A theory of formation of intermediate sulfide phases in the solid state is proposed to explain the phenomena observed in studying the phase equilibria of such systems as: Cu 2 S-CuS, Cu 2 S-Sb 2 S 3 , Cu 2 S-As 2 S 3 , and Cu 2 S-FeS. The theory of formation (and the reverse process of "ex-solution") is based upon the simultaneous and chemically-equivalent interchange of two different kinds of metal ions, which migrate in opposite directions through the relatively stable sulfur network of a developing ("ex-solving") intermediate phase.The rates of intermediate phase formation in the systems studied were observed to be very rapid and to obey Tammann's parabolic rate rule for diffusion. The diffusion process is apparently dependent on the prevailing interstitial disorder of the copper atoms of chalcocite. The role of solid-state diffusion is stressed with regard to the formation of sulfosalt and complex sulfide minerals in ore bodies located at moderate to greater depths, as well as those that undergo thermal metamorphism.

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