Nipalarsite, Ni8Pd3As4, a new platinum-group mineral from the Monchetundra Intrusion, Kola Peninsula, Russia

Nipalarsite, Ni8Pd3As4, is a new platinum-group mineral discovered in the sulfide-bearing orthopyroxenite of the Monchetundra layered intrusion, Kola Peninsula, Russia (67 degrees 52'22 '' N, 32 degrees 47'60 '' E). Nipalarsite forms anhedral grains (5-80 mu m in size) in intergrowths with sperrylite, kotulskite, hollingworthite, isomertieite, menshikovite, palarstanide, nielsenite and monchetundtraite enclosed in pentlandite, anthophyllite, actinolite and chlorite. Nipalarsite is brittle, has a metallic lustre and a grey streak. In plane-polarised light, nipalarsite is light grey with a blue tinge. Reflectance values in air (in %) are: 46.06 at 470 nm, 48.74 at 546 nm, 50.64 at 589 nm and 54.12 at 650 nm. Values of VHN20 fall between 400.5 and 449.2 kg.mm(-2), with a mean value of 429.9 kg.mm(-2), corresponding to a Mohs hardness of similar to 4. The average result of 27 electron microprobe wavelength dispersive spectroscopy analyses of nipalarsite is (wt.%): Ni 44.011, Pd 28.74, Fe0.32, Cu 0.85, Pt 0.01, Au 0.05, As 25.42, Sb 0.05, Te 0.39, total 99.85. The empirical formula (normalised to 15 atoms per formula unit) is: (Ni8.10Fe0.06)(Sigma 8.16)(Pd2.94Cu0.18)(Sigma 3.12)(As3.68Te0.03)(Sigma 3.71) or, ideally, Ni8Pd3As4. Nipalarsite is cubic, space group Fm (3) over barm, with a = 11.4428(9) angstrom, V= 1498.3(4) angstrom(3) and Z= 8. The strongest lines in the powder X-ray diffraction pattern of synthetic Ni8Pd3As4 [d, angstrom (I) (hkl)1 are: 2.859(10)(004), 2.623(6)(313), 2.557(6)(024), 2.334(11)(224), 2.201(35)(115,333), 2.021(100)(044), 1.906(8)(006,244) and L429(7)(008). The crystal structure was solved and refined from the single-crystal X-ray diffraction data of synthetic Ni8Pd3As4. The relation between natural and synthetic nipalarsite is illustrated by an electron back-scattered diffraction study of natural nipalarsite. The density calculated on the basis of the empirical formula of nipalarsite is 9.60 g.cm(-3). The mineral name corresponds to the three main elements: Ni, Pd and As.