Rapid exsolution behaviour in the bornite-digenite series, and implications for natural ore assemblages

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-Cu ₉ S ₅ join, synthesised from component elements in sealed quartz capsules, were water-quenched from 600 degrees 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.