Dalnegroite, Tl_5–xPb_2x(As,Sb)_2l–xS₃₄, a new thallium sulphosalt from Lengenbach quarry, Binntal, Switzerland

Abstract

Dalnegroite, ideally Tl4Pb2(As12Sb8)Σ20S34, is a new mineral from Lengenbach, Binntal, Switzerland. It occurs as anhedral to subhedral grains up to 200 μm across, closely associated with realgar, pyrite, Sb-rich seligmanite in a gangue of dolomite. Dalnegroite is opaque with a submetallic lustre and shows a brownish-red streak. It is brittle; the Vickers hardness (VHN25) is 87 kg mm-2(range: 69—101) (Mohs hardness ∼3—3½). In reflected light, dalnegroite is highly bireflectant and weakly pleochroic, from white to a slightly greenish-grey. In cross-polarized light, it is highly anisotropic with bluish to green rotation tints and red internal reflections.According to chemical and X-ray diffraction data, dalnegroite appears to be isotypic with chabournéite, Tl5-xPb2x(Sb,As)21-xS34. It is triclinic, probable space groupP1, witha= 16.217(7) Å,b= 42.544(9) Å,c= 8.557(4) Å, α = 95.72(4)°, β = 90.25(4)°, γ = 96.78(4)°,V= 5832(4) Å3,Z= 4.The nine strongest powder-diffraction lines [d(Å) (I/I0) (hkl)] are: 3.927 (100) (10 0); 3.775 (45) (22); 3.685 (45) (60); 3.620 (50) (440); 3.124 (50) (2); 2.929 (60) (42); 2.850 (70) (42); 2.579 (45) (02); 2.097 (60) (024). The mean of 11 electron microprobe analyses gave elemental concentrations as follows: Pb 10.09(1) wt.%, Tl 20.36(1), Sb 23.95(1), As 21.33(8), S 26.16(8), totalling 101.95 wt.%, corresponding to Tl4.15Pb2.03(As11.86Sb8.20)S34. The new mineral is named for Alberto Dal Negro, Professor in Mineralogy and Crystallography at the University of Padova since 1976.

Te-RICH CANFIELDITE, Ag8Sn(S,Te)6, FROM THE LENGENBACH QUARRY, BINNTAL, CANTON VALAIS, SWITZERLAND: OCCURRENCE, DESCRIPTION AND CRYSTAL STRUCTURE

“…Over the nearly 200 years of quarrying activity at this locality, over 100 mineral specimens have been described, including 28 type-locality minerals (Graeser et al 2008, Nestola et al 2010. Note that neither geochemical nor mineralogical data have ever mentioned the presence of tellurium-bearing minerals at Lengenbach.…”

Modular Crystal Chemistry of Thallium Sulfosalts

“…Interesting is the close spatial relationship of large Tl + and small Mn 2+ polyhedra in the former array and along its contact with the latter array. The thallium polyhedra have trigonal prismatic Tl-S distances between 3.120 Nestola et al [88] described an As-dominant member of the chabournéite homeotypic series, which they named dalnegroite, with ideal composition Tl 4 Pb 2 (As 12 Sb 8 ) Σ20 S 34 . Bindi et al [85] described its crystal structure in the space group P1, a The approximately 7.8 Å thick SnS-like layer of dalnegroite alternates with the 13.6 Å thick PbS-like layer.…”

“…In some of these structures (e.g., écrinsite), this set is differentiated into Tl-rich positions and Tl-poor positions. Such differentiation is pronounced especially for the homeotypic chabournéite family [82][83][84][85][86][87][88]. When stoichiometry requires, the prism columns host Tl-Sb sequencies.…”

The Lengenbach Quarry in Switzerland: Classic Locality for Rare Thallium Sulfosalts

“…Dalnegroite was described as a new mineral species from Lengenbach in 2009 [8]. It is considered the As-analogue of chabournéite.…”