Gladkovskyite, MnTlAs3S6, a new thallium sulfosalt from the Vorontsovskoe gold deposit, Northern Urals, Russia

Gladkovskyite (IMA2018-098), MnTlAs3S6, is a new sulfosalt from the Vorontsovskoe gold deposit, Sverdlovsk Oblast', Northern Urals, Russia. The new mineral occurs in limestone breccias cemented by abundant realgar, orpiment, baryte and pyrite, as well as minor clinochlore, fluorapatite, quartz and talc. Gladkovskyite forms long-prismatic crystals and grains up to 0.2 mm; it has dark cherry-red color and red streak. It is transparent with adamantine luster. The Vickers microhardness is 94 kg/mm(2). The new mineral is brittle, with uneven fracture; neither cleavage nor parting was observed. The density calculated based on the empirical formula is 4.356 g/cm(3). In reflected light, gladkovskyite is grayish-white, strongly anisotropic with rotation tints varying from light-grey to brown. Its pleochroism is moderate, from orange-red to dark-red, and bireflectance is very weak, Delta R = 0.58 % (589 nm). The empirical formula of gladkovskyite is Mn1.01Tl0.99Pb0.01As2.86Sb0.13S6.00 (based on 11 atoms pfu). Prominent features in the Raman spectrum include bands of Mn-S and As-S stretching vibrations as well as numerous low-frequency bands related to phonons. Gladkovskyite is trigonal, R31c, a = 9.6392(2), c= 6.4560(15)angstrom, V= 519.49(12) angstrom(3) and Z = 2. The seven strongest powder X-ray diffraction lines are [d(obs), angstrom (I,%)(hkl)]: 5.11 (80)(101),4.83 (70)(110), 3.49 (50)(201), 3.23 (30)(002), 2.86 (100) (-2-11, 211), 2.68 (30)(112), 2.55 (60) (301, 202). The crystal structure of gladkovskyite was refined from the single-crystal X-ray diffraction data to R= 0.0178 for 679 observed reflections with I-obs > 3 sigma(I). It contains infinite chains of face-sharing MnS6 octahedra, TlS9 polyhedra and pyramidal AsS3 groups; the structure belongs to the group of seldom occurring cyclic sulfosalts. The new mineral honors Russian geologist Boris Aleksandrovich Gladkovsky who discovered the Vorontsovskoe gold deposit.