The sphalerite-pyrite oxidation by Acidithiobacillus ferrooxidans was studied to analyze how the formation of the elemental sulfur layers occurs around sphalerite grains. Two possible mechanisms of formation have been raised. One mechanism corresponds to the formation of sulfur pseudomorphs where, both, iron and zinc have been leached of the sphalerite, whereas compound sulfur is oxidized, in situ, to elemental sulfur, leaving an unreacted core of sphalerite that remains in the center. Another mechanism consists in the dissolution of iron, zinc and sulfur presents. When the attack by Fe3+ as by H + broken the S-metal bonds, sulfur is then oxidized to a series of sulfur intermediate compounds. These compounds in solution then are oxidized to elemental sulfur, which precipitates on sphalerite grains. © 2007 Trans Tech Publications.20-21 134138Da Silva, G., Lastra, M.R., Budden, J.R., (2003) Minerals Engineering, 16, p. 199Rodriguez, Y., Ballester, A., Blázquez, M.L., Gonzáles, F., Muñoz, J.A., (2003) Hydrometallurgy, 71, p. 57Rossi, G., (1990) Biohydrometallurgy., , McGraw-Hill Book Company GmbH, HamburgThomas, J.E., Skinner, W.M., Smart, R., (1998) Geochimica et Cosmochimica Acta, 62, p. 1555Weisener, C., Smart, R., Gerson, A., (2003) Geochimica et Cosmochimica Acta, 67, p. 823Ahonen, L., Tuovinen, O., (1993) Geomicrobiology Journal, 10, p. 207Da Silva, G., (2004) Hydrometallurgy, 73, p. 313Ossa, D.M., Márquez, M.A., Zapata, D.M., (2007) Submitted to Revista Colombiana de BiotecnologíaCrundwell, F., (1987) Hydrometallurgy, 33, p. 137Crundwell, F., (1988) AIChE Journal, 34, p. 1128Fowler, T., Crundwell, F., (1999) Applied and Environmental Microbiology, 65, p. 5285Schippers, A., Sand, W., (1999) Applied and Environmental Microbiology, 65, p. 319