Lead-antimony sulfosalts from Tuscany (Italy). XXIII. Andreadiniite, CuAg7HgPb7Sb24S48, a new oversubstituted (Cu,Hg)-rich member of the andorite homeotypic series from the Monte Arsiccio mine, Apuan Alps

The new mineral species andreadiniite, CuAg₇HgPb₇Sb₂₄S₄₈, was discovered in a quartz vein embedded in metadolostone from the Sant'Olga tunnel, Monte Arsiccio mine, Stazzema, Apuan Alps, Tuscany, Italy. It occurs as black anhedral grains, up to some mm in size, with a metallic luster, associated with sphalerite and stibnite. Under the ore microscope, andreadiniite is white, with a slightly yellow-bronze tint. Pleochroism was not observed. Anisotropism is weak, in shades of gray to bluish-gray. Reflectance percentages for the four COM wavelengths are [R_min, R_max (%), (λ)]: 34.8, 36.4 (470 nm); 33.5, 35.1 (546 nm); 32.9, 35.0 (589 nm); and 31.8, 32.4 (650 nm). Electron-microprobe analysis gave (in wt% − average of seven spot analyses): Cu 1.06(2), Ag 11.25(18), Tl 0.45(9), Hg 2.76(14), Pb 19.95(16), As 1.55(5), Sb 40.45(21), S 22.23(11), total 99.70(42). On the basis of ΣMe = 40 atoms per formula unit, the chemical formula is Cu_1.14Ag_7.12Tl_0.15Hg_0.94Pb_6.57(Sb_22.68As_1.41)_Σ24.09S_47.33, ideally CuAg₇HgPb₇Sb₂₄S₄₈. The main diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 3.719 (ms), 3.406 (s), 3.277 (s), 2.885 (s), 2.740 (ms), 2.131 (ms), 2.055 (s), and 1.788(s). The crystal structure study gave a monoclinic pseudo-orthorhombic unit cell, space group P2₁/c, with a = 19.0982(14), b = 17.0093(11), c = 13.0008(10) Å, β = 90.083(4)°, V = 4223.3(5) ų, Z = 2. The crystal structure was solved and refined to R₁ = 0.067 on the basis of 9756 reflections with F_o > 4σ(F_o) and 424 refined parameters. Andreadiniite is a new ^4,4L homologue belonging to the andorite sub-series of Sb-rich members within the lillianite homologous series. Antimony substituting Pb gives an ideal substitution percentage n = 106.25%. Distribution of minor cations (Hg, Cu, As) is detailed. Mercury may play a critical role for the stabilization of andreadiniite, through a complex substitution rule implying three cation sites: Pb²⁺ + Sb³⁺ + Ag⁺ → Sb³⁺ + Ag⁺ + Hg²⁺. Copper is subordinate to the two Hg-rich sites. The name honors Andrea Dini (b. 1966) for his contribution to the knowledge of ore deposits from Tuscany and, in particular, the ore geology and mineralogy of Hg ores from Apuan Alps.