The speciation of thallium in (Tl,Sb,As)-rich pyrite

Three samples of (Tl,Sb,As)-rich pyrite from the ore deposits of the southern Apuan Alps (Tuscany, Italy) were studied through a multi-technique approach in order to constrain the speciation and incorporation mechanism of thallium in pyrite. High concentrations of Tl, Sb, and As were detected in all the studied samples through laser ablation inductively-coupled plasma mass spectrometry (LA-ICP-MS). Average Tl contents were 1299 ppm, 1967 ppm, and 2623 ppm in samples from Sennari, Canale della Radice, and Fornovolasco, respectively. The LA-ICP-MS time-resolved down-hole ablation profiles were smooth indicating that Tl, Sb, and As are dissolved in the pyrite matrix, or occur in homogeneously distributed nanoparticles (NPs). X-ray absorption spectroscopy (XAS) data revealed that Tl, Sb, and As occurs as Tl + , Sb 3+ , As 3+ , and As − . In all the studied samples, bond distances and coordination numbers for Sb 3+ and As 3+ are constant, whereas Tl displays a range of coordination numbers (∼3 to ∼6), revealing the occurrence of different local environments. Moreover, no hints of features corresponding to coordination shells higher than the first were observed, suggesting an extremely disordered environment around Tl. Transmission electron microscopy (TEM) investigations revealed the presence of dislocations and As-rich “fluid” inclusions, whereas no Tl-rich NPs were observed. The study suggests that Tl + could occur in structural defects in pyrite, lacking any long-range order. The ‘loose’ nature of Tl in pyrite favours its ready loss during the metamorphic recrystallization of pyrite and its redistribution within ore bodies and country rocks, with important implications from an economic and environmental point of view.