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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 0.74 Tl 0.10 ) Σ0.84 (Al 0.84 Fe 0.14 ) Σ0.98 S 2.03 O 8 ·12H 2 O. The occurrence of minorNH   4  +$\text{NH}_{4}^{+}$    was detected through FTIR spectroscopy. Its unit-cell parameter is  a  = 12.2030(2) Å,  V  = 1817.19(9) Å 3 , space groupP  a   3  ¯   .    $Pa\bar{3}.$   Its crystal structure has been refined down to  R  1  = 0.0351 for 648 reflections with  F  o  &gt; 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)∑  2.22(Fe    3.57    2  +Mg    0.94Mn    0.55)∑  5.06Fe    3.06    3  +Al    0.98S   11.92O   48    ⋅  18   H  2   O    ${{\left( {{\text{K}}_{1.94}}\text{T}{{\text{l}}_{0.28}} \right)}_{\sum 2.22}}{{\left( \text{Fe}_{3.57}^{2+}\text{M}{{\text{g}}_{0.94}}\text{M}{{\text{n}}_{0.55}} \right)}_{\sum 5.06}}\text{Fe}_{3.06}^{3+}\text{A}{{\text{l}}_{0.98}}{{\text{S}}_{11.92}}{{\text{O}}_{48}}\cdot 18{{\text{H}}_{\text{2}}}\text{O}$   and(    K   2.04Tl    0.32)∑  2.36(Fe    3.83    2  +Mg    0.91Mn    0.29)∑  5.03Fe    3.05    3  +Al    0.97S   11.92O   48    ⋅  18   H  2   O  ,    ${{\left( {{\text{K}}_{2.04}}\text{T}{{\text{l}}_{0.32}} \right)}_{\sum 2.36}}{{\left( \text{Fe}_{3.83}^{2+}\text{M}{{\text{g}}_{0.91}}\text{M}{{\text{n}}_{0.29}} \right)}_{\sum 5.03}}\text{Fe}_{3.05}^{3+}\text{A}{{\text{l}}_{0.97}}{{\text{S}}_{11.92}}{{\text{O}}_{48}}\cdot 18{{\text{H}}_{\text{2}}}\text{O},$   respectively. Infrared spectroscopy confirmed the occurrence of minorNH   4  +$\text{NH}_{4}^{+}$   also in voltaite. Its unit-cell parameter is  a  = 27.2635 Å,  V  = 20265(4) Å 3 , space groupF  d   3  ¯   c  .    $Fd\bar{3}c.$   The crystal structure was refined down to  R  1  = 0.0434 for 817 reflections with  F  o  &gt; 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.

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