Museumite, ideally Pb5AuSbTe2S12, is a newly identified mineral from the gold-telluride deposit of Sacarîmb, Metaliferi Mountains, western Romania. The mineral occurs as anhedral to subhedral grains up to 300 μm in cavities and vugs of large nagyágite crystals and it does not show any inclusion or intergrowth of other minerals. The associated minerals are nagyágite, 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 (VHN15) is 42 kg/mm2. Museumite is greyish white in reflected light, with very low bireflectance and pleochroism. With crossed polars it shows distinct anisotropism, similar to nagyágite, but slightly stronger. Internal reflections are absent. Reflectance percentages for Rmin and Rmax 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 P21 or P21/m, with the following unit-cell parameters: a = 4.361(2) Å, b = 6.618(3) Å, c = 20.858(9) Å, β = 92.71(5)°, V = 601.3(5) Å3. The strongest five powder-diffraction lines [d in Å (I/I0) (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 Pb5.00Au1.08Sb1.01Te1.83S12.08.

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