Formation of natural IrO2 through Os–Ir–Ru alloy oxidation

We studied a rare platinum-group element (PGE)-bearing phase occurring as inclusions in Pt Fe alloys from placer deposits, previously referred to as “iridium oxide,” but lacking confirmation of actual Ir oxide. X-ray absorption spectroscopy confirms that the studied Ir O phase contains genuine IrO 2 , with iridium in a formal oxidation state of 4+ and an Ir O bond distance of 1.95 ± 0.02 Å, matching synthetic rutile-type IrO 2 . Atom probe tomography reveals that IrO 2 forms thin films along boundaries between metallic PGE-rich domains, which are separated by oxide-rich regions composed of Ca, Mn, Fe, K, and V oxides. Mass-balance constraints indicate that Rh, Ru, and Pt occur mainly as metals, while Os is partly oxidized. Formation of IrO 2 from Os–Ir–Ru alloys requires extremely oxidizing conditions, estimated at approximately fayalite–magnetite–quartz (FMQ) + 5 to +10. While such conditions could potentially develop during hydrothermal alteration, there is limited geological evidence for such high fO 2 values in natural environments. The coexistence of iridium oxide and metallic Ru contradicts thermodynamic predictions, which suggest Ru should oxidize prior to Ir. This discrepancy indicates that conventional equilibrium thermodynamics, where pressure, temperature, and chemical potential are the controlling intensive variables, may not predict natural redox behaviour. Alternative mechanisms, including kinetically controlled reactions, extremely limited length-scales of equilibrium, or a role for electrochemical work, may permit localized Ir oxidation. The results provide rare in-situ evidence for PGE oxidation and Os Ir mobility within the lithosphere, with implications for PGE distribution and Os isotope systematics in mantle and crustal rocks. • Synchrotron XAS and APT document natural IrO 2 inclusions in Pt Fe alloys. • EXAFS yields Ir O distances consistent with a rutile-type IrO 2 structure. • Os is removed from Os-Ir-Ru alloys as a volatile oxide phase during oxidation. • Residual Ir in these alloys is oxidized to IrO 2 at very high f O 2 . • Coexisting IrO 2 and metallic Ru indicate nanoscale, non-equilibrium redox conditions.