The recrystallization behavior and deformation of synthetic chalcocite (Cu2S) in the temperature range 400–725 °C was studied microscopically after the compound was annealed in evacuated silica glass capsules and heated under differential pressures in sealed gold capsules. The temperature of recrystallization and grain growth ascribed to the hexagonal cubic inversion, at sulfur vapor pressures much less than 1 atmosphere, was determined at 465 ± 5 °C, with annealing time of .Experiments involving differential pressures of 8 000 p.s.i. show that chalcocite in the solid state becomes considerably more mobile above 563 ± 10 °C and can readily be injected as a plastic mass to form veins without preservation of deformational textures.Natural bornite and natural galena may also be injected under differential pressure at 640 °C, but some unhealed fractures remain. Flow structure is well-preserved in galena and, under certain conditions, in bornite.Mixtures of bornite and pyrite flowed and recrystallized to chalcopyrite and bornite with exsolved chalcopyrite. No evidence of flowage remained.Chalcopyrite and pyrrhotite are resistant to injection under similar differential pressures and temperatures.