Grundmannite, CuBiSe2, the Se-analogue of emplectite, a new mineral from the El Dragón mine, Potosí, Bolivia

Abstract

Grundmannite, ideally CuBiSe 2 , is a new mineral species from the El Dragon mine, Department of Potosi, Bolivia. It either fills small shrinkage cracks or interstices in brecciated krut’aite – penroseite solid solutions or forms independent grains in the matrix. Grain size of the anhedral to subhedral crystals is usually in the range 50–150 μm, but may approach 250 μm. Grundmannite is usually intergrown with watkinsonite and clausthalite; other minerals occasionally being in grain-boundary contact comprise quartz, dolomite, native gold, eldragonite, eskebornite, umangite, klockmannite, Co-rich penroseite, and three unnamed phases of the Cu–Hg–Pb–Bi–Se system, among which is an as-yet uncharacterized species with the ideal composition Cu 4 Pb 2 HgBi 4 Se 11 . Grundmannite is non-fluorescent, black and opaque with a metallic lustre and black streak. It is brittle, with an irregular fracture and no obvious parting. Some grains display a perfect {001} cleavage. The VHN 20 values range from 45 to 61 (mean 53) kg mm −2 , which equates to a Mohs hardness of 2 to 2½. In plane-polarized incident light, grundmannite is weakly bireflectant and weakly pleochroic, from cream to light grey, and shows no internal reflections. Between crossed polarisers, grundmannite is distinctly anisotropic, with light-brown to brown rotation tints. The reflectance values in air for the COM standard wavelengths are: 41.0–43.4 (470 nm), 41.8–45.1 (546 nm), 42.1–45.7 (589 nm), and 42.5–46.2 (650 nm). Electron probe micro-analyses yielded a mean composition Cu 14.88, Pb 1.23, Hg 0.07, Ni 0.05, Bi 44.90, Se 38.92, total 100.05 wt.%. The mean empirical formula, normalized to 4 atoms per formula unit ( apfu ), is Cu 0.99 (Bi 0.91 Pb 0.02 ) ∑0.93 Se 2.08 ( n = 19). The ideal formula is CuBiSe 2 , which requires (in wt.%) Cu 14.76, Bi 48.55, Se 36.69, sum 100.00. Grundmannite is orthorhombic, space group Pnma , with a = 6.6362(5), b = 4.2581(3), c = 15.3691(9) A, V = 434.29(5) A 3 , and Z = 4. Density, calculated on the basis of the mean chemical composition and unit-cell parameters derived from the single-crystal X-ray study, is 6.582 g cm −3 . The five strongest X-ray powder-diffraction lines [ d in A ( I/I 0 ) ( hkl )] are: 3.4901 (50) (111), 3.3180 (70) (200), 3.2746 (100) (013), 2.4923 (45) (015), and 2.3307 (50) (213). The crystal structure of grundmannite is topologically identical to that of emplectite, CuBiS 2 , with the two independent sulfur positions occupied by Se, thus being the Se-isotype of emplectite. In the structure, Bi forms BiSe 3 trigonal pyramids (with two additional longer distances) and Cu nearly regular CuSe 4 tetrahedra. Grundmannite is a primary mineral, deposited from an oxidizing low- temperature hydrothermal fluid at the waning stage of selenide-mineral formation at El Dragon, at a f Se 2 / f S 2 ratio greater than unity and in the presence of hematite, conditions typically prevailing during the formation of telethermal vein-type selenide deposits. The mineral was named after Gunter Grundmann, in recognition of his pioneering work on the El Dragon mine.