ABSTRACT

The volume variable cluster model method, which is an improvement proposed by Rustad and coworkers, has been successfully used to calculate equilibrium isotope fractionations between solids and solutions. The biggest difference between these two cluster model methods is that the fixed layer of atoms in the method of Rustad et al. has been removed and the whole cluster is freely optimized under the “VVCM” (the volume variable cluster model method) treatment. In this paper, several sulfides have been chosen whose sulfur isotope fractionations have been theoretically studied by previous researchers but with disagreements on calculating sulfur isotope fractionations. HS^-_(aq) and H₂S_(aq) solutions are used to model sulfur-bearing aqueous solutions under different conditions. The calculated results of the first-principles DFT method are quite different from those based on Mössbauer data. The VVCM-based method is completely different from both of the above methods and can therefore provide an independent evaluation of their results. The results in this article show that the β-factors of sulfides decrease in the order of pyrite > sphalerite > galena and that the sulfur isotope fractionation magnitudes are in good agreement with previous results.