LEAD-ANTIMONY SULFOSALTS FROM TUSCANY (ITALY). XIII. PROTOCHABOURNEITE,   Tl2Pb(Sb9-8As1-2) 10S17, FROM THE MONTE ARSICCIO MINE: OCCURRENCE, CRYSTAL STRUCTURE AND RELATIONSHIP WITH CHABOURNEITE

Abstract

The new mineral species protochabourneite, ~ Tl2Pb(Sb9–8As1–2)∑10S17, has been discovered in the barite + pyrite + iron oxide ore deposit exploited at the Monte Arsiccio mine, near Sant’Anna di Stazzema (Apuan Alps, Tuscany, Italy). It occurs as compact masses associated with barite, boscardinite, calcite, cymrite, dolomite, pyrite, realgar, routhierite, sphalerite, and stibnite. Protochabourneite is metallic black. Minimum and maximum reflectance data for COM wavelength are [λ (nm): R air (%)]: 470: 36.4/39.5; 546: 34.2/36.7; 589: 33.0/35.4; 650: 31.2/33.4. Electron microprobe analyses collected from two different grains gave (wt.%): Tl 16.81(27), Pb 10.65(17), Sb 41.75(17), As 6.59(4), S 23.43(11), total 99.24(36) and Tl 15.05(26), Pb 13.04(32), Sb 45.49(29), As 3.07(4), S 22.77(14), total 99.42(39). On the basis of ∑ Me = 13 apfu , the chemical formulae are Tl1.89Pb1.18Sb7.90As2.03S16.83 and Tl1.74Pb1.48Sb8.81As0.97S16.75, respectively, in agreement with the general formula Tl2− x Pb1+2 x (Sb10− x − y As y )S17 ( x = 0.10–0.24; y = 1–2). Protochabourneite is triclinic P 1, with a 8.150(2), b 8.716(2), c 21.579(4) A, α 85.18(1)°, β 96.94(1)°, γ 88.60(1)°, V 1515.4(6) A3, and Z = 2. The main diffraction lines of the powder diagram, corresponding to multiple ( hkl ) indices, are (relative visual intensity): 3.608 (s), 2.824 (s), 2.790 (ms), 2.170 (vs). The crystal structure of protochabourneite has been solved by X-ray single-crystal studies on the basis of 3401 reflections with a final R 1 = 0.044. It agrees with the general features of chabourneite, but with a primitive unit cell, without any visible superstructure. It presents mixed (Tl,Pb), (Pb,Sb), and (Sb,As) sites, together with pure Tl, Pb, and Sb sites. The structure is composed of two kinds of layers. The first one, derived from the SnS archetype, is closely related to sartorite and parapierrotite, whereas the second one is derived from the PbS archetype. The high Sb:As atomic ratio of protochabourneite appears as the main factor precluding the 2 a × 2 c superstructure visible in chabourneite as well as in dalnegroite. Protochabourneite, like dalnegroite, belongs to the chabourneite homeotypic series.