Rietveld refinement of the crystal structure of the new zeolite mineral gobbinsite

Abstraet.The crystal structure of the new zeolite mineral gobbinsite, Cao.6Na2.6K2.2Al6Si¡o032 . 12H20, has been determined in the space groupPmn2¡ with a = 10.108(1), b = 9.766(1) and e = 10.171(1) Á. The topology of the framework is the same as that of gismondine and can be describedas a stacking of2-dimensional arrays of double crankshaft chains. The structure was refined by the Rietveld technique using X-ray powder data, and converged with RF = 0.126 and Rwp = 0.136 (statistically expectedRe = 0.121). Fifty soft restrictions on the bond distances and angles ofthe aluminosilicate framework atoms were included in the least-squares refinement as additional (geometric) observations. This facilitated the locationofthe non-framework cations and water molecules, and improved the stability of the refinement. As in the closely-related synthetic zeolite NaP2, the pseudo-tetragonal axis of the gismondine-type framework is parallelto one of the near-equivalent unit cell dimensions (c-axis), not to theunique one as expected. The arrangement of cations within the zeolitic channelssuggests an explanation for this distortion ofthe ideally tetragonal framework.