First-principles investigation of the effect of crystal structure on sulfur isotope fractionation in sulfide polymorphs

In order to study the effect of crystal structure on S isotope fractionation properties of sulfide polymorphs, the reduced partition function ratios of ³⁴S/³²S (10³lnβ_34–32) for FeS₂, CoAsS, ZnS, CdS and MnS polymorphs are calculated in the present study. Our calculations show that 10³lnβ_34–32 decreases in the order of pyrite > marcasite > cobaltite > alloclasite > sphalerite ≈ wurtzite > γ-MnS > hawleyite ≈ greenockite > alabandite. The S coordination number exerts a strong influence on the equilibrium partitioning of S isotopes in these sulfides, and there exist significant S isotope fractionations between the sulfides with different S coordination numbers (CN_S), such as alabandite (CN_S = 6) and other sulfides (e.g., FeS₂, CoAsS, ZnS and CdS polymorphs) (CN_S = 4). There is a broad negative correlation between the average bond lengths of all the bonds formed by S and 10³lnβ_34–32 values. The compactness of atom packing plays an important role in determining S isotope fractionation properties of sulfides, and ³⁴S generally tends to concentrate in the phases with higher atomic packing factor (APF). The linear correlation between APF and 10³lnβ_34–32 can predict the change of 10³lnβ_34–32 with increasing pressure, but pressure has a negligible effect on S isotope fractionation factors between sulfides.