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MIESSIITE, Pd₁₁Te₂Se₂, A NEW MINERAL SPECIES FROM MIESSIJOKI, FINNISH LAPLAND, FINLAND

Kari K. Kojonen^1,, Mahmud Tarkian², Andrew C. Roberts³, Ragnar Törnroos⁴ and Stefanie Heidrich⁵

¹ Geological Survey of Finland, P.O. Box 96, Espoo FIN–02150, Finland
² Insitute of Mineralogy and Petrology, University of Hamburg, D–20146 Hamburg, Germany
³ Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada
⁴ Department of Geology and Mineralogy, P.O. Box 64, University of Helsinki, FIN–00014 Helsinki, Finland
⁵ Institute of Mineralogy and Petrology, University of Hamburg, D–20146 Hamburg, Germany

E-mail address:: kari.kojonen{at}gsf.fi

Miessiite, a new mineral species of ideal composition Pd₁₁Te₂Se₂, is cubic, space group Fd3m (#227), a 12.448(2) Å, V 1929.0 (4) ų, Z = 8. The strongest six lines of the X-ray powder-diffraction pattern [d in Å(I)(hkl)] are: 2.395(80)(511,333), 2.197(100)(440), 1.875(25)(622), 1.555(25)(800), 1.305(25)(931) and 1.271(30)(844). The mineral was discovered in a platinum-group-mineral (PGM) placer sample taken from the Miessijoki river, Lemmenjoki area, Inari Commune, Finnish Lapland, Finland. Associated PGM and other heavy minerals include stillwaterite, isomertieite, mertieite-II, cooperite, braggite, kotulskite, vincentite, tantalite, thorianite, pyrite, magnetite, chromite, and isoferroplatinum with inclusions of laurite and Os–Ir–Ru alloy. Miessiite is black with a metallic luster. The fragments are malleable and silvery grey in color. The average micro-indentation hardness VHN₁₀₀ is 362 (n = 4) and corresponds to a Mohs hardness of 2–2. Under reflected plane-polarized light, the mineral is isotropic with a light grey color in comparison with the associated PGM grains. Measured values of reflectance are tabulated, obtained in air and in oil for a single grain. The reflectance values in air for the standard COM wavelengths are: 48.88 (470 nm), 51.63 (546 nm), 53.91 (589 nm) and 56.82% (650 nm). D (calc) is 10.94 g/cm³ (for the empirical formula and unit-cell parameter refined from powder data). The electron-microprobe analyses yielded (wt.%) Pd 75.17, Se 9.61, Te 17.06, total of 101.84 wt.%, which corresponds to Pd_11.02 Te_2.09 Se_1.90, based 15 atoms. The mineral is named after the locality, the Miessi river (in Saami language, Miessijohka).

Keywords: miessiite, new mineral species, palladium telluride selenide, X-ray data, electron-microprobe data, reflectance data, Miessijoki River, Lemmenjoki area, Finnish Lapland, Finland.