Lead-antimony sulfosalts from Tuscany (Italy). XIV. Disulfodadsonite, Pb11Sb13S30(S2)0.5, a new mineral from the Ceragiola marble quarry, Apuan Alps: occurrence and crystal structure

The new mineral species disulfodadsonite, Pb11Sb13S30(S-2)(0.5), has been discovered in a cavity of the Liassic marbles quarried in the Ceragiola area, near the town of Seravezza, Apuan Alps, Tuscany, Italy. It occurs as acicular crystals, up to 3-4 mm in length and a few micrometers in width, associated with boulangerite, calcite, and sphalerite. Disulfodadsonite is metallic black. Under the ore microscope, it is white with red internal reflections visible on the grain edges; no pleochroism could be distinguished. Anisotropy is weak but distinct, with rotation tints from brown to dark blue. Electron microprobe analyses collected on two different grains give (wt %): Pb 46.42(20), Sb 32.29(32), As 0.41(2), S 20.19(13), Cl 0.03(2), total 99.34(58), and Pb 46.76(55), Sb 32.30(32), As 0.40(4), S 20.64(9), Cl 0.02(1), total 100.12(35). On the basis of Sigma Me = 48 apfu, the structural formulae are respectively Pb-21.74(8)(Sb25.73(8)As0.53(2))(Sigma=26.26)S61.11(24)Cl0.08(4) and Pb-21.83(25)(Sb25.66(25)As0.51(4))(Sigma=26.17)S62.27(41)Cl0.06(2), close to Pb22Sb26S62 (against Pb23Sb25S60Cl for dadsonite). The crystal structure study gives a triclinic unit cell, space group P-1, with a 4.1192(3), b 17.4167(14), c 19.1664(16) angstrom, alpha 96.127(6), beta 90.015(7), gamma 91.229(7)degrees, V 1366.9(2) angstrom(3). Main diffraction lines of the powder diagram, corresponding to multiple hkl indices, are (relative visual intensity): 3.820 (ms), 3.649 (s), 3.416 (s), 3.381 (vs), 2.857 (ms), 1.897 (ms). The mean crystal structure of disulfodadsonite has been solved by X-ray single-crystal study on the basis of 3389 reflections with a final R-1 = 0.102. It agrees with the general features of dadsonite but without any visible superstructure. The unit-cell content is Pb11Sb13S30(S-2)(0.5) (Z = 1). There are five pure Pb and five pure Sb sites, two mixed (Sb/Pb) sites, and sixteen S positions. The structure can be described as formed by rod-layers, with two types of rod-layers alternating along the b stacking direction. Disulfodadsonite is the Cl-free homeotype of dadsonite, Pb23Sb25S60Cl, stabilized by the disulfide ion (S-2)(2-), and related by the coupled substitution Pb2+ + Cl -> Sb3+ + (S-2)(2-). Its formation is dependent upon a high value of f(S-2), like other associated minerals at Seravezza (native sulfur, enargite, moeloite, etc).