Electron microscope study of phase transformations in cubanite

Direct observations of the phase transformations in cubanite both on heating and cooling have been made in a transmission electron microscope. Low temperature orthorhombic cubanite undergoes a cation disordering process at about 200° C resulting in a hexagonal wurtzite type structure. The reordering process takes place by the growth of independently nucleated regions of short range order. Long range order is difficult to attain in short term experiments due to the lack of order correlation between regions. At a slightly higher temperature a transformation from hexagonal close-packing to cubic close-packing takes place within the sulphur structure, by the propagation of partial dislocations along alternate close-packing planes. This transformation is irreversible. On cooling the disordered cubic structure, cation ordering takes place as an alternative to the transformation back to hexagonal close-packing, but in the absence of an ordering scheme at this composition based on the cubic subcell, the ordering process results in the exsolution of chalcopyrite. This transformation is interpreted in terms of metastable, kinetically controlled behaviour.