LEAD-ANTIMONY SULFOSALTS FROM TUSCANY (ITALY). XII. BOSCARDINITE, TlPb4(Sb7As2) 9S18, A NEW MINERAL SPECIES FROM THE MONTE ARSICCIO MINE: OCCURRENCE AND CRYSTAL STRUCTURE

Abstract

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 [λ (nm), R air (%)] 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 ∑ Me = 14 apfu , they lead to the formulae Ag 0.36 Tl 1.23 Pb 2.92 (Sb 7.50 As 2.00 ) ∑9.50 S 17.88 Se 0.01 and Ag 0.33 Tl 1.13 Pb 3.20 (Sb 7.09 As 2.25 ) ∑9.34 S 17.76 , respectively. A single-crystal X-ray study of boscardinite indicates triclinic symmetry, space group P 1, with a 8.0929(4), b 8.7610(5), c 22.4971(11) A, α 90.868(4), β 97.247(4), γ 90.793(4)°, V 1582.0(2) A 3 , Z = 2. The d values (A) 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 R 1 = 0.045. It can be described in the sartorite homologous series as formed by the 1:1 alternation of sartorite-type and dufrenoysite-type layers. The simplified structural formula is based on 18 sulfur atoms and can ideally be written as TlPb 4 (Sb 7 As 2 ) ∑9 S 18 . The name boscardinite honors Matteo Boscardin for his contribution to knowledge of the regional mineralogy of Italy.