Phase Relations in the Cu-Co-S System and Minerai Associations of the Carrollite (CuCo ₂S ₄)- Linnaeite (Co ₃S ₄) Series

Phase relations in the system Cu-Co-S have been studied using sealed silica capsule methods between 400° and 900°C. The high-temperature portion of the system is dominated by the coexistence of the chalcocite-digenite solid solution with cobalt sulfides (CoS ₂, Co _1-xS, Co ₄S ₈). As temperature is decreased to 880° ± 5°C, the dominant ternary phase, the carrollite-linnaeite solid solution, appears with a composition of ∼Cu _0.5Co _2.5S ₄. The copper-rich compositional limit is extended on cooling, reaching a maximum (CuCo ₂S ₄) at -500°C. Below 507°C covellite is stable and coexists in the sulfur-rich portion of the system with copper-bearing cattierite. Below 500°C the major changes involve a change in stable assemblages (at ∼450°C), in which the high-temperature pair Co + Cu ₂S is replaced by the pair Cu +Co ₉S ₈, the disappearance of Co _1-xS (≤460°C) and digenite (≤70°C), and the appearance of low chalcocite (103°C), djurle-ite (93°C), and anilite (70°C). Optical and electron microprobe studies of natural assemblages permit speculation on phase relations at 25°C and suggest the topology of the system is similar to that at 400°C except for the appearance of the low-temperature copper sulfides. Similar studies also permit speculation on the low-temperature phase relations in the relevant portion of the Cu-Co-Fe-S system in which carrollite frequently occurs in close association with chalcopyrite or bornite and, in some cases, cobalt-pyrite. More cobalt-rich "carrollites" are found to occur in assemblages with more cobalt-rich "pyrites" and this corresponds to decreasing a _S2. Assemblages such as carrollite-chalco-pyrite-bornite-cobaltian pyrite and carrollite-cobaltian pyrite-bornite-digenite correspond to a further decrease in a _S2. The actual a _S2 values during formation of assemblages containing these phases are also considered using thermochemical data for sulfidation equilibria. In the carrollite-linnaeite series, the systematic decrease in unit cell parameter and increase in reflectivity (at 589 nm) as a function of composition, as well as the actual compositional variations in the natural phases, are consistent with crystal chemical models.