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

Abstract

Abstract The crystal structure of the new zeolite mineral gobbinsite, Ca0.6Na2.6K2.2Al6Si10O32 · 12H2O, has been determined in the space group Pmn21 with a = 10.108(1), b = 9.766(1) and c = 10.171(1) Å. The topology of the framework is the same as that of gismondine and can be described as a stacking of 2-dimensional arrays of double crankshaft chains. The structure was refined by the Rietveld technique using X-ray powder data, and converged with R F = 0.126 and R wp = 0.136 (statistically expected R e = 0.121). Fifty soft restrictions on the bond distances and angles of the aluminosilicate framework atoms were included in the least-squares refinement as additional (geometric) observations. This facilitated the location of the non-framework cations and water molecules, and improved the stability of the refinement. As in the closely-related synthetic zeolite Na – P2, the pseudo-tetragonal axis of the gismondine-type framework is parallel to one of the near-equivalent unit cell dimensions (c-axis), not to the unique one as expected. The arrangement of cations within the zeolitic channels suggests an explanation for this distortion of the ideally tetragonal framework.