Improved increment method to calculate sulfur isotope fractionation factors in sulfides

Increment method was successfully developed to calculate thermodynamic isotope fractionation factors for oxygen in silicates, oxides, carbonates and sulfates. In this paper, we modified the increment method to calculate thermodynamic isotope fractionation factors of sulfur in sulfides, based on chemical features of sulfur-metal bonds and crystal features of sulfides. To approximate the bond strength of sulfides, a new constant, Madelung constant, was introduced. The increment method was then extended to calculate the reduced partition function ratios of sphalerite, chalcopyritc, galena, pyrrholite and greenockite as well as the isotope fractionation factors between them over the temperature range from 0 degrees C to 1000 degrees C. The order of S-34 enrichment in these five minerals is pyrrhotite > greenockite > sphalerite > chalcopyrite > galena. A comparison between published experimental data and our calculated results and the linear relationships among S-33, S-34 and S-36 isotopes indicate that our improved method could be taken as another model for approximately calculating the thermodynamic isotope fractionation factors of sulfur in sulfides of geochemical interest.