Ferrostalderite, CuFe₂TlAs₂S₆, a new mineral from Lengenbach, Switzerland: occurrence, crystal structure, and emphasis on the role of iron in sulfosalts

The new mineral species ferrostalderite, CuFe₂TlAs₂S₆, was discovered in the Lengenbach quarry, Binn Valley, Wallis, Switzerland. It occurs as minute, metallic, black, equant to prismatic crystals, up to 50 μm, associated with dolomite, realgar, baumhauerite (?) and pyrite. Minimum and maximum reflectance data for COM wavelengths in air are [λ (nm): R (%)]: 471.1: 24.2/25.4; 548.3: 23.7/24.7; 586.6: 22.9/23.8; 652.3: 21.0/22.0. Electron microprobe analyses give (wt.%): Cu 6.24(25), Ag 4.18(9), Fe 9.95(83), Zn 4.46(91), Hg 1.22(26), Tl 26.86(62), As 19.05(18), Sb 0.63(6), S 25.39(47), total 97.98(72). On the basis of 12 atoms per formula unit, the chemical formula of ferrostalderite is Cu_0.75(2)Ag_0.30(1)Fe_1.36(10)Zn_0.52(11)Hg_0.05(1)Tl_1.00(1)[As_1.94(4)Sb_0.04(1)]_Σ1.98(4)S_6.04(4). The new mineral is tetragonal, space group I2 m, with a = 9.8786(5), c = 10.8489(8) Å, V = 1058.71(11) ų, Z = 4. The main diffraction lines of the calculated powder diagram are [d (in Å), intensity, hkl]: 4.092, 70, 211; 3.493, 23, 220; 3.396, 35, 103; 3.124, 17, 310; 2.937, 100, 222; 2.656, 19, 321; 2.470, 19, 400; 2.435, 33, 303. The crystal structure of ferrostalderite has been refined by X-ray single-crystal data to a final R₁ = 0.050, on the basis of 1169 reflections with F₀ > 4σ(F₀). It shows a three dimensional framework of (Cu,Fe)-centred tetrahedra (1 M1 + 2 M2), with channels parallel to [001] hosting disymmetric TlS₆ and (As,Sb)S₃ polyhedra. Ferrostalderite is derived from its isotype stalderite ^M1Cu^M2Zn₂TlAs₂S₆ through the homovalent substitution ^M2Zn²⁺ → ^M2Fe²⁺. The ideal crystal-chemical formula of ferrostalderite is ^M1Cu^M2Fe₂TlAs₂S₆.