Arsenic K-edge and Thallium L3-edge XANES, with XFM imaging and LA-ICP-MS to determine oxidation states and distribution in sphalerite

XANES, XFM imaging and LA-ICP-MS were employed to reveal the localisation of elements and the oxidation states of As and Tl in highly banded MVT sphalerite from Wiesloch, Germany. Arsenic K-edge and Tl L3-edge XANES spectra were retrieved, and results were compared to reference standards that represented different oxidation states. Fingerprinting analyses indicated As³⁺ and Tl⁺ in the Wiesloch sphalerite. Substitution mechanisms are provided that include the coupled incorporation of As and Tl in sphalerite, and a two-step series of reactions that creates and consumes vacant cation sites to better accommodate the larger size of Tl⁺ in tetrahedral coordination. XANES spectra also revealed a different edge position for As in Porgera (Papua New Guinea) pyrite and XFM imaging showed As localised entirely in the pyrite, rather than the adjacent sphalerite. A pH sensitive measure of redox in sphalerite-pyrite coprecipitated mineral assemblages is therefore suggested.