Raberite, Tl₅Ag₄As₆SbS₁₅, a new Tl-bearing sulfosalt from Lengenbach quarry, Binn valley, Switzerland: description and crystal structure

Raberite, ideally Tl₅Ag₄As₆SbS₁₅, is a new mineral from Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs very rarely as euhedral crystals up to 150 μm across associated with yellow needle-like smithite, realgar, hatchite and probable trechmannite, edenharterite, jentschite and two unidentified sulfosalts. Raberite is opaque with a metallic lustre and has a dark brown–red streak. It is brittle with a Vickers hardness (VHN₁₀) of 52 kg mm⁻² (range 50–55) corresponding to a Mohs hardness of 2½–3. In reflected light raberite is moderately bireflectant and very weakly pleochroic from light grey to a slightly greenish grey. It is very weakly anisotropic with greyish to light blue rotation tints between crossed polars. Internal reflections are absent. Reflectance percentages for the four COM wavelengths [listed as R_min, R_max, (λ)] are 30.6, 31.8 (471.1 nm), 28.1, 29.3 (548.3 nm), 27.1, 28.0 (586.6 nm), and 25.8, 26.9 (652.3 nm).

Raberite is triclinic, space group P1İ, with a = 8.920(1), b = 9.429(1), c = 20.062(3) Å, α = 79.66(1), β = 88.84(1), γ = 62.72(1)°, V = 1471.6(4) ų and Z = 2. The crystal structure [R₁ = 0.0827 for 2110 reflections with I > 2σ(I)] consists of columns of nine-coordinate Tl atoms forming irregular polyhedra extending along [001] and forming sheets parallel to (010). The columns are decorated by corner-sharing MS₃ pyramids (M = As, Sb) and linked by AgS₃ triangles. Of the seven M positions, one is dominated by Sb and the others by As; the mean M–S bond distances reflect As↔Sb substitution at these sites.

The eight strongest lines in the powder diffraction pattern [d_calc in Å (I) (hkl)] are: 3.580 (100) (11İ3); 3.506 (58) (1İ2İ3); 3.281 (73) (006); 3.017 (54) (1İ23); 3.001 (98) (133); 2.657 (51) (226); 2.636 (46) (300); 2.591 (57) (330). A mean of 9 electron microprobe analyses gave Tl 39.55(13), Ag 18.42(8), Cu 0.06(2), As 17.08(7), Sb 5.61(6), S 19.15(11); total 99.87 wt.%, which corresponds to Tl_4.85Ag_4.28Cu_0.02As_5.72Sb_1.16S_14.97 with 31 atoms per formula unit. The new mineral has been approved by the IMA-CNMNC Commission (IMA 2012-017) and is named for Thomas Raber, an expert on Lengenbach minerals.