Arsenmarcobaldiite, Pb₁₂(As_3.2Sb_2.8)_Σ=6S₂₁, a new N = 3.5 jordanite homologue from the Sant’Anna tectonic window, Apuan Alps (Tuscany, Italy)

The new mineral species arsenmarcobaldiite, Pb₁₂(As_3.2Sb_2.8)_Σ=6S₂₁, occurs as black anhedral grains, up to 0.5 mm in size, with metallic lustre, associated with pyrite and minor sphalerite, within layers of granoblastic quartz and accessory baryte embedded in pyrite-rich schist from a small baryte + pyrite prospect at Verzalla, Stazzema, Apuan Alps, Tuscany, Italy. Under the ore microscope, arsenmarcobaldiite is white. It is weakly pleochroic in shades of grey. Anisotropy is distinct, both in air and oil, with rotation tints from brownish to bluish-grey. Reflectance percentages for the four COM wavelengths are [R_min, R_max (%) (λ)]: 32.8, 38.3 (470 nm); 32.2, 37.5 (546 nm); 31.7, 36.9 (589 nm); 31.4, 36.6 (650 nm). Polysynthetic twinning is common and characteristic. Electron microprobe analysis gave (in wt% – average of 19 spot analyses): Ag 0.20(1), Pb 66.34(38), As 6.36(10), Sb 8.93(9), Bi 0.30(5), S 18.00(14), total 100.13(48). On the basis of ΣMe = 18 atoms per formula unit, the chemical formula is Ag_0.07(1)Pb_11.96(5)(As_3.17(4)Sb_2.74(3)Bi_0.05(1))_Σ5.96S_20.97(17), ideally Pb₁₂(As_3.2Sb_2.8)_Σ=6S₂₁. The main diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 3.595 (m), 3.429 (m), 3.214 (ms), 3.027 (ms), 2.233 (s), 2.125 (ms), 1.839 (ms). The X-ray diffraction study gave a triclinic unit cell, space group P$1¯$⁠, with a = 8.9736(9), b = 29.334(3), c = 8.4925(10) Å, α = 98.369(6), β = 118.705(6), γ = 90.874(6)°, V = 1930.5(4) Å³, Z = 2. The crystal structure was solved and refined to R₁ = 0.063 on the basis of 6325 reflections with F_o > 4σ(F_o) and 364 refined parameters. Arsenmarcobaldiite is a new N = 3.5 homologue belonging to the jordanite homologous series and it is the As-dominant isotype of marcobaldiite.