TISCHENDORFITE, Pd8Hg3Se9, A NEW MINERAL SPECIES FROM TILKERODE, HARZ MOUNTAINS, GERMANY

Abstract

Tischendorfite, ideally Pd8Hg3Se9, is a new mineral species from the Eskaborner Stollen (Eskeborn Adit) at Tilkerode, Harz Mountains, Germany. It occurs as aggregates in a carbonate matrix, together with the associated metallic minerals clausthalite, tiemannite, chrisstanleyite, stibiopalladinite and gold. It is opaque with a metallic luster, has a black streak, is brittle, and the fracture is uneven. In plane-polarized reflected light, tischendorfite has a cream or slightly beige color in association with chrisstanleyite (light buff to slightly gray-green buff), clausthalite (off-white) and tiemannite (gray). It is not pleochroic, and displays a weak bireflectance. It does not possess internal reflections. The anisotropy is weak to moderate, with rotation tints in weak shades of steel blue and greenish brown. Reflectance data and color values are tabulated. Average results of four electron-microprobe analyses are: Pd 39.4, Pt 0.1, Ag 1.0, Cu 0.2, Hg 24.9, Pb 1.4, Se 32.0, total 99.0 wt.%. The empirical formula, (Pd8.05Pt0.01)∑8.06(Hg2.70Ag0.20Pb0.15Cu0.07) ∑3.12Se8.82, is based on 20 atoms per formula unit. The calculated density is 9.13 g/cm3 (on the basis of the empirical formula and unit-cell parameters refined from powder data). Tischendorfite is orthorhombic, with a 7.219(3), b 16.782(7), c 6.467(5) A, V 783.6(8) A3, a:b:c 0.4302:1:0.3854, and Z = 2. The probable space-groups are Pmmn (59), P 21 mn (31) or Pm 21 n (31) (diffraction aspect P ** n ). The strongest seven X-ray powder-diffraction lines [ d in A( I )( hkl )] are: 4.819(40)(101), 4.373(40)(130), 2.797(60)(032,122), 2.743(100)(151,231,240), 2.325(40)(052), 2.116(40)(062) and 2.091(100)(261). The mineral’s name honors Gerhard Tischendorf, geochemist and mineralogist, in recognition of his contributions to understanding the genesis of selenide mineral deposits.