Crystal-chemistry of sulfates from the Apuan Alps (Tuscany, Italy). VI. Tl-bearing alum-(K) and voltaite from the Fornovolasco mining complex

Thallium-bearing samples of alum-(K) and voltaite from the Fornovolasco mining complex (Apuan Alps, Tuscany, Italy) have been characterized through X-ray diffraction, chemical analyses, micro-Raman, infrared (FTIR), Mössbauer, and X-ray absorption spectroscopy (XAS). Alum-(K) occurs as anhedral colorless grains or rarely as octahedral crystals, up to 5 mm. Electron-microprobe analysis points to the chemical formula (K_0.74Tl_0.10)_Σ0.84(Al_0.84Fe_0.14)_Σ0.98S_2.03O₈·12H₂O. The occurrence of minor $NH4+$ was detected through FTIR spectroscopy. Its unit-cell parameter is a = 12.2030(2) Å, V = 1817.19(9) ų, space group Pa3. Its crystal structure has been refined down to R₁ = 0.0351 for 648 reflections with F_o > 4σ(F_o) and 61 refined parameters. The crystal structure refinement agrees with the partial substitution of K by 12 mol% Tl. This substitution is confirmed by XAS data, showing the presence of Tl⁺ having a first coordination shell mainly formed by 6 O atoms at 2.84(2) Å. Voltaite occurs as dark green cubic crystals, up to 1 mm in size. Voltaite is chemically zoned, with distinct domains having chemical formula (K_1.94Tl_0.28)_Σ2.22(⁠$Fe3.572+$Mg_0.94Mn_0.55)_Σ5.06$Fe3.063+$Al_0.98S_11.92O₄₈·18H₂O and (K_2.04Tl_0.32)_Σ2.36(⁠$Fe3.832+$Mg_0.91Mn_0.29)_Σ5.03$Fe3.053+$Al_0.97S_11.92O₄₈·18H₂O, respectively. Infrared spectroscopy confirmed the occurrence of minor $NH4+$ also in voltaite. Its unit-cell parameter is a = 27.2635 Å, V = 20265(4) ų, space group Fd3c. The crystal structure was refined down to R₁ = 0.0434 for 817 reflections with F_o > 4σ(F_o) and 87 refined parameters. The partial replacement of K by Tl is confirmed by the structural refinement. XAS spectroscopy showed that Tl⁺ is bonded to six O atoms, at 2.89(2) Å. The multi-technique characterization of thallium-bearing alum-(K) and voltaite improves our understanding of the role of K-bearing sulfates in immobilizing Tl in acid mine drainage systems, temporarily avoiding its dispersion in the environment.