Ba. Grguric et A. Putnis, Rapid exsolution behaviour in the bornite-digenite series, and implications for natural ore assemblages, MINERAL MAG, 63(1), 1999, pp. 1-12
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.