Structural crystallography of sulfate minerals in thallium cycling

Abstract

In the extreme thallium (Tl) environments, such as oxidation zones of Tl-bearing sulfide deposits including mining wastes and technosols, Tl is retained mostly through formation of secondary Tl minerals or Tl-adsorption onto micaceous phyllosilicates and illite-like clay minerals. To better understand Tl-retention processes and phase formation in such environments we have synthesized high-quality single crystals of three mineral-like Tl salts using hydrothermal methods. These are thallium(I) triiron(III) bis(sulfate) hexahydroxide doped by small amounts of K+ and Al3+, (Tl0.91K0.09)(Fe2.90Al0.10)(SO4)2(OH)6 (I), aluminium thallium bis(sulfate) dodecahydrate Tl+Al(SO4)2·12H2O (II) and ferric iron thallium bis(sulfate) Tl+Fe3+(SO4)2 (III). They were characterized through single-crystal X-ray diffraction (SCXRD), Fourier-transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDX) and partly by transmission electron microscopy (TEM). The compounds I and III crystallise in trigonal centrosymmetric space groups: I in R – 3 m with a = 7.3330(4), c = 17.7118(6) Å and III in R – 3 with a = 4.8375(2), c = 24.7905(9) Å, whereas II is cubic, space group Pa – 3 , with a = 12.2727(14) Å. I and II, are synthetic analogues of the minerals dorallcharite and lanmuchangite, respectively. Synthetic dorallcharite (I) is isostructural with minerals in the jarosite–alunite series, KFe3+3(SO4)2(OH)6–KAl3(SO4)2(OH)6, while synthetic lanmuchangite (II), nominally TlAl(SO4)2·12H2O, is a cubic thallium alum, a type of hydrated double sulfate salt isostructural with the β-alum, CsAl(SO4)2·12H2O. The crystal structure of III, TlFe3+(SO4)2, is like that of the mineral steklite, KAl(SO4)2. II and III chemically belong to the group of alums with different degrees of hydration. The O–H···O hydrogen bonds in I and II assemble water molecule H2O and hydroxide OH– ions into several various hydrogen bond networks. The structural connections to the related minerals and inorganic compounds of I–III have been discussed. Our findings provide new insights into the chemical and structural properties of dorallcharite and lanmuchangite, enhancing our understanding of their formation in Nature. The findings also suggest the potential natural occurrence of TlFe(SO4)2, offering valuable implications for the role of sulfates in the environmental cycling of Tl.