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On the existence of a Na-deficient monoclinic trinepheline with composition Na_7.85Al_7.85Si_8.15O₃₂

Predrag Vuli^*, Volker Kahlenberg and Jürgen Konzett

Institute for Mineralogy and Petrography, University of Innsbruck, Innrain 52, A-6020 Innsbruck, Austria

Correspondence: ^* E-mail: Predrag.Vulic{at}uibk.ac.at

The crystal structure of a new synthetic Na_8–rAl_8–rSi_8+rO₃₂ (r = 0.15) modification has been solved by direct methods and refined to an R index of 0.0289 for 11 643 independent reflections and 764 parameters. The compound is monoclinic with space group symmetry P112₁, a = 9.9897(6) Å, b = 9.9622(6) Å, c = 24.979(2) Å, and = 119.788(4)°. The twinning of three individuals related by rotation around the c direction for 120° was accounted for in the calculation. The phase was named monoclinic trinepheline because the length of its c lattice parameter is 3x the length of the c parameter in nepheline, whereas the a parameter is almost the same in both structures, similarly to a previously investigated trinepheline of hexagonal symmetry. Actually, the present compound represents an intermediate state between these two phases. The characteristic of the crystal structure are layers of six-membered rings built up of regularly alternating AlO₄ and SiO₄ tetrahedra. The layers are perpendicular to [001] and are connected through apical O atoms. The resulting spatial network contains channels in which the Na cations are situated. The structure of monoclinic trinepheline can be described as a consecutive stacking of basic nepheline like modules (containing two tetrahedral sheets) and one sheet corresponding to those observed in the hexagonal trinepheline structure. The topology of the new compound corresponds to that of tridymite. The coordination sequences are identical for all T atoms in the asymmetric unit: 4-12-25-44-67-96-130-170-214-264. The vertex symbols for the tetrahedral centers are 6₂·6₂·6₂·6₂·6₂·6₂. The framework density of the structure is equal to 22.2 T atoms/1000 ų.

Key Words: Monoclinic trinepheline • Na_7.85Al_7.85Si_8.15O₃₂ • flux method • nepheline