CRYSTAL-STRUCTURE AND MICROTWINNING OF K-RICH NENADKEVICHITE

Potassium-rich nenadkevichite, 5{(Nb0.6Ti0.4)(Nb0.5Ti0.5)[Si4O12](O,OH )2}.1.6H2O, M = 582.9, monoclinic, space group Cm, a = 14.692(4), b = 14.164(4), c = 7.859(3) angstrom, beta = 117.87(2)-degrees, V = 1445.7 (1) angstrom3, Z = 4, rho(c) = 2.67 g/cm3, lambda(MoKalpha) = 0.71073 angstrom, mu = 22.16 cm-1, dimensions (mm) 0.2 x 0.3 x 0.4, F(000) = 1 120.32, room temperature, R = 0.055 for 1443 reflections with Absolute value of F > 3sigma(F), twinned crystal, the twinning index n = 2. Th e crystal structure, like the other members of the labuntsovite nenadk evichite group, contains columns formed by (Nb,Ti)-octahedra with shar ed trans-vertices. The octahedral columns are linked by tetrahedral fo ur-membered rings [Si4O12]. The statistically distributed K, Na-ions a nd water molecules are located in the channels of the mixed framework.