Natural SnGeS3 from Radvanice near Trutnov (Czech Republic): its description, crystal structure refinement and solid solution with PbGeS3

Natural SnGeS3 is monoclinic, with space group P2(1)/c and with a = 7.2704( 15) Angstrom, b = 10.197(2) Angstrom, c = 6.8463(14) Angstrom, beta = 105.3 4(3)degrees V= 489.5 Angstrom (3), a:b:c = 0.7130:1:0.6714, Z = 4 and D-cal c = 3.98 g/cm(3). The strongest lines in the X-ray powder diffraction patte rn are (d in Angstrom /I/I-o (hkl)): 7.006/100(100), 4.135/49(120), 3.077/4 7(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 c ross-section 2-5 x 20-30 mum and up to 1 cm in the length, which produce ra ndom or fan-shaped clusters on rock fragments and on black crumbly ash. Tt has an orange to yellowish red colour, very light yellowish brown streak, v itreous to adamantine lustre; VHN10g, microhardness is 55 kp/mm(2) (539 MPa ); two types of perfect cleavage were established - perpendicular or obliqu e 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 bro wn internal reflections, anisotropic rotation tints are from dark brownish grey to reddish brown or brownish violet. Chemical analysis yields an avera ge composition of Sn 40.47, Pb 0.29, Ge 22.73, Fe 0.03, Bi 0.27, Sb 0.10, A s 0.10, S 31.42, Se 2.62, total 98.03 wt. %, leading to an empirical formul a Sn1.02Ge0.94(S2.93Se0.10)(Sigma3.03) SnGeS3 was also observed in multicom ponent aggregates on which the above described acicular crystals grow. Phas es of the SnGeS3-PbGeS3 series in these aggregates form irregular grains 2- 100 mum in size with a more variable composition. They exhibit an extensive solid solution between Pb and Sn limited to 70 mel. % Pb in the Sn positio n. SnGeS3 formed under reducing conditions by direct crystallization from h ot gasses (250-300 degreesC) at a depth of 30-60 cm under the surface of a burning coal mine dump of the Katerina mine at Radvanice near Trutnov, Czec h Republic. It was found in close association with phases of the solid solu tions Bi-Sb, Bi2S3-Sb2S3 and Bi2S3-Bi2Se3; Bi3S2, Cd4GeS6, SnS, CdIn2S4, Ge AsS and GeS2. The crystal structure of natural SnGeS3 was determined from s ingle crystal X-ray diffraction data: (R-1 = 3.28 % for 1026 reflections wi th F-o > 4 sigma [F-o]); it is isostructural with synthetic SnGeS3. GeS4 fo rms maximally collapsed tetrahedral chains along c which are interconnected via Sn-S bonds which form wavy layers parallel to (100); layers are interc onnected 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 s imilar Sn-S distances define the base of a square pyramid. Inclusion of two distant S atoms complete the SnS7 monocapped trigonal prismatic coordinati on where the prism body hosts the lone electron-pair of Sn2+.