R. W. Potter
An electrochemical investigation of the system copper-sulfur
Economic Geology and the Bulletin of the Society of Economic Geologists (December 1977), 72(8):1524-1542

Abstract:
The phase equilibria of the copper-sulfur system were investigated using electrochemical cells from 0 degrees to 250 degrees C over the composition range Cu/S = 0.95-2.10. Vapor-saturated aqueous cupric sulfate and cuprous chloride solutions were used as the electrolytes. The two blaubleibender covellites, low digenite, tetragonal phase, and protodjurleite were found to be metastable. The Cu (sub 1.9) S phase was demonstrated not to be a real phase in the system.The stable invariant points (Cu/S in parentheses) were as follows:Covellite (1.000 + or - 0.001) --> high digenite (1.732 + or - 0.005) + liquid sulfur, T = 507 degrees + or - 2 degrees C;high chalcocite (2.000 + or - 0.004) + Cu --> high digenite (2.000 + or - 0.004) + Cu, T = 435 degrees + or - 8 degrees C;low chalcocite (2.000 + or - 0.002) + Cu --> high chalcocite (2.000 + or - 0.002) + Cu, T = 103.5 degrees + or - 0.5 degrees C;djurleite (1.942 + or - 0.002) --> high digenite (1.834 + or - 0.002) + high chalcocite (1.988 + or - 0.005), T = 93 degrees + or - 2 degrees C;low chalcocite (1.993 + or - 0.002) --> djurleite (1.960 + or - 0.002) + high chalcocite (1.990 + or - 0.005), T = 90 degrees + or - 2 degrees C;anilite (1.750 + or - 0.003) --> covellite (1.000 + or - 0.001) + high digenite (1.765 + or - 0.002), T = 75 degrees + or - 3 degrees C; andhigh digenite (1.805 + or - 0.002) --> anilite (1.750 + or - 0.003) + djurleite (1.934 + or - 0.002), T = 72 degrees + or - 3 degrees C.The standard free energies of formation determined for each stable and metastable phase as a function of temperature are as follows:Covellite: 273.15 to 388.36K, delta G ^o _f = -12,726 - 0.54T + or - 50 cal/mole and 388.36 to 717.73K, delta G ^o _f = -13,548 + 1.587T + or - 50 cal/mole;blaubleibender covellite I (1.1): 273.15 to 423.15K, delta G ^o _f = -13,463 - 0.45T + or - 100 cal/mole;blaubleibender covellite II (1.4): 273.15 to 423.15K, delta G ^o _f = -14,675 - 2.36T + or - 100 cal/mole;anilite (1.750): 273.15 to 348.15K, delta G ^o _f = -18,169 - 2.02T + or - 70 cal/mole;low digenite (1.765): 273.15 to 348.15K, delta G ^o _f = -18,366 - 1.17T + or - 100 cal/mole;djurleite (1.934): 273.15 to 345.15K, delta G ^o _f = -19,063 - 3.28T + or - 100 cal/mole;djurleite (1.965): 273.15 to 363.15K, delta G ^o _f = -19,178 - 3.53T + or - 100 cal/mole;low chalcocite (2.000): 273.15 to 376.65K, delta G ^o _f = -19,226 - 4.112T + or - 120 cal/mole; andhigh chalcocite (2.000): 376.65 to 708.15K, delta G ^o _f = -18,386 - 6.01T - 0.000941T ² + or - 300 cal/mole;where the numbers in parentheses are the Cu/S ratio of the phase and sulfur is the phase stable at the respective temperature and 1 atmosphere pressure.