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Natural bornite (Cu5FeS4) leached in ferric chloride or in sulfuric or hydrochloric acid solutions with oxygen showed several differences. In ferric chloride 80% of the copper was leached in two stages. The first (30% copper removal) requiring one minute was controlled by mixed kinetics (liquid ferric ion diffusion and chemical surface reaction). The second was controlled by formation of a surface sulfur layer and then diffusion through the sulfur. With oxygen in sulfuric acid reaction was slow and only the first stage was studied. Copper removal had an induction period. The bornite structure disappeared at 12% copper removal (also found with ferric chloride) and covellite formed on the surface with a species...