'Bond strengths in sulfides are estimated from heats and free energies of formation, lattice energies, and other indirect, largely empirical methods.' Sequences, from greatest to least enrichment in S-34, predicted from bond strength, are pyrite, sphalerite, chalcopyrite, galena; and pyrite, pyrrhotite, chalcopyrite, bornite, chalcocite. Where a reasonable amount of data exists, the systematic and consistent patterns in naturally occurring coexisting sulfides indicate an approach to isotopic equilibrium, and validate the use of estimates of bond strength to predict fractionation patterns among sulfides formed under equilibrium conditions.

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