URANOPOLYCRASE, THE URANIUM-DOMINANT ANALOG OF POLYCRASE-(Y), A NEW MINERAL FROM ELBA ISLAND, ITALY, AND ITS CRYSTAL-STRUCTURE

Uranopolycrase, with chemical formula 03)(Sigma=1.04)(Ti1.46Nb0.36Ta0. 12)(Sigma=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, El ba Island, Tuscany, Italy. It occurs as tabular {100} crystals, elonga ted in the [001] direction. The mineral is opaque, with brown-red colo r and adamantine luster, mean VHN20 = 659. In reflected light, color i s pale grey with bluish tones; pleochroism, anisotropism and bireflect ance were not observed. Reflectance values, measured in air, range fro m 21.5% to 25.1%. Uranopolycrase is orthorhombic, space group Pbcn, wi th a = 14.51(1) Angstrom, b = 5.558(5), c = 5.173(4). The strongest li nes in the X-ray powder pattern are: 2.99 Angstrom, 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 uran opolycrase with other natural and synthetic compounds with the same st ructural topology are briefly discussed.