Eckerite, Ag2CuAsS3, a new Cu-bearing sulfosalt from Lengenbach quarry, Binn valley, Switzerland: description and crystal structure

Eckerite, ideally Ag2CuAsS3, is a new mineral from the Lengenbach quarry in the Binn Valley, Valais, Switzerland. It occurs as very rare euhedral crystals up to 300 mu m across associated with realgar, sinnerite, hatchite, trechmannite and yellow, fibrous smithite. In thick section eckerite is opaque with a metallic lustre and shows a dark orange-red streak. It is brittle; the Vickers hardness (VHN25) is 70 kg/mm(2) (range: 64-78) (Mohs hardness of similar to 21/2-3). In reflected light, eckerite is moderately bireflectant and weakly pleochroic from light grey to a slightly bluish grey. Internal reflections are absent. Under crossed nicols, it is weakly anisotropic with greyish to light blue rotation tints. Reflectance percentages for R-min and R-max are 27.6, 31.7 (471.1 nm), 22.8, 26.1 (548.3 nm), 21.5, 24.5 (586.6 nm) and 19.4, 22.3 (652.3 nm), respectively. Eckerite is monoclinic, space group C2/c, with a = 11.8643(3), b = 6.2338(1), c = 16.6785(4) angstrom, beta = 110.842(3)degrees, V = 1152.81(5) angstrom(3), Z = 8. The crystal structure [R-1 = 0.0769 for 1606 reflections with F-o > 4 sigma(F-o)] is topologically identical to that of xanthoconite and pyrostilpnite. In the structure, AsS3 pyramids are joined by AgS3 triangles to form double sheets parallel to (001); the sheets are linked by Cu(Ag) atoms in a quasi-tetrahedral coordination. Among the three metals sites, Ag-2 is dominated by Cu. The mean metal-S distances reflect well the Ag <-> Cu substitution occurring at this site. The eight strongest powder X-ray diffraction lines [d in angstrom (I/I-0) (hkl)] are: 3.336 (70) ((3) over bar 12); 2.941 (100) ((3) over bar 14,114); 2.776 (80) (400,(2) over bar 06); 2.677 (40) (312); 2.134 (50) ((4) over bar 21); 2.084 (40) ((2) over bar 08,206); 2.076 (40) (420); 1.738 (40) ((2) over bar 28,226). A mean of five electron microprobe analyses gave Ag 52.08(16), Cu 11.18(9), Pb 0.04(1), Sb 0.29(3), As 15.28(11), S 20.73(13), total 99.60 wt.%, corresponding, on the basis of a total of 7 atoms per formula unit, to Ag-2.24Cu0.82As0.94Sb0.01S2.99. The new mineral has been approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (2014-063) and named for Markus Ecker, a well known mineral expert on the Lengenbach minerals for more than 25 years.