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Crystal Structures and Crystal Chemistry |
1 Department of Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7, Canada
2 Geological Institute, Kola Science Center, Russian Academy of Sciences, 14 Fersman Street, 184200 Apatity, Russia
3 United Institute of Geology, Geophysics and Mineralogy, Siberian Branch of the Russian Academy of Sciences, 3 Koptyug Avenue, 630090 Novosibirsk, Russia
E-mail address: barkov{at}eps.mcgill.ca
Menshikovite, ideally Pd3Ni2As3, is a new mineral species found in two localities in Russia. It occurs in a pod of coarse-grained, entirely altered gabbronorite rich in sulfides (up to 30 vol.%), hosted by a microgabbronorite, in the Vostok deposit, Lukkulaisvaara intrusion, in Karelia. There, menshikovite (up to 0.2 mm) is associated with chalcopyrite, pentlandite, calcic amphibole (mostly actinolite), clinochlore, merenskyite, kotulskite, sobolevskite, michenerite, hollingworthite, hessite, and a Re-rich sulfide. Menshikovite (up to 0.1 mm) also occurs in mineralized quartz–feldspar sandstones, which are metasomatically altered and recrystallized. They are located at the lower (exo)-contact of the Chiney intrusion in Siberia, and contain amphibole(s), mica(s), chlorite, disseminated or massive sulfides (mostly chalcopyrite), Pd-rich maucherite, nickeline, sperrylite, mertieite-II or stibiopalladinite (or both), isomertieite, majakite, and michenerite. We also report a third occurrence of menshikovite from the Oktyabr’ deposit, Noril’sk complex, in Siberia. In reflected light, menshikovite is pink and opaque. A cleavage is not observed. There is no evidence of bireflectance. The anisotropy is weak, from light gray to brownish gray. Reflectance percentages in air (and in oil) are, for R1 and R2, at 470 nm 48.4, 50.2 (38.45, 39.3), at 546 nm 51.2, 53.2 (41.0, 41.8), at 589 nm 53.2, 55.3 (42.3, 43.3), and at 650 nm 56.6, 58.7 (46.6, 47.8). Fine twins are present. The average value of microhardness VHN40, 50, 65 is 517.1 kg/mm2; it has Mohs hardness of ~5 and is brittle. The average of two sets of multiple wavelength-dispersion electron-microprobe analyses of menshikovite from Lukkulaisvaara gave Pd 48.18, Pt 0.07, Ni 17.77, Fe 0.07, As 33.91, the total is 100.0 wt.% (not normalized), corresponding to Pd2.99(Ni2.00Fe0.01)
2.01As2.99 (basis: atoms = 8). Compositions of menshikovite from Chiney vary from Pd3.00(Ni1.98Co0.01)1.99As2.99 to Pd3.15(Ni1.97Co0.01)1.98As2.86. A solid-solution series probably exists between menshikovite and majakite [i.e., (Pd,Ni)2As of the ß-Pd2As type]. The powder pattern of menshikovite from Lukkulaisvaara was indexed on a hexagonal cell with a 8.406(4), c 6.740(4) Å, and V 412.4(6) Å3. The strongest eight lines in this pattern [d in Å (I)(hkl)] are 2.626(10)(112), 2.477(10)(202), 2.429(8)(300), 2.283(7)(301), 2.150(6)(103), 1.978(7)(113), 1.818(7)(400) and 1.757(6)(401). Menshikovite formed in volatile-rich environments relatively late in the crystallization history of the layered complexes. The name honors Yurii P. Men’shikov, of the Kola Science Center, Apatity, Russia.
Keywords: menshikovite, new mineral species, Pd–Ni–As system, arsenide, platinum-group mineral, platinum-group elements, layered intrusion, mafic–ultramafic rocks, Lukkulaisvaara, Chiney, Noril’sk, Russia.
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