Stabilization of pyrite (FeS2), marcasite (FeS2), arsenopyrite (FeAsS) and loellingite (FeAs2) surfaces by polymerization and auto-redox reactions

Abstract

Abstract A comparative X-ray photoelectron spectrometric (XPS) study of the Fe 2p, S 2p and As 3d spectra of pristine, fractured surfaces of pyrite, marcasite, arsenopyrite and loellingite indicates systematic and predictable change to the surfaces of these minerals. Auto-redox and polymerization reactions stabilize uncompensated surfaces of these 3d transition metal-bearing dichalcogenides. The route to stabilization is predictable through consideration of bulk structure and electronegativity of the elements constituting the dichalcogenides. Stabilization occurs through reconstruction via polymerization of surface ligands, unless inhibited by intervening atoms or by large distances separating adjacent ligand dimers. The cubic structure of pyrite results in adjacent S dimers being separated by a distance about 50% greater than the S dimer bond length and Fe atoms are located sufficiently close to surface S atoms to inhibit S-polymer formation on (0 0 1) surfaces; polymerization on this surface consequently is minimal (if it occurs). The pyrite surfaces are consequently stabilized via an auto-redox reaction. Adjacent As dimers of loellingite (orthorhombic) are separated by a distance only 16% greater than the As dimer bond length and Fe atoms do not separate surface dimers from bulk dimers. Surface stabilization occurs via reconstruction with migration of surface As dimers toward the bulk and bonding with bulk As dimers to produce near-surface tetramers. Polymerization proceeds to such an extent that no surface monomers or dimers are detected at fracture surfaces. The crystal structure of marcasite is orthorhombic like loellingite but contains strongly electronegative S atoms (rather than As atoms). Its fracture surfaces are stabilized by both polymerization reactions (just as observed on loellingite surfaces) and by surface redox reactions (as on pyrite surfaces). The shortest distance separating adjacent S dimers is about 50% greater than the S–S dimer bond length in marcasite. This distance apparently diminishes appreciably the probablility of polymerization of surface S atoms thus allowing auto-redox reactions to occur. Polymerization and redox reactions stabilize Fe dichalcogenide surfaces. The extent to which each type of reaction proceeds is dependent upon bulk structure and electronegativity of the constituent atoms of the compound and favorable redox reaction energetics.