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

Abstract

The crystal structure of a natural Na-rich gobbinsite from Bundoora, a northern suburb of Melbourne, Victoria, Australia [(Na 4.97 K 0.07 Ca 0.48 ) Σ5.52 (Al 5.62 Si 10.29 ) Σ15.91 O 32 ·11.91H 2 O, Z = 1, a = 10.1035(15), b = 9.7819(10), c = 10.1523(9) Å, and V = 1003.37(20) Å 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 anisotropic 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 o > 4σ(F o ), and 114 refined parameters. The residuals in the final difference-Fourier maps are less than ±0.6 e -/Å 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 (W1, 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 H 2 O molecules), with at least five possible mutually exclusive configurations, whereas the coordination shell of the Ca site is represented by a distorted polyhedron with CN = 7 (5 framework O atoms + 2 H 2 O molecules), with at least two possible mutually exclusive configurations.

On the high-pressure behavior of gobbinsite, the natural counterpart of the synthetic zeolite Na–P2

“…The diffraction pattern was found to be metrically orthorhombic with lattice parameters: a = 10.0875(6) Å, b = 9.7718(5) Å, c = 10.1381(6) Å and V = 999.3(1) Å 3 . Systematic absences were compatible with the space group Pmnb, as reported by Gatta et al [6] (Table 1). No evidence of twinning was found.…”

“…A single crystal (220 Â 180 Â 80 lm 3 ) free of defects at the optical scale was selected from the same millimetric sample of natural gobbinsite from Bundoora (Melbourne, Australia) already studied by Gatta et al [6] and Gatta and Lotti [35] . Further details about the sample mineralogy and the chemical analyses protocol are given in Gatta et al [6].…”

“…Further details about the sample mineralogy and the chemical analyses protocol are given in Gatta et al [6].…”

“…Gobbinsite is a rare zeolite belonging to the gismondine group with ideal chemical formula between Na 5 (Al 5 Si 11 O 32 )Á11H 2 O and Na 4 Ca(Al 6 Si 10 O 32 )Á11H 2 O, Z = 1 [1,2] and references therein, which can be found as late hydrothermal product in basalt cavities often in close association with phillippsite [3][4][5][6][7]. The other natural zeolites belonging to the gismondine group [8] are amicite [9] and garronite [10].…”

“…Gobbinsite structure was first refined by Rietveld method [5,19] due to the paucity of single crystals. Later, the finding of a millimetric sample from Bundoora (Victoria, Australia) allowed Gatta et al [6] to refine the crystal structure from single-crystal X-ray diffraction data (space group: Pmnb with a $ 10.09, b $ 9.77, and c $ 10.14 Å). The refinement showed that Na (i.e.…”