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

Abstract

The new mineral species disulfodadsonite, Pb11Sb13S30(S2)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 ∑ Me = 48 apfu, the structural formulae are respectively Pb21.74(8)(Sb25.73(8)As0.53(2))∑=26.26S61.11(24)Cl0.08(4) and Pb21.83(25)(Sb25.66(25)As0.51(4))∑=26.17S62.27(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) A, α 96.127(6), β 90.015(7), γ 91.229(7)°, V 1366.9(2) A3. 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(S2).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 (S2)2−, and related by the coupled substitution Pb2+ + Cl− → Sb3+ + (S2)2−. Its formation is dependent upon a high value of f (S2), like other associated minerals at Seravezza (native sulfur, enargite, moeloite, etc ).