Crystal structure of undersubstituted Sb-rich vikingite Vik40, Ag2.85Pb12.35(Bi9.52Sb1.27)Σ=10.80S30: site population and comparison with structure of vikingite Vik50, Ag3.5Pb11.0Bi11.5S30

Abstract

Crystal structure of Sb-rich vikingite with lillianite substitution percentage L % below 50 % from Kutná Hora ore district, Czech Republic, was solved and refined from single-crystal diffraction data to determine the site populations of metal sites concerning a) the decreasing “lillianite“ substitution 2 Pb2+ = Ag+ + Bi; b) Sb content not known in vikingite from other localities throughout the world. Vikingite is monoclinic, C2/m, with a = 13.5394(10), b = 4.0992(3), c = 25.506(3) Å, β = 95.597(8)°, V = 1408.9(2) Å3, Z = 1, Dc = 7.0412 g/cm3. The structural formula derived from the refinement is Ag2.85Pb12.35(Bi9.52Sb1.27)Σ=10.80S30, corresponding to Vik40.0. The structure of vikingite is composed of thinner slabs (4L) of four octahedra Me2–Me1–Me1–Me2 and thicker slabs (7L) of seven octahedra Me4–Me5–Me6–Me7–Me6–Me5–Me4 separated by Pb atoms Me3 in trigonal prismatic coordination. The refinement showed differences between the structures of Vik40 and the previously published structure of Vik50. The drop of L % below 50 % shows most profoundly in the marginal octahedral site Me2 of the thinner 4L slabs, which becomes a Bi–Pb–Ag site with 28.6 % of silver next to 50 % of Bi and 21.4 % of Pb. The central Me1 site from 4L slabs which is almost a pure Bi site in Vik50 (0.97 Bi + 0.3 Ag) becomes a Bi-Pb site with minor Sb (0.54 Bi + 0.06 Sb + 0.40 Pb) in Vik40. The Sb for Bi substitution was found to take place in the semimarginal site Me5 (0.74 Bi + 0.26 Sb) in the thicker 7L slabs, which is a pure Bi site in Sb-free Vik50. Another important change against Vik50 occurs in central octahedral site Me6 (pure Pb site in Vik50), which becomes – despite the decrease in Bi content with decreasing L % – a Pb–Bi mix site. The correctness of the refined structural model was verified and the occupancies of mixed sites were fine-tuned employing charge distribution calculations in program ECoN21. In Vik40 weighted average bond lengths RAV of the marginal sites Me2 and Me4 and of the central site Me1 are significantly larger than in Vik50, reflecting the lower Ag content and the presence of Pb, while the Bi site Me5, which is partly substituted by Sb and the site Me6 with minor Bi at the expense of Pb exhibit adequately shortened RAV values.