Lead-antimony sulfosalts from Tuscany (Italy). XII. Boscardinite, TlPb4(Sb7As2)S18, a new mineral species from the Monte Arsiccio mine: occurrence and crystal structure

The new mineral species boscardinite was discovered in the barite ??? pyrite ??? iron oxides deposit of Monte Arsiccio, near Sant???Anna di Stazzema, in the Apuan Alps, Tuscany, Italy. It forms a millimetric compact mass in a quartz vein embedded in dolomitic rocks. Other associated sulfides are stibnite and zinkenite. Boscardinite is metallic grey. Under the ore microscope, it is white; pleochroism is not discernible. Anisotropism is distinct, with an ubiquitous polysynthetic twinning; rotation tints are in shades of grey. Minimum and maximum reflectance data for COM wavelengths [l (nm), Rair (%)] are: 470: 33.8/39.3, 546: 32.1/38.0, 589: 31.2/36.9, 650: 29.7/35.3. The hardness was not measured owing to the scarcity of the available material. Electron-microprobe analyses of two samples gave (wt. %, result mean of five analyses): 1) sample 4977: Ag 1.48(4), Tl 9.72(26), Pb 23.36(20), Sb 35.25(60), As 5.78(10), S 22.14(45), Se 0.04(1), total 97.77(90); 2) sample 4989: Ag 1.37(7), Tl 8.96(19), Pb 25.74(20), Sb 33.46(32), As 6.54(8), S 22.08(29), Se 0.01(1), total 98.16(63). On the basis of SMe = 14 apfu, they lead to the formulae Ag0.36Tl1.23Pb2.92(Sb7.50As2.00)S9.50S17.88Se0.01 and Ag0.33Tl1.13Pb3.20(Sb7.09As2.25)S9.34S17.76, respectively. A single-crystal X-ray study of boscardinite indicates triclinic symmetry, space group P1, with a 8.0929(4), b 8.7610(5), c 22.4971(11) Å, a 90.868(4), b 97.247(4), g 90.793(4)°, V 1582.0(2) Å3, Z = 2. The d values (Å) of the main powder-diffraction lines, corresponding to multiple hkl indices, are (relative intensity visually estimated): 3.705 (ms), 3.540 (ms), 3.479 (m), 3.085 (m), 2.977 (ms), 2.824 (vs), 2.707 (s), 2.324 (ms), and 2.176 (ms). Boscardinite is the Tl???Sb homeotype of baumhauerite; its crystal structure has been solved by X-ray single-crystal study on the basis of 4319 observed reflections with a final R1 = 0.045. It can be described in the sartorite homologous series as formed by the 1:1 alternation of sartorite-type and dufrénoysite-type layers. The simplified structural formula is based on 18 sulfur atoms and can ideally be written as TlPb4(Sb7As2)S9S18. The name boscardinite honors Matteo Boscardin for his contribution to knowledge of the regional mineralogy of Italy.