NEW INSIGHT INTO THE EXTRA-FRAMEWORK CONTENT OF ZEOLITE LEVYNE

Levyne is a rare zeolite [ideal chemical formula (Ca0.5,Na,K)6(Al6Si12O36)•18H2O] discovered almost two centuries ago in the Fær Øer Islands by Brewster [1]. It is usually found as secondary mineral into cavities of andesitic and basaltic rocks. Merlino et al. solved its crystal structure in the space group R-3m from an untwined crystal [2]. Later, Sacerdoti reported new refinements of the crystal structure of two different levynes, Na and Ca-rich respectively [3]. In all the aforementioned studies, all the extra framework cations (except the site C1 site), were described with isotropic thermal parameters [2,3]. The crystal chemistry of a large number of natural levynes was studied by Galli et al [4]. The framework of this zeolite can be described as a sequence of six-membered rings (AABCCABBC…), which build up the so-called “levyne-cage” and is characterized by the presence of three equivalent channel systems perpendicular to the threefold axis, confined by eight-membered rings. The six-membered single ring represents the secondary building unit (SBU) of this framework type. The topological symmetry is R-3m and corresponds to the general symmetry of the crystal structure. In the present work we give new insights into the description of the extra framework population of two levynes, Ca-rich and Na-rich respectively, based on anisotropic structure refinement from single-crystal X-ray diffraction, wave-length dispersive electron microprobe analysis and laser-ablation ICP-MAS spectroscopy. The chemical composition of our crystals slightly differs from those of Sacerdoti [3]. In order to describe the thermal behaviour of the extra framework content, we collected intensity data for both the levynes at 298 and 150K. The extra-framework content, represented by Ca, Na, K and water molecules, lies in the levyne-cages. At least five independent cation sites (C1, C2, C3, C4, C5, [2,3]) have been located. C1 is fully [2] or almost fully [3] occupied by Ca only. The C2, C3 and C4 sites are occupied by a mixture of Ca, Na and K. The C5 site is considered to be occupied by Ca, Na and K [2] or by K only [3]. Compared to previous studies, we observe different occupancies for the extra-framework sites that are in good agreement with the chemical analysis of the two samples. The high quality of the structural refinements allow a reliable description of the bonding environments based on a very good determination of bond-distances and bond-angles. Moreover, the low temperature data show that the thermo-elastic deformation of the structure is mainly accommodate by a re-arrangement of the extra-framework population. The isotopes analysis performed by laser-ablation ICP-MAS show a high selection of trace elements with 71Ga, 85Rb, 88Sr, 137Ba and 24Mg above the detection limit. Among the major isotopes detected, levyne behaves as a “geochemical trap” for 88Sr displaying a selective character for Sr, with a content of 299 and 1497 ppm for Ca and Na-rich levyne respectively (ratio of about 1:5). --------------------------- References. [1] M. Brewster, Edinb. J. Sci., 2, 332-334, 1825; [2] S. Merlino, E. Galli, A. Alberti, Tscher. Miner. Petrog., 22, 117-129, 1975; [3] M. Sacerdoti, Neues Jb. Miner. Monat., 3, 114-124, 1996; [4] E. Galli, R. Rinaldi, C. Modena, Zeolites, 1, 157-160, 198