Spaltiite, Tl₂Cu₂As₂S₅, one more new thallium sulfosalt mineral from Lengenbach quarry, Binn, Switzerland

Spaltiite is a new thallium sulfosalt with the ideal formula of Tl₂Cu₂As₂S₅. It was found on a dump of the famous mineral locality Lengenbach (Binntal, Canton Valais, Switzerland). A small piece of pure white Triassic dolomite belonging to the Penninic Monte Leone Nappe hosts three euhedral long prismatic to lathlike spaltiite crystals, each approximately 2mm in length but only ~0:2mm thin. The hand specimen contains small quantities of pyrite, drechslerite and hatchite. The spaltiite crystals are greyish to black in colour and extremely soft. The Mohs’ hardness is 1.5–2 (VHN₁₅ ranges from 30 to 65, mean 47 kg mm^-2). The monoclinic crystals have a perfect cleavage parallel to {100}, which produces minute and plastic slabs. Reflectance measurements in air yield the following R_min/R_max values based on the standard wavelengths (Commission on Ore Mineralogy, COM): 27.0% / 32.6% (470 nm); 26.8% / 32.1% (546 nm); 26.0% / 31.1% (589 nm); and 24.8% / 29.3% (650 nm). Averaged electron-microprobe analyses (n = 10) gave (in wt %) Tl 47.41(19), Cu 15.46(12), Ag 0.15(6), As 17.36(14), Sb 0.41(5) and S 19.20(8), total 99.99(32). The empirical formula is Tl_1.94Cu_2.04Ag_0.01As_1.95Sb_0.03S_5.03, calculated based on 11 apfu. The large crystals exhibit a remarkably homogeneous composition. Spaltiite crystallises in space group P21/c (a = 15:791(8), b = 10:000(5), c = 6:323(3) Å,  β = 99:25(8)°, V = 985:5(8)ų). The crystal structure was determined from single-crystal X-ray diffraction data (R₁ = 12:18% for 4753 data, with F_o > 4σ (F_o) and 101 variable parameters). Spaltiite exhibits a pronounced layered atomic arrangement: two polar Cu–As layers in (1/4 y z) and (3/4 y z), respectively, are related by inversion symmetry. Sandwiched between them are the Tl atoms. These two layers are centred in (0 y z) and (1/2 y z), centrosymmetric but topologically and crystallographically distinct. The eight strongest intensities in the X-ray powder diagram are [d in Å (intensity) hkl]: 3.914 (40) 021; 2.988 (63) 510; 3.496 (45) 311; 2.869 (45) 511; 2.652 (36) 331; 3.646 (34) 221; 2.506 (29) 040; 2.762 (26) 202. The name of the new mineral originates from the nickname “spalti”, which was used during laboratory studies, illustrating the extremely pronounced cleavage (in German, “spalten” means cleave).