Data di Pubblicazione:
2014
Citazione:
Effects of low temperature and high pressure on the structure of gobbinsite / G.D. Gatta, P. Lotti, N. Rotiroti. ((Intervento presentato al 9. convegno Zeolite 2014 - 9th International Conference on the Occurrence, Properties and Utilization of Natural Zeolites tenutosi a Belgrade, Serbia nel 2014.
Abstract:
Introduction
Gobbinsite is a rare zeolite, typically found in amygdaloidal vugs of massive volcanic rocks, where it crystallizes as a hydrothermal post-magmatic product. Usually, gobbinsite forms radiating aggregates of sub-microscopic elongated crystals, often associated with phillipsite, gmelinite, natrolite and calcite. It was first described from the type locality of Co. Antrim, N. Ireland, but other occurrences were later reported from a few other localities. Its ideal formula is Na5[Al5Si11O32]•12H2O, but evidence of Ca,K-enriched form has been reported, so that the general chemical formula can be re-written as: (Na2-2x,Cax)2K2[Al6Si10O32]•12H2O.
Gobbinsite has a GIS (gismondine) framework type, which can be described as a stacking of two perpendicular double crankshaft chains, the first running along [100], the second along [010]. Two secondary building units are present, 8- and 4-membered rings, with two perpendicular and interconnecting sets of channels: 8mR[100] and 8mR[010]. The ideal symmetry for the GIS framework type is tetragonal I41/amd (idealized unit cell constants: a = 9.8 Å, c = 10.2 Å), but zeolites with GIS topology often show a lower symmetry (e.g. gismondine; garronite; amicite; gobbinsite; Na-P1; Na-P2; TMA-gismondine). The lack of crystals large enough (and free of twinning and defects), forced early studies on the crystal structure of gobbinsite only by X-ray powder diffraction technique. Solution and refinement of the structure led to an orthorhombic unit cell (a = 10.108(1), b = 9.766(1) and c = 10.171(1) Å) and space group Pmn21. However, such a model showed unusually low coordination for the extra-framework cations (CN = 4 for K and 5 for Na). Later, findings of sub-millimetric crystals from Bundoora, Melbourne, Victoria, Australia, allowed us to re-investigate the gobbinsite crystal structure by X-ray single-crystal technique. The analysis of reflection conditions and statistics of distribution of normalized structure factors suggested the centrosymmetric space group Pmnb as highly likely, with unit cell a = 10.1035(15), b = 9.7819(10) and c = 10.1523(9) Å. The single-crystal structure refinement showed two extra-framework sites partially occupied by Na and Ca, respectively (though inter-sites chemical disorder could not be ruled out), five partially occupied sites for H2O molecules and a (Si,Al)-disordered distribution in the tetrahedral framework.
In addition to the re-investigation of the crystal structure of gobbinsite from Bundoora (Australia), we have investigated the low-temperature (LT) and the high-pressure (HP) behavior of this zeolite by means of in-situ single-crystal X-ray diffraction. The LT experiments have been performed in order to minimize the effects of atomic thermal libration and positional disorder. The presence of many centers of motion for a single site (positional disorder) results in large and unusual anisotropic displacement parameters. LT conditions are expected to reduce such effects, leading to a clearer picture of the extra-framework configuration. Moreover, comprehension of LT-behavior could provide more information about chemical disorder in extra-framework cationic sites. The HP experiments have been performed following a series of experiments on microporous materials aimed to describe the response of this open-framework materials under hydrostatic pressure of the order of GPa.
Experimental Methods
Chemical analysis, by means of electron microprobe in the wavelength dispersive mode and elemental CHN analysis, yelded to the following chemical formula:
(Na4.97K0.07Ca0.48)Σ5.52[Al5.62Si10.29]Σ15.91O32•11.9H2O (Z = 1) (E(%) = -6.3).
Two single-crystals, free of defects under polarized microscope, has been used for the LT and HP experiments, respec
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Elenco autori:
G.D. Gatta, P. Lotti, N. Rotiroti
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