Search

The crystal structure of a natural Na-rich gobbinsite from Bundoora, a northern suburb of Melbourne, Victoria, Australia [(Na4.97K0.07Ca0.48)Σ5.52(Al5.62Si10.29)σ15.91O32·11.91 H2O, Z = 1, a = 10.1035(15), b = 9.7819(10), c = 10.1523(9) Å, and V = 1003.37(20) ų, 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: R1(F) = 0.0608, 1362 unique reflections with Fo > 4σ(Fo), and 114 refined parameters. The residuals in the final difference-Fourier maps are less than ±0.6 e-/Å3. Previous structure solutions based on powder diffraction data were performed in the space group Pmn21, 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 H2O 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 H2O molecules), with at least two possible mutually exclusive configurations. [ABSTRACT FROM AUTHOR]

The crystal-structure, crystal-chemistry, and low-temperature behavior of a natural phillipsite-Na from the "Newer Volcanic Suite," Richmond, Melbourne district, Victoria, Australia [K0.75(Na0.88Ca0.57)Σ1.45 (Al2.96Ti0.01Si5.07)Σ8.04O16·6.2H2O (Z = 2), a = 9.9238(6), b = 14.3145(5), c = 8.7416(5) Å, β = 124.920(9)°, and V = 1018.20(9) Å3, space group P21/m], have been investigated by means of in situ single-crystal X-ray diffraction, thermogravimetric analysis, and electron microprobe analysis in the wavelength dispersive mode. Two accurate structural refinements have been obtained on the basis of single-crystal X-ray diffraction data collected at 298 and 100 K, with: R1(F)298K = 0.035, 3678 unique reflections with Fo > 4σ(Fo) and 195 parameters, and R1(F)100K = 0.035, 3855 unique reflections, Fo > 4σ(Fo) and 195 parameters. In both refinements, the residuals in the final difference Fourier maps are <1 e-/Å3. A configuration of the extra-framework population different from that reported in previous studies is found at room temperature, with two possible sites for potassium (K1 and K2), one sodium/calcium site (Ca), and seven independent sites partially occupied by water molecules (W1, W2, W3, W4, W4', W5, and W6). The low-temperature refinement shows that the framework component of the phillipsite structure is maintained within the T-range investigated. However, a change in the configuration of the extra-framework content occurs at low temperature: the occupancy of site K2 drastically decreases, while that of site K1 increases, the Ca site is split into two sub-sites (Ca1 and Ca2) and the number of water molecule sites decreases to six (W1, W2, W3, W4, W5, and W6). The rearrangement of the extra-framework population at low temperature is likely due to the change in shape (and size) of the micropores by tetrahedral tilting. The evolution of the "free diameters" with temperature shows that an "inversion" of the ellipticity of the eight-membered ring channel along [010] occurs. The evolution of the unit-cell parameters with T (measured at 298, 250, 200, 150, and 100 K) shows a continuous and linear trend, without evident thermo-elastic anomalies. The axial and volume thermal expansion coefficients (aj = lj-1·∂lj/∂T, αV = V-1·∂V/∂T) between 100 and 298 K, calculated by weighted linear regression, yield the following values: αa = 1.8(1) × 10-5, αb = 1.2(1) × 10-5, αc = 1.1(1) × 10-5, and αV = 3.7(1) × 10-5K-1. The thermal expansion of phillipsite is significantly anisotropic (αa:αb:αc = 1.64:1.09:1). [ABSTRACT FROM AUTHOR]