Thermal Stability of Assemblages in the Cu--Fe--S System

The phase relations in the Cu-Fe-S system were determined from 700° C to approximately 200° C in most portions of the system and below 100° C in restricted areas. Approximate solid solution limits for bornite, chalcopyrite, and pyrrhotite were determined at elevated temperatures. At low temperatures emphasis was placed on establishing the stable assemblages and less on determining the compositions of coexisting phases. At 700° C two extensive ternary solid solutions dominate the phase relations in this system. One of these solid solutions (bornite) includes the compositions Cu2S, Cu1·8S, and Cu5FeS4and the other (chalcopyrite) lies with in the area bounded by the compositions CuFeS2 CuFe2S3, and CU3Fe4S4. The two fields are separated by approximately 10 weight per cent copper at 700° C. The chalcopyrite volume, as seen in a trigonal prism representing temperature and composition, is intersected by a miscibility gap below approximately 600°C. Below this temperature the two one-phase volumes are referred to as chalcopyrite and cubanite. Chalcopyrite is tetragonal at low temperature but isometric above approximately 550°C. The temperature of the transformation is a function of composition. Cubanite is isometric above 252°C, tetragonal from 252° to at least 213°C, and orthorhombic at lower temperature. The temperature of the second transformation is unknown because the tetragonal-to-orthorhombic transformation has not been achieved in the laboratory. Bornite and pyrite become stable together at 568°C and coexist down to 228°C. Covellite appears with lowering temperature at 507°C, and idaite at 501°C. Idaite—pyrite and idaite—bornite are stable assemblages below 501° C. The composition of bornite coexisting with idaite changes gradually towards digenite with decreasing temperature, thus permitting the change from the bornite—pyrite tie-line to the digenite—chalcopyrite tie-line at 228°C. Other major tie-line changes are bornite—iron to pyrrhotite—copper below 475°C and cubanite—pyrite to chalcopyrite—pyrrhotite below 334°C. A new synthetic phase, x-bornite, which has a composition close to bornite (Cu5FeS4) but contains about 0·4 weight per cent more sulfur, forms when sulfur-rich bornite synthesized at high temperature is annealed between 62° and 140°C. Optically this new phase is very similar to bornite, and their X-ray powder diffraction patterns are given for comparison. o The determined phase relations are applicable to numerous deposits. The tie-line changes involving bornite³pyrite reacting to produce digenite³chalcopyrite below 228° C and cubanite (isometric)³pyrite going to chalcopyrite³pyrrhotite below 334° C are of considerable geological interest. The rates of these reactions are sufficiently slow to allow the higher temperature assemblages to be observed in some ores. The cubic—tetragonal inversion in chalcopyrite is often deduced in ores by inversion twins. However, twinning is also commonly produced through deformation. Geological application of the inversion therefore depends on correct interpretation of the twinning. Because of the considerable solubility of copper in pyrrhotite the pyrrhotite—pyrite solvus of the pure Fe—S system cannot be applied indiscriminately to ores that also contain chalcopyrite or cubanite, or both. The new x-bornite phase was identified with the natural ‘anomalous bornites’, which when heated exsolve chalcopyrite and, depending on their composition, also digenite. The experimental results indicate that the mineral commonly identified as chalcopyrrhotite is in reality tetragonal or even isometric cubanite. Experimental evidence could not be obtained for the existence of a phase of Cu2Fe4S7 or Cu2Fe4S7 composition, the older formulae given foor valleriite. The thermal breakdown of natural material supports the idea that valleriite is a low-temperature polymorph of chalcopyrite. The relatively uncommon occurrence of idaite in comparison to covellite is attributed to the greater difficulty in nucleating idaite. The possibility of stable coexistence of chalcocite and pyrite was investigated but was found to be prohibited by tie-lines between bornite and digenite even as low as 100° C.