Uranopolycrase, the uranium-dominant analogue of polycrase-(Y), a new mineral from Elba Island, Italy, and its crystal structure

Abstract

Uranopolycrase, with chemical formula (U 0.62 Y 0.29 Th 0.07 Nd 0.01 Ca 0.02 Mn 0.03 ) Σ=1.04 (Ti 1.46 Nb 0.36 Ta 0.12 ) Σ=1.94 O 6 (ideally (U,Y)(Ti,Nb,Ta) 2 O 6 ), was found in the «Fonte del Prete» pegmatitic vein, near S. Piero in Campo village, Elba Island, Tuscany, Italy. It occurs as tabular {100} crystals, elongated in the [001] direction. The mineral is opaque, with brown-red color and adamantine luster, mean VHN 20 = 659. In reflected light, color is pale grey with bluish tones; pleochroism, anisotropism and bireflectance were not observed. Reflectance values, measured in air, range from 21.5% to 25.1%. Uranopolycrase is orthorhombic, space group Pbcn, with a = 14.51(1) A, b = 5.558(5), c = 5.173(4). The strongest lines in the X-ray powder pattern are: 2,99 A, 1.90, 1.48, 1.77, 2.78. The structure of uranopolycrase has been refined up to R = 0.047 with single-crystal X-ray diffraction data. The relationships of uranopolycrase with other natural and synthetic compounds with the same structural topology are briefly discussed