Quadratite Ag(Cd,Pb)(As,Sb)S-3 - a new sulfide mineral from Lengenbach, Binntal (Switzerland)

With the mineral quadratite another new sulfide phase has been identified i n the famous Lengenbach locality (Binntal, Switzerland). Like the numerous other, mainly As-sulfide minerals at this locality, quadratite occurs in sm all vugs in a white dolomitic rock of presumably Triassic age. Different fr om the new minerals discovered within the last few years, which all belong to the very special group of thallium sulfosalts and therefore always occur in close association with As-rich phases, quadratite exclusively occurs to gether with minerals poor in arsenic. Therefore it is closely associated wi th minerals like jordanite and galena, whereas realgar is clearly absent. I n general, quadratite forms crystals of characteristic quadratic, sometimes octogonal shape. always tabular along the basis (001). The size is normall y about 0.1 mm across, crystals with a diameter of more than 1 mm are extre mely rare. With Sew exceptions the mineral is untwinned, one or two samples show an oriented intergrowth which may be interpreted as contact twins alo ng (016). Small crystals are brown-red translucent, larger specimens are op aque with grey metallic luster, streak is reddish-brown. Single-crystal X-r ay diffraction yielded a tetragonal cell with a = 5.499 (5) and c = 33.91 ( 4) Angstrom, V = 1025 (2) Angstrom(3),space group I4/amd, Z = 8. The strong est lines in the powder diagram are (d(obs) in Angstrom, I-obs, hkI): 2.77 (100) (200), 1.960 (80) (2.0.12), 1.679 (70) (2.0.16), 1.598 (70) (2.2.12;3 .0.11), 1.274 (60) (4.0.10), 3.19 (50) (116). Chemical composition(electron microprobe, mean of 6 analyses, wt%) is: Ag 26.39, Cu 0.05, Tl 0.49, Cd 21 .42, Pb 10.17, As 18.06, Sb 0.27, S 23.61, total 100.37 wt%. The simplified formula is close to AqCdAsS(3). The calculated density D-calc = 5.31 g/cm( 3). From the microhardness VHN (load 10 g) of 57-72 kg/mm(2) a Mohs hardnes s of less than or equal to 3 may be derived. Quadratite has a perfect cleav age along {001} and distinct along {110}. The name is derived from the conspicous quadratic shape of the mineral, uni que among Lengenbach sulfosalts, so far.