Museumite, Pb5AuSbTe2S12, a new mineral from the gold-telluride deposit of Sacarimb, Metaliferi Mountains, western Romania

Abstract

Museumite, ideally Pb 5 AuSbTe 2 S 12 , is a newly identified mineral from the gold-telluride deposit of Sacarimb, Metaliferi Mountains, western Romania. The mineral occurs as anhedral to subhedral grains up to 300 μm in cavities and vugs of large nagyagite crystals and it does not show any inclusion or intergrowth of other minerals. The associated minerals are nagyagite, hessite, sylvanite, petzite and coloradoite, whereas the gangue minerals are calcite and quartz. Museumite is dark silver-grey in colour and shows a grey-black streak. It has a perfect {001} cleavage, the fracture is hackly and the Vickers hardness (VHN 15 ) is 42 kg/mm 2 . Museumite is greyish white in reflected light, with very low bireflectance and pleochroism. With crossed polars it shows distinct anisotropism, similar to nagyagite, but slightly stronger. Internal reflections are absent. Reflectance percentages for R min and R max are 38.4, 40.3 (471.1 nm), 38.1, 40.1 (548.3 nm), 37.5, 39.4 (586.6 nm), and 35.9, 38.0 (652.3 nm), respectively. Museumite is monoclinic, space group P 2 1 or P 2 1 / m , with the following unit-cell parameters: a = 4.361(2) A, b = 6.618(3) A, c = 20.858(9) A, β = 92.71(5)°, V = 601.3(5) A 3 . The strongest five powder-diffraction lines [ d in A ( I/I 0 ) ( hkl )] are: 4.80 (52) (013); 3.56 (100) (111); 3.47 (58) (112); 2.99 (50) (023); 2.56 (41) (116). The average of 25 electron microprobe analyses gave Pb 52.0(4), Au 10.7(1), Sb 6.2(2), Te 11.7(2), S 19.4(2), total 100.0 wt. %, corresponding, on the basis of total atoms = 21, to Pb 5.00 Au 1.08 Sb 1.01 Te 1.83 S 12.08 .