Structure refinement of natural non-metamict polycrase-(Y) from Zomba-Malosa complex, Malawi

Polycrase-(Y)from the Zomba-Malosa complex, Malawi, is the first recorded e xample of natural non-metamict crystals of the euxenite-polycrase series su itable for structure refinement. It forms aggregates of needle-shaped almos t colourless 0.25 x 0.01 x 0.01 mm crystals. They are optical negative with beta parallel to the needle axis, 2V alpha = 31(4)degrees, n(gamma) = 2.27 (3), n(beta) = 2.26(3) and n(alpha) = 2.22(3). Microprobe analysis gives th e empirical formula: (Y0.87Dy0.05Er0.04Yb0.02Pb0.01Th0.01)(Sigma = 1.00)(Ti1.20Nb0.79Ta0.02)(Sig ma=2.01) (O-5.82(OH)(0.18))(Sigma=6) X-ray powder pattern is presented. Crystal structure was solved by direct m ethods and refined to R(F) = 6.85% based on single-crystal data. The refine d unit cell parameters are a = 14.6673(12) Angstrom, b = 5.5925(4) Angstrom and c = 5.1888(4) Angstrom, Z = 4; space group Pbcn. Polycrase-(Y) is isos tructural with a large group of AB(2)O(6) compounds, as to structural detai ls closely related to uranopolycrase (AURISICCHIO et al. 1993). The structu re is disordered; the most prominent extra peaks are located 0.28 Angstrom away from Y and 0.59 Angstrom from Ti. The disorder is interpreted as the v ery first sign of a breaking metamictisation.