Thermodynamic stability reversal of iron sulfides at the nanoscale: Insights into the iron sulfide formation in low-temperature aqueous solution

초록

Understanding the formation of iron sulfides at low temperatures is essential for interpretation of geochemical signatures recorded in sedimentary sulfide minerals related to the evolution of the Earth's surface environment. Observed variations in the precipitation of nanoparticulate metastable phases prior to the formation of the stable-phase pyrite have been discussed from a kinetic perspective only. Here, using the ab-initio thermodynamic scheme to integrate surface thermodynamics with solid-aqueous equilibria, we demonstrate that stability relations among iron sulfides are dependent on particle size. The lower surface energies of metastable mackinawite and greigite than pyrite can drive reversals in relative thermodynamic stability at the nanoparticle scale and also lead to faster nucleation rates of the metastable phases, which are dependent on aqueous Eh-pH and Fe(II)/H2S conditions. This study emphasizes the role of surface energy in the formation pathways of iron sulfides and their particle-size dependent redox equilibria in anoxic aqueous solutions. (C) 2022 Elsevier Ltd. All rights reserved.

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