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

Intermediate compositions along the bornite-digenite join exsolve during qu enching from above-solvus temperatures. This involves vacancy clustering an d cation ordering processes, and is facilitated by fast ration diffusion ra tes in the presence of a large (10-25%) metal vacancy population. Samples o f six different compositions across the bornite-Cu9S5 join, synthesised fro m component elements in sealed quartz capsules, were water-quenched from 60 0 degrees C and analysed using high-resolution neutron powder diffraction ( HRPD). Time-of-flight spectra measured at room temperature showed all inter mediate compositions had exsolved into mixtures of bornite and low digenite with a 5.0a superstructure. No evidence for the presence of any other phas e was found. Variations in the lattice parameters of the exsolved bornite p hase were observed for different bulk compositions across the join, and asc ribed to variations in the degree of order. Bornite exsolved from digenite- rich compositions may not be fully ordered due to the much lower solvus tem peratures at the Cu-rich end of the solid solution. As only slight differen ces were observed between the diffraction patterns of a visibly exsolved an d 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 quenchin g process. The rapid kinetics of exsolution at geologically low temperature s, explains the lack of authenticated natural occurrences of intermediate c ompositions in the solid solution in nature, and the limited degree of stoi chiometric variation observed in end-members.