Journal of

GEOsciences

  (Formerly Journal of the Czech Geological Society)

Original paper

Anatoly V. Kasatkin, Jakub Plášil, Emil Makovicky, Nikita V. Chukanov, Radek Škoda, Atali A. Agakhanov, Sergey Y. Stepanov, Roman S. Palamarchuk

Auerbakhite, MnTl₂As₂S₅, a new thallium sulfosalt from the Vorontsovskoe gold deposit, Northern Urals, Russia

Journal of Geosciences, volume 66 (2021), issue 2, 89 - 96

DOI: http://doi.org/10.3190/jgeosci.321

Auerbakhite (IMA 2020-047), MnTl₂As₂S₅, is a new sulfosalt from the Vorontsovskoe gold deposit, Sverdlovsk Oblast’, Northern Urals, Russia. The new mineral occurs in limestone breccias composed of calcite, dolomite, baryte, clinochlore, fluorapatite and quartz, and cemented by major realgar, orpiment and pyrite. Other minerals directly associating with auerbakhite include alabandite, bernardite, christite, cinnabar, coloradoite, dalnegroite, gillulyite, gold, hutchinsonite, imhofite, lorándite, metacinnabar, philrothite, rebulite, routhierite, sphalerite, and vrbaite. Auerbakhite forms rare and very small short-prismatic crystals and irregular grains which do not exceed 15  5 µm. It is transparent, has a bright red color and adamantine luster. Auerbakhite is brittle, with an uneven fracture. Cleavage is not observed. The calculated density is 5.245 g/cm³. In reflected light, auerbakhite is light gray, weakly bireflectant. In crossed polars, it is distinctly anisotropic, in light gray and brown tones, with abundant bright red internal reflections. The empirical formula of auerbakhite is Mn_1.04Tl_1.97Pb_0.02As_1.95S_5.02 (based on 10 atoms pfu). Prominent features in the Raman spectrum include bands of Mn-S and As-S stretching vibrations and numerous low-frequency bands related to mixed soft modes involving bending and Tl-S stretching vibrations. Auerbakhite is orthorhombic, space group Cmce, a = 15.3280(15)Å, b = 7.662(7)Å, c = 16.6330(14)Å, V = 1953.40(18)ų and Z = 8. The crystal structure of auerbakhite was refined from the single-crystal X-ray diffraction data to R = 0.0723 for 307 observed reflections with I_obs > 3σ(I). Auerbakhite is isostructural to the synthetic phase Tl₂MnAs₂S₅. Its crystal structure consists of complex (001) layers based on a set of MnS₄ columns parallel to [010] and are composed of edge-sharing slightly irregular MnS₆ octahedra. The columns are connected via Tl1 and are framed by paired AsS₃ pyramids with lone electron pairs oriented into interlayer space. Highly irregular Tl(2)S₆ coordination polyhedra are situated in interlayers and share ligands with the layers. The new mineral honors Russian mining engineer, manufacturer and mineralogist Alexander Andreevitch Auerbakh whose activities were closely related to Northern Urals.