Pyrite (FeS2) isotopic compositions (δ56Fe and δ34S) are widely used as paleoenvironmental tracers, but the interpretations are complicated by isotopic fractionation during transformation of metastable Fe–sulfide precursors (amorphous FeS/cubic FeS/mackinawite/greigite/marcasite → pyrite). While pyrite formation typically proceeds via metastable precursors, these transient phases are rarely preserved in geological records, which makes the determination of Fe–S isotopes during the phase transition process difficult. Herein, density functional theory (DFT) and DFT + U calculations were employed to determine the equilibrium Fe and S isotopic fractionation factors among these minerals across a temperature range of 0–700 °C. Our results establish distinct fractionation levels: marcasite preferentially enriches heavy S (19.97‰) and heavy Fe (10.54‰) isotopes, while greigite hosts the lightest isotopes. Phase transitions induce significant isotopic fractionation, with the largest S isotope fractionation occurring between greigite and pyrite (−10.60‰ at 25 °C) and the significant Fe isotopic fractionation also observed between these two phases (−5.59‰ at 25 °C). Analyses of bond lengths and force constants confirm that 1000 ln β values are inversely proportional to Fe–S bond lengths and positively correlated with force constants. This study provides atomic-scale quantification of equilibrium Fe–S isotope fractionation during the transformation of pyrite precursors, yielding critical insights for interpreting isotopic records in sedimentary diagenesis, hydrothermal mineralization, and paleoredox reconstruction.
Li et al. (Thu,) studied this question.