Reference: ▪ Biagioni, C., Moëlo, Y., Orlandi, P., Paar, W.H. (2018): 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. European Journal of Mineralogy, 30, 1021-1035.
Abstract: 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 mmin size, with a metallic luster, associatedwith 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 [Rmin, Rmax (%), (l)]: 34.8, 36.4 (470 nm); 33.5, 35.1 (546 nm); 32.9, 35.0 (589 nm); and 31.8, 32.4 (650nm). 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 Cu1.14Ag7.12Tl0.15Hg0.94Pb6.57(Sb22.68As1.41)Σ24.09S47.33, 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 (s), 2.131 (ms), 2.055 (s), and 1.788(s). The crystal structure study gave amonoclinic 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)Å3, Z = 2. The crystal structure was solved and refined to R1 = 0.067 on the basis of 9756 reflections with Fo>4σ(Fo) 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.