Ralphcannonite, AgZn₂TlAs₂S₆, a new mineral of the routhierite isotypic series from Lengenbach, Binn Valley, Switzerland

The new mineral species ralphcannonite, AgZn₂TlAs₂S₆, was discovered in the Lengenbach quarry, Binn Valley, Wallis, Switzerland. It occurs as metallic black equant, isometric to prismatic crystals, up to 50 μm, associated with dufrénoysite, hatchite, realgar and baryte. Minimum and maximum reflectance data for COM wavelengths in air are [λ (nm): R (%)]: 471.1: 25.8/27.1; 548.3: 25.2/26.6; 586.6: 24.6/25.8; 652.3: 23.9/24.8. Electron microprobe analyses give (wt.%): Cu 2.01(6), Ag 8.50(16), Zn 10.94(20), Fe 3.25(8), Hg 7.92(12), Tl 24.58(26), As 18.36(19), Sb 0.17(4), S 24.03(21), total 99.76(71). On the basis of 12 atoms per formula unit, the chemical formula of ralphcannonite is Ag_0.63(2)Cu_0.25(2)Zn_1.35(5)Fe_0.47(1)Hg_0.32(2)Tl_0.97(3)[As_1.97(6)Sb_0.01(1)]_Σ1.98(5)S_6.03(8). The new mineral is tetragonal, space group I4İ2m, with a = 9.861 (2), c = 11.125(3) Å, V = 1081.8(4) ų, Z = 4. The main diffraction lines of the calculated powder diagram are [d(in Å), intensity, hkl]: 4.100, 85, 211; 3.471, 40, 103; 2.954, 100, 222; 2.465, 24, 400; 2.460, 39, 303. The crystal structure of ralphcannonite has been refined by X-ray single-crystal data to a final R₁ = 0.030, on the basis of 140 observed reflections [F_o > 4σ(F_o)]. It shows a three dimensional framework of (Ag,Zn)-centred tetrahedra (1 M1 + 2 M2), with channels parallel to [001] hosting TlS₆ and (As,Sb)S₃ disymmetric polyhedra. Ralphcannonite is derived from its isotype routhierite ^M1Cu^M2Hg₂TlAs₂S₆ through the double heterovalent substitution ^M1Cu⁺ + ^M2Hg²⁺ → ^M1Zn²⁺ + ^M2Ag⁺. This substitution obeys a steric constraint, with Ag⁺, the largest cation relative to Zn²⁺ and Cu⁺, entering the largest M2 site, as observed in arsiccioite. The ideal crystal-chemical formula of ralphcannonite is ^M1Zn^M2(Zn_0.5Ag_0.5)₂TlAs₂S₆.