Revealing the Chemical Form of “Invisible” Gold in Natural Arsenian Pyrite and Arsenopyrite with High Energy-Resolution X-ray Absorption Spectroscopy
Résumé
The structural chemistry of gold in arsenian pyrite (FeS2) and arsenopyrite (FeAsS) is as yet unknown, despite the economic importance of this element and its potential recovery from low-grade ores and mine tailings. The systematic co-occurrence of Au and As poses a challenge for measuring the Au L3-edge X-ray absorption spectra because the Au Lα emission line is partly (pyrite) to totally (arsenopyrite) obscured by the intense As Kα line. Utilizing a newly developed high luminosity multi-crystal analyzer, in combination with the capabilities of a synchrotron-based microprobe, the Au Lα and As Kα lines were separated and the Au L3-edge XANES and EXAFS spectra of chemically-bound Au in arsenian pyrite and arsenopyrite from gold deposits measured for the first time. High energy-resolution XANES (HR-XANES) shows that gold has a formal oxidation state of 1+ in the two sulfides, but a distinct bonding environment. In arsenian pyrite with a point Au/As atomic ratio of 0.14 (0.37 wt ratio) and lacking geochemical correlation between the concentrations of Au and As, Au occupies the Fe site and is bonded to six S atoms at 2.41 Å, as 2 determined by HR-EXAFS. In contrast, in arsenopyrite with a point Au/As atomic ratio of 7.1×10-3 (0.02 wt ratio) and exhibiting a Au-As correlation, octahedrally coordinated Au is bonded only to As atoms at 2.52 Å. The results reveal the atomic-scale structure of gold and its intricate geochemical association with arsenic in auriferous Fe sulfides. This work demonstrates that high energy-resolution X-ray absorption spectroscopy on synchrotron X-ray microprobes can illuminate the structural chemistry of trace elements in chemically complex Earth and planetary materials.
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