Arsenic incorporation in pyrite at ambient temperature at both tetrahedral S-I and octahedral Fe+II sites: evidence from EXAFS-DFT analysis

International audiencePyrite is an ubiquitous mineral in reducing environments, and is well-known to incorporate trace elements such as Co, Ni, Se, Au and commonly arsenic. Indeed, As-bearing pyrite is observed in a wide variety of sedimentary environments, making it a major sink for this toxic metalloid. Based on the observation of natural hydrothermal pyrites, As-I is usually assigned to the occupation of tetrahedral S sites, with the same oxidation state as in arsenopyrite (FeAsS), even if rare occurrences of AsIII or AsII are reported. However, the modes of As incorporation into pyrite during its crystallization in low-temperature diagenetic conditionsare not elucidated yet since arsenic acts as an inhibitor for pyrite nucleation at ambient temperature. Here, we bring evidence from X-ray absorption spectroscopy for AsII,III incorporation into pyrite at octahedral FeII sites and to As-I at tetrahedral S sites during crystallization at ambient temperature. Extended X-Ray absorption fine structure (EXAFS) spectra of these As-bearing pyrites are explained by local structure models obtained using density functional theory (DFT), assuming incorporation of As at the Fe and S sites as well as local clustering of arsenic. Such observation of As-I incorporation at ambient temperature may help understanding the early formation of authigenic arsenian pyrite in subsurface sediments. Moreover, evidence for AsII,III for Fe substitution in our synthetic samples questions both the possible occurrence and geochemical reactivity of such type of As-bearing pyrites in low-temperature subsurface environments