Quijarroite, Cu 6 HgPb 2 Bi 4 Se 12 , a New Selenide from the El Drag????n Mine, Bolivia

摘要

Quijarroite, ideally Cu 6 HgPb 2 Bi 4 Se 12 , is a new selenide species from the El Drag????n mine, Department of Potos???-, Bolivia. It most frequently occurs as lath-shaped thin plates (up to 150 ???μm in length and 20 ???μm in width) intimately (subparallel) intergrown with hansblockite, forming an angular network-like intersertal texture. Quijarroite is occasionally also present as sub- to anhedral grains up to 200 ???μm in length and 50 ???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, quijarroite is weakly pleochroic from cream to very slightly more brownish-cream, displaying no internal reflections. Between crossed polars, quijarroite is moderately anisotropic with pale orange-brown to blue rotation tints. Lamellar twinning on {110} is common; parquet twinning occurs rarely. The reflectance values in the air for the COM (Commission on Ore Mineralogy) standard wavelengths ( R 1 and R 2 ) are: 46.7, 46.8 (470 nm), 47.4, 48.2 (546 nm), 47.1, 48.5 (589 nm), and 46.6, 48.7 (650 nm). Electron-microprobe analyses yielded a mean composition of Cu 13.34, Ag 1.02, Hg 7.67, Pb 16.87, Co 0.03, Ni 0.15, Bi 27.65, Se 33.52, total 100.24 wt %. The mean empirical formula, normalized to 25 apfu (atoms per formula unit), is (Cu 5.84 Ag 0.26 ) ???￡ = 6.10 (Hg 1.06 Ni 0.07 Co 0.01 ) ???￡ = 1.14 Pb 2.27 Bi 3.68 Se 11.81 ( n = 24). The simplified formula is Cu 6 HgPb 2 Bi 4 Se 12 . Quijarroite is orthorhombic, space group Pmn 2 1 , with a = 9.2413(8), b = 9.0206(7), c = 9.6219(8) ????, V = 802.1(1) ???? 3 , Z = 1. The calculated density is 5.771 g???·cm ?￠????3 . The five strongest X-ray powder-diffraction lines ( d in ???? ( I/I 0 ) ( hkl )) are: 5.36 (55) (111), 3.785 (60) (211), 3.291 (90) (022), 3.125 (100) (212), and 2.312 (50) (400). The crystal structure of quijarroite can be considered a galena derivative and could be derived from that of bournonite. It is a primary mineral, deposited from an oxidizing low- T hydrothermal fluid at a f S e 2 / f S 2 ratio greater than unity. The new species has been approved by the IMA-CNMNC (2016-052) and is named for the Quijarro Province in Bolivia, in which the El Drag???3n mine is located.