BOND STRENGTH AND SULFUR ISOTOPIC FRACTIONATION IN COEXISTING SULFIDES

Bond strengths in sulfides are estimated from heats and free energies of formation, lattice energies, and other indirect, largely empirical methods. The sequence from greatest to least enrichment in S 34 is predicted on the basis of bond-strength and is reflected in naturally-occurring coexisting sulfides where a reasonable amount of data exists: pyrite > sphalerite > chalcopyrite > galena. For the important iron and copperiron sulfides, the sequence is: pyrite > pyrrhotite > chalcopyrite > bornite > chalcocite. Variations in S 32/S 34 distributions among naturally-occurring coexisting sulfides are systematic and consistent with estimates of the relative strengths of metal-sulfur bonds. Such consistent patterns are indicative of at least an approach to isotopic equilibrium among the coexisting sulfides. Estimates of bond strengths in sulfides provide a valid means of predicting fractionation patterns among sulfides formed under equilibrium conditions. © 1969 Society of Economic Geologists, Inc.