The new mineral species marcobaldiite, ~Pb12 (Sb3 As2 Bi)Σ6 S21, 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
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 Pb11.98(4) Sb2.90(3) As2.33(3) Bi0.79(1) S20.80(20).
The main diffraction lines, corresponding to multiple hkl indices, are [d in A (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)Å3, Z = 2. The crystal structure has been solved and refined to R
1 = 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.
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Document Type: Research Article
Publication date: June 1, 2018
This article was made available online on December 20, 2017 as a Fast Track article with title: "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".
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