ELDRAGONITE, Cu6BiSe4(Se2), A NEW MINERAL SPECIES FROM THE EL DRAGON MINE, POTOSI, BOLIVIA, AND ITS CRYSTAL STRUCTURE

Abstract

Eldragonite, with the simplified formula Cu 6 BiSe 4 (Se 2 ), is a new mineral species discovered in a telethermal vein-type deposit with selenides at the El Dragon mine, Province of Quijarro, Department of Potosi, Bolivia. It forms inclusions in krut’aite, and is associated with clausthalite, klockmannite, umangite and tiemannite, as well as with watkinsonite, petrovicite and two unnamed phases in the system Cu–Pb–Hg–Bi–Se. The unique vein of eldragonite-bearing krut’aite is hosted within sandstones and shales of Devonian age. Eldragonite occurs in anhedral grains and polycrystalline aggregates attaining a size of up to 100 × 80 μm. Megascopically, the mineral has a brownish to light-maroon color, is opaque and lacks internal reflections. It has a metallic luster and a brownish black streak, is brittle with an uneven to conchoidal fracture, without observable cleavage. The VHN 15 values range between 212 and 243 (mean 225) kg/mm 2 , corresponding to a Mohs hardness of ~3 ½. In plane-polarized light, eldragonite is distinctly bireflectant and pleochroic, from light grayish brown to cream; it is strongly anisotropic with rotation tints in shades of orange and blue-black. The reflectances (in air and oil, respectively) for the COM standard wavelengths are: 32.5–34.5, 17.7–19.7 (470 nm), 32.95–36.3, 18.0–21.4 (546 nm), 33.3–36.8, 18.3–21.6 (589 nm), 34.0–36.9, 19.1–21.7 (650 nm). Electron-microprobe analyses gave (mean of 24 analyses): Cu 35.9, Fe 1.25, Ni 0.35, Bi 20.3, Se 42.5, total 100.3 wt.%, corresponding to (Cu 5.98 Fe 0.24 Ni 0.06 ) ∑6.28 Bi 1.03 Se 5.70 . The ideal formula is Cu 6 BiSe 4 (Se 2 ), which requires Cu 35.84, Bi 19.64, Se 44.52 wt.%. Eldragonite has an orthorhombic cell, space group Pmcn , with a 4.0341(4), b 27.056(3), c 9.5559(9) A, V 1043.0(3) A 3 , and Z = 4. The calculated density is 6.76 g/cm 3 . The strongest X-ray powder-diffraction lines [ d in A ( I ) hkl ] are: 6.547(58)031, 3.579(100)052, 3.253(48)141, 3.180(77)081, 3.165(56)013, 3.075(84)102, 3.065(75)151,112, 2.011(53)200, 1.920(76)154, 1.846(52)1103. The crystal structure was solved from single-crystal data, and was refined to R 1 = 0.026 on the basis of 1731 unique reflections. There are one Bi and six Cu positions. Among the six Se positions, two Se atoms form a Se 2 pair [d(Se–Se) = 2.413 A]; eldragonite is thus a mixed selenide–diselenide compound. The crystal structure is organized according to two slabs alternating along a . The thin slab with formula Cu 6 Se 6 is a zigzag layer derived from the CaF 2 archetype; the thick slab, Cu 6 Bi 2 Se 6 , is similar to that of wittichenite, Cu 3 BiS 3 . The Se 2 pair is at the junction between these two slabs. This new mineral species is named after the location where it was discovered.