REFINEMENT OF THE CRYSTAL-STRUCTURE OF TADZHIKITE

The crystal structure of tadzhikite, monoclinic, a 19.058(4), b 4.729( 1), c 10.321(2) Angstrom, beta 111.39(1)degrees, V 866.1(3) Angstrom(3 ), Z = 2, space group P2/a, has been refined to an R index of 4.9% for 1702 observed reflections measured with MoK alpha X-radiation. The cr ystal used to collect the X-ray-diffraction data was subsequently anal yzed with an electron microprobe. Tadzhikite is isostructural with hel landite, but shows: strong positional disorder of the REE cations that is presumably related to partial metamictization. Bond-valence analys is shows that the structure contains essential H as OH, and site popul ations were assigned on the basis of the refined site-scattering value s and the results of electron-microprobe analysis. The general formula of tadzhikite can be written as 4+,Fe3+)(R3+,square)(2)[B4Si4O16(O,OH )(6)](OH)(2).