Lead-antimony sulfosalts from Tuscany (Italy). XXII. Marcobaldiite, ∼Pb₁₂(Sb₃As₂Bi)_Σ6S₂₁, a new member of the jordanite homologous series from the Pollone mine, Valdicastello Carducci

The new mineral species marcobaldiite, ∼Pb₁₂(Sb₃As₂Bi)_Σ6S₂₁, has been discovered in a single specimen collected in a quartz vein embedded in tourmaline-bearing schist from the Pollone mine, Valdicastello Carducci, Pietrasanta, Apuan Alps, Tuscany, Italy. It occurs as a black blocky prismatic crystal, up to 1 cm in size, with a metallic luster, associated with Sb-rich tennantite. Under the ore microscope, marcobaldiite is white, with a distinct anisotropism, with grey to bluish-grey rotation tints. Polysynthetic lamellar twinning is common and characteristic. Reflectance percentages for the four COM wavelengths are [R_min, R_max (%), (λ)]: 31.6, 40.1 (470 nm), 30.9, 39.6 (546 nm), 30.4, 38.5 (589 nm), and 30.0, 37.6 (650 nm). Electron microprobe analyses gave (average of 10 spot analyses − in wt%): Pb 64.05(34), Bi 4.24(6), Sb 9.10(14), As 4.51(8), S 17.24(19), total 99.14(53). On the basis of ΣMe = 18 apfu, the empirical formula is Pb_11.98(4)Sb_2.90(3)As_2.33(3)Bi_0.79(1)S_20.80(20). The main diffraction lines, corresponding to multiple hkl indices, are [d in Å (relative visual intensity)]: 3.568 (ms), 3.202 (ms), 3.016 (ms), 2.885 (ms), 2.233 (vs), 2.125 (s), 1.848 (s), and 1.775 (s). The crystal structure study gives a triclinic unit cell, space group P$1‾$⁠, with a = 8.9248(9), b = 29.414(3), c = 8.5301(8) Å, α = 98.336(5), β = 118.175(5), γ = 90.856(5)°, V = 1944.1(3) ų, Z = 2. The crystal structure has been solved and refined to R₁ = 0.067 on the basis of 6188 reflections with F_o > 4σ(F_o) and 363 refined parameters. Marcobaldiite is a new N = 3.5 homologue of the jordanite homologous series, characterized by the 1:1 alternation of two kinds of layers of distorted octahedra, three- and four-octahedra thick, i.e. of the kirkiite and jordanite types, respectively. Layers are connected by one atomic layer of three bicapped trigonal prismatic Pb atoms and one triangular pyramidal As atom in split position. The name of this new mineral species honours the mineral amateur Marco Baldi (b. 1944) for his contribution to the knowledge of the mineralogy of the pyrite  ± baryte ± iron oxide ore deposits from southern Apuan Alps.