schist containing scattered magnetite crystals. Minerals include disseminated bornite, carrollite, chalcocite, chalcopyrite, magnetite, pyrite and quartz (R&M 97.2.188).
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