Natural SnGeS3 is monoclinic, with space group P21/c and with a = 7.2704(15) Å, b = 10.197(2) Å, c = 6.8463(14) Å, β = 105.34(3)°, V = 489.5 Å3, a:b:c = 0.7130:1:0.6714, Z = 4 and Dcalc = 3.98 g/cm3. The strongest lines in the X-ray powder diffraction pattern are (d in Å/I/I0 (hkl)): 7.006/100 (100), 4.135/49 (120), 3.077/47 (130), 2.776/38 (022), 2.699/69 (211), 2.121/31 (320), 1.724/35 (41–2). It forms transparent, elastic and flexible flattened acicular crystals with cross-section 2–5 × 20–40 μm and up to 1 cm in the length, which produce random or fan-shaped clusters on rock fragments and on black crumbly ash. It has an orange to yellowish red colour, very light yellowish brown streak, vitreous to adamantine lustre; VHN10g microhardness is 55 kp/mm2 (539 MPa); two types of perfect cleavage were established - perpendicular or oblique to elongation. No fluorescence at 254 and 366 nm of UV light was observed. In reflected light SnGeS3 is light greyish white with distinct orange brown internal reflections, anisotropic rotation tints are from dark brownish grey to reddish brown or brownish violet. Chemical analysis yields an average composition of Sn 40.47, Pb 0.29, Ge 22.73, Fe 0.03, Bi 0.27, Sb 0.10, As 0.10, S 31.42, Se 2.62, total 98.03 wt. %, leading to an empirical formula Sn1.02Ge0.94(S2.93Se0.10)Σ3.03. SnGeS3 was also observed in multicomponent aggregates on which the above described acicular crystals grow. Phases of the SnGeS3-PbGeS3 series in these aggregates form irregular grains 2–100 μm in size with a more variable composition. They exhibit an extensive solid solution between Pb and Sn limited to 70 mol. % Pb in the Sn position. SnGeS3 formed under reducing conditions by direct crystallization from hot gasses (250–300°C) at a depth of 30–60 cm under the surface of a burning coal mine dump of the Kate ina mine at Radvanice near Trutnov, Czech Republic. It was found in close association with phases of the solid solutions Bi-Sb, Bi2S3-Sb2S3 and Bi2S3-Bi2Se3; Bi3S2, Cd4GeS6, SnS, CdIn2S4, GeAsS and GeS2. The crystal structure of natural SnGeS3 was determined from single crystal X-ray diffraction data: (R1 = 3.28 % for 1026 reflections with F0 > 4σ[F0]); it is isostructural with synthetic SnGeS3. GeS4 forms maximally collapsed tetrahedral chains along c which are interconnected via Sn-S bonds which form wavy layers parallel to (100); layers are interconnected via weak Sn-S interactions thorough spaces filled by lone electron pairs. SnS5 has an unusual 1+4 coordination where four S atoms with very similar Sn-S distances define the base of a square pyramid. Inclusion of two distant S atoms complete the SnS7 monocapped trigonal prismatic coordination where the prism body hosts the lone electron-pair of Sn2+.

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