Reinvestigation of the crystal structure of the zeolite gobbinsite: A single-crystal X-ray diffraction study

The crystal structure of a natural Na-rich gobbinsite from Bundoora, a northern suburb of Melbourne, Victoria, Australia [(Na4.97K0.07Ca0.48)(Sigma 5.52)(Al5.62Si10.29)(Sigma 15.91)O-32 center dot 11.91H(2)O, Z = 1, a = 10.1035(15), b = 9.7819(10), c = 10.1523(9) angstrom, and V= 1003.37(20) angstrom(3), space group Pmnb (no. 62)], has been investigated by means of single-crystal X-ray diffraction, elemental CHN analysis, and electron microprobe analysis in the wavelength dispersive mode. For the first time, an an isotropic structural refinement has been performed on the basis of single-crystal X-ray diffraction data collected at 298 K, with: R-1(F) = 0.0608, 1362 unique reflections with F-0 > 4 sigma(F-0), and 114 refined parameters. The residuals in the final difference-Fourier maps are less than +/- 0.6 e(-)/angstrom(3). Previous structure solutions based on powder diffraction data were performed in the space group Pmn2(1), with a significantly different description of the channels' content. In this study, the Si/Al distribution in the tetrahedral framework is found to be highly disordered. A complex configuration of the extra-framework population occurs, with two possible and mutually exclusive sites for cations (Na and Ca) and five independent sites partially occupied by water molecules (W 1, W2, W2', W3, and W3'). The coordination shell of the Na site is represented by a distorted polyhedron with coordination number CN = 6 (4 framework O atoms + 2 H2O molecules), with at least five possible mutually exclusive configurations, whereas the coordination shell of the Ca site is represented by a distorted polyhedron CN = 7 (5 framework O atoms + 2 H2O molecules), with at least two possible mutually exclusive configurations.