Intermediate compositions along the bornite–digenite join exsolve during quenching from above-solvus temperatures. This involves vacancy clustering and cation ordering processes, and is facilitated by fast cation diffusion rates in the presence of a large (10–25%) metal vacancy population. Samples of six different compositions across the bornite–Cu9S5 join, synthesised from component elements in sealed quartz capsules, were water-quenched from 600°C and analysed using high-resolution neutron powder diffraction (HRPD). Time-of-flight spectra measured at room temperature showed all intermediate compositions had exsolved into mixtures of bornite and low digenite with a 5.0a superstructure. No evidence for the presence of any other phase was found. Variations in the lattice parameters of the exsolved bornite phase were observed for different bulk compositions across the join, and ascribed to variations in the degree of order. Bornite exsolved from digenite-rich compositions may not be fully ordered due to the much lower solvus temperatures at the Cu-rich end of the solid solution. As only slight differences were observed between the diffraction patterns of a visibly exsolved and a rapidly quenched sample of the same bulk composition, the formation of optically-visible exsolution lamellae on {100} is ascribed to a process of coalescence of sub-microscopic domains initially formed during the quenching process. The rapid kinetics of exsolution at geologically low temperatures, explains the lack of authenticated natural occurrences of intermediate compositions in the solid solution in nature, and the limited degree of stoichiometric variation observed in end-members.

Phase relations along the join bornite (Cu5FeS4)-digenite (Cu8.52Fe0.12S4.88) have been redefined using a combination of in situ high-resolution neutron diffraction and differential scanning calorimetry (DSC). Time-of-flight neutron diffraction patterns were collected on a synthetic sample of bn90 at 16 temperatures between 35 and 350°C. This data is compared with data from a natural end-member bornite sample obtained in an earlier study under identical conditions. Phase relations along the bornite-digenite join are inferred from the temperature evolution of the lattice parameters and the intensity of subcell and supercell reflections of coexisting phases.

The DSC scans over the temperature range 50–300°C were performed on a natural digenite sample and samples synthesized at 5 mol.% intervals along the join Cu5FeS4-Cu9S5. The thermal anomalies are correlated with structural phase transitions in componen phases and the solvus temperature for each bulk composition. A phase diagram topology is defined, which was consistent with both diffraction and calorimetric data, but in marked contrast to previous diagrams, shows a consolute point at X = Cu5FeS4 and T = 265°C. This temperature corresponds to that of the tricritical intermediate-high transition in bornite. Isothermal annealing experiments carried out on synthetic starting materials for up to 7 months showed coarsening behaviour consistent with the revised phase diagram topology.