Richardsollyite, TlPbAsS₃, a new sulfosalt from the Lengenbach quarry, Binn Valley, Switzerland

The new mineral species richardsollyite, TlPbAsS₃, was discovered in the Lengenbach quarry, Imfeld, Binn Valley, Canton Valais, Switzerland, intimately associated with hutchinsonite and baryte. It occurs as grey-black crystals, up to 750 μm, with a metallic lustre. Under the reflected-light microscope, richardsollyite is grey, with bright-red internal reflections; anisotropy is distinct, with greyish-white to bluish rotation tints. Reflectance values for the four COM wavelengths are [R_min, R_max (%), (λ)]: 27.9, 29.8 (471.1 nm); 27.8, 31.0 (548.3 nm); 27.3, 30.8 (586.6 nm); and 27.0, 30.5 (652.3 nm). Electron microprobe analysis gave (in wt%): Tl 34.72(51), Pb 35.45(20), As 12.80(14), Sb 0.04(1), S 16.22(13), total 99.24(47). On the basis of 6 atoms per formula unit, the chemical formula is Tl_1.001Pb_1.008(As_1.007Sb_0.002)_Σ1.009S_2.982. The main diffraction lines [d in Å (intensity) hkl] are: 4.23 (80) $1¯02$⁠; 3.875 (70) $2¯11$⁠; 3.762 (100) 2 1 0, 1 2 0; 3.278 (70) 1 0 2; 2.931 (70) 0 2 2; 2.714 (70) $1¯13$⁠; and 2.622 (80) $3¯12$⁠. Richardsollyite is monoclinic, space group P2₁/c, with a = 8.8925(2), b = 8.4154(2), c = 8.5754(2) Å, β = 108.665(3)°, V = 607.98(3) ų, Z = 4. The crystal structure was solved and refined to R₁ = 0.0242 on the basis of 1590 reflections with F_o > 4σ(F_o). It can be described as formed by (1 0 0) [Pb (AsS₃)]⁻ layers sandwiching Tl⁺ cations, and is isostructural with synthetic ABCX₃ (A = K, Rb, Cs; B = Eu, Ba; C = As, Sb; X = S, Se) compounds. The new mineral is named after Richard Harrison Solly (1851–1925) for his outstanding contribution to the knowledge of the Lengenbach mineralogy during the first flourishing period of Lengenbach investigations, at the beginning of the 20th Century.