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

Abstract

Miessiite, a new mineral species of ideal composition Pd11Te2Se2, is cubic, space group Fd 3 m (#227), a 12.448(2) A, V 1929.0 (4) A3, Z = 8. The strongest six lines of the X-ray powder-diffraction pattern [ d in A(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 2–2½. 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 Pd11.02 Te2.09 Se1.90, based 15 atoms. The mineral is named after the locality, the Miessi river (in Saami language, Miessijohka ).