STRUCTURAL AND CHEMICAL COMPLEXITY OF PYRITE (001) SURFACES

Pyrite, FeS₂, is a widespread sulfide mineral that is thermodynamically stable under reducing conditions and in saturated sediments provides surfaces for the adsorption of aqueous species. When exposed to oxidizing environments, pyrite oxidation leads to the generation of low pH solutions termed acid mine drainage. Detailed understanding of the reactivity of pyrite surfaces requires knowledge of their atomic structure under relevant conditions. We developed an anaerobic chemical–mechanical polishing method that creates low-roughness pyrite (001) surfaces shown by atomic force microscopy (AFM) to be dominated by irregularly shaped (001) terraces. We studied the structure of the hydrated pyrite (001) surface using the crystal truncation rod (CTR) method, obtaining reproducible results from three crystals. Optimal fitting of the CTR data requires the incorporation of two structurally distinct termination surfaces that differ in the coverage, and likely the chemical speciation, of surface sulfur atoms. In each case, surface iron atoms and disulfur groups are significantly displaced from bulk positions. The adsorption of aqueous metal ions provides a complementary approach for assessing the chemical speciation of sites on the (001) surface. We observed modulation in the CTR scattering patterns following exposure to Fe³⁺ and Pb²⁺ ions, confirming that the termination surface is accessible and chemically reactive. Grazing incidence extended X-ray absorption fine structure (EXAFS) studies of surface-adsorbed Pb²⁺ revealed at least two near-neighbor distances consistent with a fraction of lead atoms adsorbing via inner-shell interactions with surface sulfur sites. We used ambient pressure X-ray photoelectron spectroscopy (XPS) with synchrotron soft-X-ray excitation to characterize the chemical state of S and Fe on pristine (001) surfaces. The data confirm the FeS₂ (001) surface to be terminated solely by Fe²⁺ and S₂^- groups and constrain the stoichiometry. Following exposure to partial pressures of water (up to 1 mTorr) and oxygen (up to 1 µTorr) the surface exhibits both reversible and irreversible changes in S and Fe oxidation state. The studies reveal key aspects of a molecular model for pyrite surface structure and reactivity.