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Schlemaite, with the simplified formula (Cu,{square})6(Pb,Bi)Se4, is a new mineral species from the Niederschlema-Alberoda vein-type uranium deposit at Hartenstein, Erzgebirge, Germany. It occurs as anhedral to subhedral grains with no obvious forms or twinning, in aggregates of up to several hundred µm across, with berzelianite, eucairite and clausthalite in a dolomite-ankerite matrix. Schlemaite is black with a black streak and opaque with a metallic luster. It is brittle with an uneven fracture and no observable cleavage. It has a mean VHN (25 g load) of 106 kg/mm2, which roughly equates to a Mohs hardness of 3. In plane-polarized reflected light, schlemaite is grey, non-pleochroic with a very weak bireflectance. It has very weak anisotropy, with rotation tints in shades of very pale metallic orange and blue, and shows no internal reflections. Electron-microprobe analyses yielded a mean composition Cu 38.86, Ag 2.57, Au 0.07, Hg 0.09, Pb 13.75, Bi 9.12, Se 35.11, total 99.57 wt.%. The empirical formula (based on 4 Se apfu) is (Cu5.50Ag0.21){sum}5.71(Pb0.60Bi0.39){sum}0.99Se4. The calculated density is 7.54 g/cm3 (based on the empirical formula and unit-cell parameters refined from single-crystal data). Schlemaite is monoclinic, P21/m, a 9.5341(8), b 4.1004(3), c 10.2546(8) Å, ß 100.066(2)°, V 394.72(9) Å3, a:b:c 2.3252:1:2.5009, Z = 2. The crystal structure of schlemaite was solved by direct methods and refined to an R index of 4.8% using 1303 unique reflections collected on a four-circle diffractometer equipped with a CCD detector. The structure consists of intercalated ordered and disordered layers. The ordered layer consists of ladders of Pb2+ + Bi3+ coordinated by Se, the former showing strong lone-pair-stereoactive effects, and a network of Cu+ coordinated by Se anions. The disordered layer consists of an array of sites partly occupied by Cu+ and Ag+ in a variety of coordinations, and is characterized by strong short-range order. The strongest seven lines of the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are:3.189(100)(012), 3.132(100)(Formula12), 2.601(70)(Formula13), 2.505(50)(Formula11), 2.151(60)(014), 2.058(80)(020) and 1.909(50)(Formula14). Although schlemaite is chemically similar to furutobeite, (Cu,Ag)6PbS4, it is not isostructural with it. The mineral is named after the Schlema-Alberoda uranium ore field near Schneeberg in the ancient mining region of Saxony, Germany. |
