Zaykovite, Rh3Se4, a new mineral from the Kazan placer, South Urals, Russia

Zaykovite, ideally Rh3Se4, is a new mineral, the first natural rhodium selenide. It was discovered in the assemblages of platinum-group minerals from the Kazan gold placer, South Urals, Russia. The mineral occurs as crystals up to 40 mu m in size within the grains of Pt3Fe alloy, in association with unnamed Pd-Sb-Te phase and Au-Pd alloy. In reflected light, zaykovite has a grey colour with bluish-greenish tint; it shows weak bireflectance and anisotropy. Reflectance values [R-max/R-min (%) for COM approved wavelengths (nm)] are: 30.1/29.3(470), 32.2/31.0(546), 33.4/32.0(589) and 35.1/33.7(650). The chemical composition corresponds to the empirical formula (Rh2.26Pt0.46Ir0.25Ru0.01Pd0.01Fe0.01)(sigma 3.00)(Se2.77S1.21Te0.02)(sigma 4.00) Zaykovite is monoclinic, space group C2/m, a = 10.877(1), b = 11.192(1), c = 6.4796(6) & ANGS;, beta = 108.887(2)& DEG;, V = 746.3(1) & ANGS;(3), Z = 6 and D-calc = 8.32 g cm(-1). The crystal structure has been solved and refined to R-1 = 0.016 based on 858 unique observed reflections. The strongest lines of the powder X-ray diffraction pattern [d(& ANGS;), (I), (hkl)] are: 5.43(37)($\bar{1}$11), 3.275(75)(310), 3.199(100)($\bar{1}$31), 3.061(87)(002), 2.568(62)(400), 2.545(41)(041), 3.413(34)($\bar{2}$41) and 1.697(34)(441). Zaykovite is a Se analogue of kingstonite, Rh3S4. A continuous series of solid solutions between kingstonite and zaykovite was encountered in the samples from the Kazan placer. The possible sources of this unique Rh-Se mineralisation in the South Urals could be serpentinised dunite-harzburgite or gabbro-clinopyroxenite-dunite complexes in the vicinity.