Luxembourgite, AgCuPbBi4Se8, a new mineral species from Bivels, Grand Duchy of Luxembourg

Luxembourgite,ideallyAgCuPbBi4Se8,isanewselenidediscoveredatBivels,GrandDuchyofLux-embourg. The mineral forms tiny fibres reaching 200 ?m in length and 5 ?m in diameter, which are depositedon dolomite crystals. Luxembourgite is grey, with a metallic lustre and without cleavage planes; its Mohs hard- -3 Ag1.00(Cu0.82Ag0.20Fe0.01)?1.03Pb1.13Bi4.11(Se7.72S0.01)?7.73, calculated on the basis of 15 atoms per formulaness is 3 and its calculated density is 8.00 g cmunit. A single-crystal structure refinement was performed to R1 =0.0476, in the P21/m space group, with a = 13.002(1), b = 4.1543(3), c = 15.312(2)Å, ? = 108.92(1)o, V = 782.4(2)Å3, Z = 2. The crystal struc- ture is similar to that of litochlebite and watkinsonite and can be described as an alternation of two types of anionic layers: a pseudotetragonal layer four atoms thick and a pseudohexagonal layer that is one atom thick. In the pseudotetragonal layers the Bi1, Bi2 ,Bi3, Pb, and Ag1 atoms are localised, while the Cu2 and Bi4 atoms occur between the pseudotetragonal and the pseudohexagonal layers. Bi1, Bi2, and Bi3 atoms occur in weakly distorted octahedral sites, whereas Bi4 occurs in a distorted 7-coordinated site. Ag1 occupies a fairly regular octahedral site, Cu2 a tetrahedral position, and Pb occurs on a very distorted 8-coordinated site.