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), Mossbauer, 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 (K0.74Tl0.10)S-0.84(Al0.84Fe0.14)S0.98S2.03O8 center dot 12H(2)O. The occurrence of minor NH4+ was detected through FTIR spectroscopy. Its unit-cell parameter is a = 12.2030(2) angstrom, V = 1817.19(9) angstrom(3), space group Pa (3) over bar. Its crystal structure has been refined down to R-1 = 0.0351 for 648 reflections with F-o > 4 sigma(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) angstrom. Voltaite occurs as dark green cubic crystals, up to 1 mm in size. Voltaite is chemically zoned, with distinct domains having chemical formula (K1.94Tl0.28)S-2.22(Fe3.572+Mg0.94Mn0.55)S5.06Fe3.063+Al0.98S11.92O48 center dot 18H(2)O and (K2.04Tl0.32)S-2.36(Fe3.832+Mg0.91Mn0.29)S5.03Fe3.053+Al0.97S11.92O48 center dot 18H(2)O, respectively. Infrared spectroscopy confirmed the occurrence of minor NH4+ also in voltaite. Its unit-cell parameter is a = 27.2635 angstrom, V = 20265(4) angstrom(3), space group Fd (3) over barc. The crystal structure was refined down to R-1 = 0.0434 for 817 reflections with F-o > 4 sigma(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) angstrom. 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.