SELENOJALPAITE, Ag3CuSe2, A NEW MINERAL SPECIES FROM THE SKRIKERUM Cu Ag Tl SELENIDE DEPOSIT, SMALAND, SOUTHEASTERN SWEDEN

Abstract

Selenojalpaite, ideally Ag3CuSe2, is a very rare mineral newly identifi ed at the Skrikerum Cu–Ag–Tl selenide deposit, near Valdermarsvik, Smaland, southeastern Sweden. It occurs as dark grey anhedral to subhedral grains up to 200 m across, closely associated with eucairite, altaite, berzelianite and chalcopyrite. Selenojalpaite is opaque with a metallic luster; it possesses a black streak. It is brittle with an uneven fracture; the Vickers microhardness (VHN25) is 37 kg/mm 2 (range 36–40), equivalent to a Mohs hardness of 4–4½. The calculated density is 7.64 g/cm 3 (on the basis of the empirical formula). In plane-polarized refl ected light, selenojalpaite is light grey, weakly to moderately birefl ectant, and weakly pleochroic from brownish grey to a slightly darker greenish grey. Between crossed polars, it is anisotropic, without characteristic rotation-tints or internal refl ections. Values of refl ectance in the range 400–700 nm are tabulated; values of Rmin and Rmax for the four standard COM wavelengths are 33.5, 37.1 (471.1 nm), 31.8, 35.1 (548.3 nm), 30.4, 34.0 (586.6 nm), and 29.3, 32.4 (652.3 nm), respectively. Its unit-cell parameters were derived by comparison with jalpaite, Ag3CuS2, which shows strong chemical and crystallographic similarities. Selenojalpaite is tetragonal, probable space-group I41/amd, with unit-cell parameters refi ned from powder data: a 8.939(1), c 11.844(2) A, V 946.4(2) A 3 , c : a 1.3250, Z = 8. The strongest seven powder-diffraction lines [d in A(I)(hkl)] are: 4.47(60)(200),