Cerromojonite, CuPbBiSe3, from El Dragόn (Bolivia): A New Member of the Bournonite Group

Abstract

Cerromojonite, ideally CuPbBiSe3, represents a new selenide from the El Dragόn mine, Department of Potosí, Bolivia. It either occurs as minute grains (up to 30 µm in size) in interstices of hansblockite/quijarroite intergrowths, forming an angular network-like intersertal texture, or as elongated, thin-tabular crystals (up to 200 μm long and 40 μm wide) within lath-shaped or acicular mineral aggregates (interpreted as pseudomorphs) up to 2 mm in length and 200 μm in width. It is non-fluorescent, black, and opaque, with a metallic luster and black streak. It is brittle, with an irregular fracture, and no obvious cleavage and parting. In plane-polarized incident light, cerromojonite is grey to cream-white, and weakly pleochroic, showing no internal reflections. Between crossed polarizers, cerromojonite is weakly anisotropic, with rotation tints in shades of brown and grey. Lamellar twinning on {110} is common. The reflectance values in air for the COM standard wavelengths (R1 and R2) are: 48.8 and 50.3 (470 nm), 48.2 and 51.8 (546 nm), 47.8 and 52.0 (589 nm), and 47.2 and 52.0 (650 nm). Electron-microprobe analyses yielded a mean composition of: Cu 7.91, Ag 2.35, Hg 7.42, Pb 16.39, Fe 0.04, Ni 0.02, Bi 32.61, Se 33.37, total 100.14 wt %. The empirical formula (based on 6 atoms pfu) is (Cu0.89Hg0.11)Σ = 1.00(Pb0.56Ag0.16Hg0.15 Bi0.11Fe0.01)Σ = 0.99Bi1.00Se3.01. The ideal formula is CuPbBiSe3. Cerromojonite is orthorhombic (space group Pn21m), with a = 8.202(1) Å, b = 8.741(1) Å, c = 8.029(1) Å, V = 575.7(1) Å3, Z = 4. Calculated density is 7.035 g·cm−3. The five strongest measured X-ray powder diffraction lines (d in Å (I/I0) (hkl)) are: 3.86 (25) (120), 2.783 (100) (122), 2.727 (55) (212), 2.608 (40) (310), and 1.999 (25) (004). Cerromojonite is a new member of the bournonite group, representing the Se-analogue of součekite, CuPbBi(S,Se)3. It is deposited from strongly oxidizing low-T hydrothermal fluids at a fSe2/fS2 ratio >1, both as primary and secondary phase. The new species has been approved by the IMA-CNMNC (2018-040) and is named for Cerro Mojon, the highest mountain peak closest to the El Dragón mine.