Kvačekite, NiSbSe, a new selenide mineral from Bukov, Czech Republic

Kva & ccaron;ekite is a new mineral species discovered in a sample collected from the now abandoned Bukov uranium mine, western Moravia, Czech Republic. It occurs as rare anhedral grains, up to 15 mu m in size, associated with nickeltyrrellite, tyrrellite, berzelianite, hakite-(Zn), hakite-(Cd), eucairite, clausthalite, and gold in calcite gangue. In reflected light, kva & ccaron;ekite is white with a faint yellowish shade; bireflectance, pleochroism and anisotropy are absent. Internal reflections were not observed. Reflectance values for the four COM wavelengths for kva & ccaron;ekite in air [R (%) (lambda in nm)] are: 54.9 (470); 53.5 (546); 52.6 (589); and 52.2 (650). The empirical formula, based on electron-microprobe analyses (EPMA), is (Ni0.95Cu0.04Co0.03)Sigma 1.02Sb1.00(Se0.97S0.01)Sigma 0.98. The ideal formula is NiSbSe, which requires (in wt.%) Ni 22.63, Sb 46.93, Se 30.44, total 100.00. Kva & ccaron;ekite is cubic, P213, with unit-cell parameters a = 6.09013(13) & Aring;, V = 225.881(15) & Aring;3 and Z = 4. The strongest reflections in the X-ray powder diffraction pattern of synthetic kva & ccaron;ekite [d, & Aring; (I) hkl] are: 3.0458 (11) 200; 2.7242 (100) 201, 210; 2.4867 (71) 211; 1.8632(39) 311; 1.6277(29) 321, 312; and 1.3290 (13) 421. Given the similarity with ullmannite, NiSbS, the crystal structure was refined from the powder X-ray diffraction data starting from those atomic coordinates using the synthetic analogue of kva & ccaron;ekite. Its crystal structure is formed by corner-sharing [NiSb3Se3] octahedra which form a three-dimensional network. The identity of the natural kva & ccaron;ekite and synthetic cubic NiSbSe were confirmed by a study of their chemical composition, reflectance measurements, Raman spectroscopy and electron back-scattered diffraction (EBSD) measurements on the mineral. Kva & ccaron;ekite is named after Milan Kva & ccaron;ek (1930-1993), a prominent Czech mineralogist. The mineral and its name have been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA2023-095).