This first X-ray Photoelectron Spectroscopy (XPS) study of loellingite (FeAs2) reveals a strong Fe 2p3/2 singlet peak at 707.0 eV signifying that fully coordinated Fe of the bulk phase has the same oxidation state of bulk Fe in marcasite, arsenopyrite, and pyrite. The As 3d peak of loellingite (41.1 eV) demonstrates that As atoms of As dimers in loellingite have the same oxidation state as As atoms of As-S dimers in arsenopyrite. These XPS data consequently corroborate previous Mössbauer studies and confirm theoretical considerations; Fe of loellingite, arsenopyrite, and marcasite is divalent (formally) rather than being trivalent (arsenopyrite) or tetravalent (loellingite). Arsenic is present as As1− (formal oxidation state) in both loellingite and arsenopyrite.

The presence of an intense Fe 2p3/2 singlet peak for loellingite, rather than a multiplet set of peaks, demonstrates that all valence electrons of Fe in the bulk phase are paired; loellingite is a diamagnetic material thus resolving issues related to magnetic studies of the mineral. A multiplet set of peaks explains the weak high binding energy tail of the Fe 2p3/2 spectrum. This set of peaks is derived from Fe surface species. As for pyrite surfaces, Fe surface sites are of C4v symmetry due to loss of a ligand during fracture. The symmetry stabilizes the dz2 orbital leading to an intermediate spin state with unpaired electrons in Fe valence orbitals and a multiplet set of peaks. A second, weak set of multiplet peaks, probably representing Fe3+ bonded to As are also present, suggesting that a minor proportion of surface Fe2+ and arsenic undergo an auto-redox reaction in response to fracture.

The As 3d spectrum includes two spin-orbit-split doublets. One doublet represents As of As-As dimers located within the bulk phase (fully coordinated). The second is located at somewhat greater binding energy than the bulk contribution and may represent a surface polymeric As contribution.

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