Brodtkorbite, Cu2HgSe2, from Příbram, Czech Republic: crystal structure and description

The crystal structure and second world occurrence of the rare selenide brodtkorbite, ideally Cu₂HgSe₂, are reported in this paper. Brodtkorbite occurs in the Příbram uranium and base-metal ore district, central Bohemia (Czech Republic) as anhedral aggregates up to 150 μm across, in intergrowths with tiemannite and eucairite in a calcite gangue. Other associated minerals include berzelianite, umangite and uraninite. In reflected light, brodtkorbite is cream-white somewhat darker than the associated tiemannite and moderately pleochroic with light pink or bluish tints. Between crossed polarisers, the anisotropy is strong, with violet-grey to yellow-grey rotation tints. Reflectance values in air (R_min, R_max in %, λ nm) are 29.4–36.9 (470), 27.8–34.4 (546), 27.2–33.7 (589), 26.4–33.4 (650). Its average composition (14 electron microprobe analyses) is Ag 0.49, Hg 43.08, Cu 23.86, Se 32.25, Te 0.04, S 0.05, total 99.78 wt%. The resulting empirical formula, on the basis 5 atoms per formula unit (apfu), is (Cu_1.87Ag_0.02)_Σ1.89Hg_1.07(Se_2.03S_0.01)_Σ2.04. Brodtkorbite is monoclinic, a 7.5048(12), b 4.1851(9), c 8.0131(12) Å, β 124.430 (16)°, V 207.59(8) ų, space group P2₁/c, Z = 2. The crystal structure of brodtkorbite was solved by the direct method and refined to R = 8.05% on the basis of 416 unique reflections [I > 3σ(I)] collected on a Rigaku (Oxford Diffraction) SuperNova diffractometer with an Atlas S2 CCD detector and using MoKα radiation. The crystal structure contains one special position of Hg and two general positions of Cu and Se. The crystal structure consists of layers of edge-sharing [CuSe₄] distorted tetrahedra where Cu atoms show a 3+1 bond scheme. The [CuSe₄] tetrahedra form mackinawite-like layers parallel to (100). The layers show an AA-type of stacking and are connected by linear Se–Hg–Se bonds. Each Cu atom shows three short contacts with three adjacent Cu atoms forming a pseudohexagonal net of metal-metal interactions within one layer. The brodtkorbite crystal structure shows similarities to that of imiterite (Ag₂HgS₂), but the two minerals are not isostructural.