The structure of the linnaeite group consists of a cubic close packed array of X. Within the array of Xs there are two types of interstices, one type tetrahedrally co-ordinated and one type octahedrally co-ordinated. One eighth of the tetrahedal sites A are typically occupied by 2+ cations, and half of the octahedral sites B by 3+ cations. Charnock et al. confirmed that carrollite contains Cu wholly within the tetrahedral sites. Thus, the idealformula one would expect for a spinel like carrollite is Cu2+Co3+2S2−4, but as in the case of copper sulfides in general the oxidation state of the copper atom is 1+, not 2+. An assignment of valences as Cu+Co3+2S1.75−4 is more appropriate; this was confirmed in a study of 2009. The one missing electron per four sulfur atoms is delocalized, leading to metallic conductivity and even superconductivity at very low temperatures, combined with a complicated magnetic behavior.
Solid solutions
A solid solution results when one cation can substitute for another across an appreciable composition range. In carrollite Co2+ can substitute for Cu+ at the A sites, and when the substitution is complete the mineral formed is called linnaeite, Co2+Co3+2S4. This means that there is a solid solution series between carrollite and linnaeite. Also, Ni substitutes for both Co and Cu in the carrollite structure, giving a solid solution from carrollite to cuprian siegenite. Siegenite, Co2+Ni3+2S4, is itself a member of the solid solution series between linnaeite and polydymite, Ni2+Ni3+2S4..
Environment
Carrollite occurs in hydrothermal vein deposits associated with tetrahedrite, chalcopyrite, bornite, digenite, djurleite, chalcocite, pyrrhotite, pyrite, sphalerite, millerite, gersdorffite, ullmannite, cobaltoan calcite, and with linnaeite group members linnaeite, siegenite and polydymite. Phase relations in the Cu-Co-S system have been investigated. At temperatures around 900 °C a chalcocite-digenite solid solution coexists with cobalt sulfides. With decreasing temperature, at 880 °C a carrollite-linnaeite solid solution develops, becoming more copper-rich on cooling, with the carrollite composition at about 500 °C. Below 507 °C covellite is stable and coexists with copper-bearing cattierite. Low chalcocite appears at 103 °C, djurleite appears at 93 °C, and digenite disappears and anilite appears around 70 °C. There is some evidence for supergene replacement of an intermediate member of the linnaeite-carrollite series by djurleite.
Distribution
Carrollite is found worldwide; reported in Australia, Austria, Azerbaijan, Brazil, Bulgaria, Canada, Chile, China, the Czech Republic, the Democratic Republic of Congo, France, Germany, Japan, Morocco, Namibia, North Korea, Norway, Oman, Poland, Romania, Russia, Slovakia, Sweden, Switzerland, US and Zambia.
