Buckhornite, AuPb 2 BiTe 2 S 3 , a new mineral species from Boulder County, Colorado, and new data for aikinite, tetradymite and calaverite

Abstract

Buckhomite, ideally AuPb2BiTe2S3, is a new mineral from the long abandoned Buckhom mine (408N, 105'24n[, near Jamestown, Boulder County, Colorado. Occuning in vugs in pyrite as splendent black blades up to 1.5 x 0.2 x 0.01 mm, it is associated with aikinite, tetradymite, calaverite, gold and minor chalcopyrite and covellite. It is sectile and flexible, has a perfect cleavage, a gray streak and a VHN19 of 54. D01 8.43 (for the empirical formula) or 8.34 g/cml (for the idealized formula). Electron-microprobe analyses give Cu 0.1, Au 17.0, Pb 34.5, Bi | 8.4, Te 22.3, S 7 .8, sum 100.1 wtVo. Tlte empirical formula of buckhomite, calculated on the basis of 9 atoms in the formula unit, is Cu6.s2Au1.62Pb1.e7Bi1.e4Te2.66S2.69. In reflected light, buckhomite is opaque, bireflectant, pleochroic (brownish gray -pale bluish gray) and distinctly anisotropic (blue to mauve rotation tints). Color values relative to CIE illuminant C, for R7 and R2, respectively, are: .r 0.3 I l, y 0.318, YVo 40.95,?va57l, PVo 0.7 , and .r 0.300, y 0.308, YVa ! .4, lva 48O, PVo 4.4. The symmetry is orthorhombic, space group Pmmm or P222, with a 4.092(2), b 12.245(4), c 9.322(4) A, V 467 .l Ar . The selection of a nonstandard orientation for the cell emphasizes the observed relationship to a tetragonal subcell. The strongest seven reflections of the X-ray powder-diffraction pattem [d in A (f (hkD] er.e