Carrollite and related minerals of the linnaeite group: Solid solutions and nomenclature in the light of new data from the Siegerland District, Germany

Carrollite, ideally CuCo2S4, is a major component of "rejuvenation stage" m ineralization in the Siegerland ore province, Germany. Carrollite from seve n representative deposits has been analyzed. Combining this dataset with pu blished data on carrollite and related minerals, the existence of a solid-s olution series in the Cu-Co-Ni-S system, and the mechanisms of substitution responsible, are established. The characteristic feature of carrollite at Siegerland is a pronounced incorporation of Ni in the structure, reaching a maximum of 30.20 wt.% in the Eiserner Spies deposit, corresponding to 1.55 Ni atoms per formula unit (Me3S4). Concentrations of Ni display an ideal n egative correlation with Co and Cu. Nickel thus apparently substitutes for both Co and Cu over the entire range of compositions represented by our dat aset. This solid-solution series extends from end-member carrollite, CuCo2S 4, toward what we consider as cuprian siegenite, itself a member of an exte nsive solid-solution between linnaeite and polydymite. Our data show no evi dence for solid-solution series between carrollite and fletcherite. The sub stitution scheme for the observed linear correlation trends relating Ni, Co and Cu involves at least two different mechanisms of substitution, each op erating independently. Two alternative models are proposed: (1) substitutio n of Ni3+ for Co3+ at octahedral sites and of Ni2+ for Cu2+ at tetrahedral sites, and (2) substitution of Ni3+ for Co3+ at octahedral sites combined w ith the coupled substitution Cu2+ + Co3+ --> Ni3+ + Ni2+. Our data suggest that the substitution of Ni3+ for Co3+ is dominant. Nickelian carrollite co ntains Ni/(Ni + Co + Cu) > 0.05, up to a maximum halfway along the CuCo2S4 - CoNi2S4 join at Cu0.5Co1.5N1.0 Phases With Ni > 1.0 and Cu < 0.5 should b e termed cuprian siegenite, so long as Cu/(Cu + Ni + Co) exceeds 0.05.