Structural and chemical characterization of Tl-bearing sulphates in the Tl–Fe–S–H₂O system: insights into dorallcharite formation and thallium retention

Abstract

Minerals of the jarosite-group significantly influence the mobility and fate of trace elements in acidic, sulphate-rich environments, such as those impacted by mining. However, the interactions between these minerals and the highly toxic element thallium are not well understood. In the natural Tl(I)–Fe(III)–S(VI)–H 2 O system, the formation of small (up to 10 μm) rhombohedral crystals of dorallcharite, a Tl-dominant member of the jarosite group, has been observed. Dorallcharite, nominally TlFe 3+3(SO 4) 2 (OH) 6 , is the main Tl sink in the Tl and sulphate-rich, acidic environments affected by mining, as well as in associated soils and sediments. Due to its poor crystallinity, its structural and compositional characteristics have remained only partially understood. To address this, we synthesized high-quality single crystals of [Tl 0.94 (H 3 O) 0.06 )] + Fe 3+3 (SO 4 ) 2(OH) 6 (I) and Tl + Fe 3+ (SO 4) 2 (II) using hydrothermal methods. These crystals were thoroughly characterized through single- crystal X-ray diffraction (SC-XRD), Raman spectroscopy, scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). Both sulphates crystallize in trigonal centrosymmetric space groups: I in RC3 m (No. 166) with a = 7.3330(4), c = 17.7118(6) Å, V = 824.81(9) Å3 , Z = 3 and II in RC3 (No. 148) with a = 4.8375(2), c = 24.7905(9) Å, V = 502.41(5) Å3 and Z = 3. SEM-EDS and TEM-EDS investigations have been shown that K + substitute Tl + and Al 3+ substitute Fe 3+ in very small amounts at mixed atom sites. The chemical composition of II corresponds to the formula Tl + (Fe 0.8 Al0.2 ) 3+ (SO 4 ) 2 based on 2S atoms p.f.u., which agrees with the chemical compositions obtained from crystal-structure refinements. These comprehensive chemical and structural analyses provide new insights into dorallcharite’s chemical composition and crystal structure, advancing our understanding of how it forms in nature. Additionally, II is a compound that may also be present in natural environments. These findings offer valuable insights into the role of sulphates in the environmental cycling of Tl, contributing to a deeper understanding of this complex system.