Luxembourgite, AgCuPbBi 4 Se 8 , a new mineral species from Bivels, Grand Duchy of Luxembourg
2020
Abstract. Luxembourgite, ideally AgCuPbBi4Se8 , is a new selenide
discovered at Bivels, Grand Duchy of Luxembourg. The mineral forms tiny
fibres reaching 200 µ m in length and 5 µ m in diameter, which are
deposited on dolomite crystals. Luxembourgite is grey, with a metallic
lustre and without cleavage planes; its Mohs hardness is 3 and its calculated
density is 8.00 g cm −3 . Electron-microprobe analyses indicate an
empirical formula Ag1.00(Cu0.82Ag0.20Fe0.01)Σ1.03Pb1.13Bi4.11(Se7.72S0.01)Σ7.73 ,
calculated on the basis of 15 atoms per formula unit. 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) A , β = 108.92 ( 1 ) ∘ , V=782.4(2) A3 ,
Z=2 . The crystal structure 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.
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