Search Results (1)

The new mineral species oyonite, ideally Ag₃Mn₂Pb₄Sb₇As₄S₂₄, has been discovered in the Uchucchacua base-metal deposit, Oyon district, Catajambo, Lima Department, Peru, as very rare black metallic subhedral to anhedral crystals, up to 100 μm in length, associated with orpiment, tennantite/tetrahedrite, menchettiite, and other unnamed minerals of the system Pb-Ag-Sb-Mn-As-S, in calcite matrix. Its Vickers hardness (VHN₁₀₀) is 137 kg/mm² (range 132–147). In reflected light, oyonite is weakly to moderately bireflectant and weakly pleochroic from dark grey to a dark green. Internal reflections are absent. Reflectance values for the four COM wavelengths [R_min, R_max (%) (λ in nm)] are: 33.9, 40.2 (471.1); 32.5, 38.9 (548.3), 31.6, 38.0 (586.6); and 29.8, 36.5 (652.3). Electron microprobe analysis gave (in wt %, average of 5 spot analyses): Cu 0.76 (2), Ag 8.39 (10), Mn 3.02 (7), Pb 24.70 (25), As 9.54 (12), Sb 28.87 (21), S 24.30 (18), total 99.58 (23). Based on 20 cations per formula unit, the chemical formula of oyonite is Cu_0.38Ag_2.48Mn_1.75Pb_3.79Sb_7.55As_4.05S_24.12. The main diffraction lines are (d in Å, hkl and relative intensity): 3.34 (−312; 40), 3.29 (−520; 100), 2.920 (−132; 40), 2.821 (−232; 70), 2.045 (004; 50). The crystal structure study revealed oyonite to be monoclinic, space group P2₁/n, with unit-cell parameters a = 19.1806 (18), b = 12.7755 (14), c = 8.1789 (10) Å, β = 90.471 (11)°, V = 2004.1 (4) ų, Z = 2. The crystal structure was refined to a final R₁ = 0.032 for 6272 independent reflections. Oyonite belongs to the Sb-rich members of the andorite homeotypic sub-series within the lillianite homologous series. The name oyonite is after the Oyon district, Lima Department, Peru, the district where the type locality (Uchucchacua mine) is located. Full article