PASAVAITE, Pd_3Pb_2Te_2, A NEW PLATINUM-GROUP MINERAL SPECIES FROM THE NORIL'SK-TALNAKH Ni-Cu CAMP, RUSSIA

摘要

Pasavaite, Pa_2Pb_2Te_2, is a new palladium-bearing mineral species discovered at the Talnakh deposit, Noril'sk-Talnakh Ni-Cu camp, Taimyr Autonomous District, Russia. It was observed in two polished sections, where it occurs as subhedral grains not exceeding 20 mu m in diameter embedded in polarite, and accompanied by unknown Pd-Pb-Bi-Te phases and sperrylite or intergrown with Au-Ag phases. The other associated minerals are: taimyrite, froodite, galena, and the chalcopyrite + pentlandite matrix. Synthetic pasavaite is megascopically grey in color, has a metallic luster and a grey streak. The mineral is brittle, with a weak {001} cleavage. Values of VHN_(25) (15 indentations on five grains of synthetic Pd_3Pb_2Te_2) range between 173 and 281, for a mean value of 233 kg/mm~2, which corresponds to a Mohs hardness of approximately 2. In plane-polarized light, pasavaite is pale pink with brownish tinge, has strong bireflectance, pleochroic from brownish to light pink, distinctly to strongly anisotropic, and exhibits no internal reflections. Reflectance values of synthetic pasavaite in air (R_(max), R_(min) are: 49.9, 42.4 at 470 nm, 51.8, 44.6 at 546 nm, 52.2, 45.7 at 589 nm, and 52.8, 46.9 at 650 nm. The average result of four electron-microprobe analyses of pasavaite is: Pd 31.51, Pb 41.54, Bi 0.19, and Te 25.75, total 98.99 wt. percent , corresponding to an empirical formula (based on 7 atoms) Pd_(2.96)(Pb_(2.01)Bi_(0.01))Te_(2.02); synthetic pasavaite (average of seven analyses) gave: Pd 32.17, Pb 41.78, and Te 25.93, total 99.88 wt. percent , corresponding to Pd2.99Pb2.00Te2.01- The mineral is orthorhombic, space group Pmmn, with a 8.599(1), b 5.9381(6), c 6.3173(8) A, V 322.6(1) A~3, and Z = 2. The crystal structure was solved and refined from the powder X-ray-diffraction data of synthetic Pd_3Pb_2Te_2. Pasavaite is structurally related to shandite and parkerite. The strongest lines in the X-ray powder-diffraction pattern of synthetic pasavaite [d in A(I)(M/)] are: 6.3152(34)(001), 3.1572(33)(002), 3.0495(100)(211), 2.5456(63) (202), 2.4424(34)(220), 2.2786(42)(221), 2.1637(71)(022), 2.1496(30)(400), 1.8906(42)(203), and 1.5248(31)(422). The mineral is named after Jan Pasava, in recognition of his important contributions to the geochemistry of platinum-group elements in anoxic environments and related ore deposits.