Lead-antimony sulfosalts from Tuscany (Italy); XII, Boscardinite, TlPb (sub 4) (Sb (sub 7) As (sub 2) ) (sub Sigma 9) S (sub 18) , 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 [lambda (nm), R (sub 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 Sigma Me=14 apfu, they lead to the formulae Ag (sub 0.36) Tl (sub 1.23) Pb (sub 2.92) (Sb (sub 7.50) As (sub 2.00) ) (sub Sigma 9.50) S (sub 17.88) Se (sub 0.01) and Ag (sub 0.33) Tl (sub 1.13) Pb (sub 3.20) (Sb (sub 7.09) As (sub 2.25) ) (sub Sigma 9.34) S (sub 17.76) , 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) Aa, alpha 90.868(4), beta 97.247(4), gamma 90.793(4) degrees , V 1582.0(2) Aa (super 3) , Z=2. The d values (Aa) 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 (sub 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 (sub 4) (Sb (sub 7) As (sub 2) ) (sub Sigma 9) S (sub 18) . The name boscardinite honors Matteo Boscardin for his contribution to knowledge of the regional mineralogy of Italy.