Petříčekite, CuSe2, a New Member of the Marcasite Group from the Předbořice Deposit, Central Bohemia Region, Czech Republic

Abstract

Petříčekite, ideally CuSe2, is a new mineral from the Předbořice deposit, Central Bohemia Region, Czech Republic. It occurs as rare inclusions, up to 150 μm across, in large eucairite grains closely associated with athabascaite/klockmannite and unknown selenide phases. Petříčekite is opaque with a metallic luster and shows a black streak. It is brittle; the Vickers hardness (VHN15) is 33 kg/mm2 (range: 28–40 kg/mm2) (Mohs hardness of ~2–2½). In reflected light, petříčekite is pale blue grey to pale pinkish, weakly pleochroic and weakly bireflectant from slightly blue-grey to slightly pinkish-grey. Under crossed polars, it is anisotropic with light grey-blue to light pink rotation tints. Internal reflections are absent. Reflectance percentages for the four COM (Commission on Ore Mineralogy) wavelengths (Rmin and Rmax) are 42.35, 41.8 (470 nm), 42.0, 42.2 (546 nm), 41.9, 42.35 (589 nm) and 42.05, 42.85 (650 nm), respectively. Petříčekite is orthorhombic, space group Pnnm, with a = 4.918(2) Å; b = 6.001(2) Å; c = 3.670(1) Å; V = 108.31(1) Å3; Z = 2. The crystal structure (R1 = 0.0336 for 159 reflections with I > 2σ(I)) belongs to the marcasite-type structure. It consists of edge-sharing chains of CuSe6 octahedra parallel to [001] linked by sharing Se2 dimers. The Se–Se bonds are all parallel to (001). The five strongest powder-diffraction lines (d in Å (I/I0) (hkl)) are: 2.938 (70) (101); 2.639 (100) (111); 2.563 (85) (120); 1.935 (70) (211); 1.834 (30) (002). The mean of nine electron-microprobe analyses on the crystal used for the structural study gave Ag 0.22(13), Cu 15.39(15), Hg 0.01(3), Pb 0.03(2), Fe 12.18(10), Pd 0.11(4), S 0.09(1), Se 71.61(29) and total 99.64(41) wt %, corresponding on the basis of a total of three atoms, to (Cu0.53Fe0.48)Σ1.01(Se1.98S0.01)Σ1.99. Additional crystals exhibiting higher Cu contents (up to 0.74 a.p.f.u.) were also investigated. The new mineral has been approved by the IMA-NMNC Commission (2015-111) and named after Václav Petříček, renowned crystallographer of the Institute of Physics of the Czech Academy of Sciences, Prague. Optical, compositional and structural properties confirm that nearly pure petříčekite also formed as late-stage mineral in the Se mineralization at El Dragón, Bolivia. It has end-member composition, Cu0.99Se2.00 (n = 5), and is typically associated with krut’aite of ideal composition, native selenium and goethite. Finally, optical and chemical data indicate that pure petříčekite is likely present also at Sierra de Cacheuta, Argentina.