STRUCTURAL VARIATIONS IN THE ZEOLITE PAULINGITE. A. Bieniok, W. Joswig and W. H. Baur, Univ. Frankfurt, Institut f. Kristallographie, Senckenberganlage 30, 60054 Frankfurt, Germany.

The Ca- and Ba-rich forms of paulingite were studied by X-ray single crystal methods. The refinements¹ converged to wR 0.085 and 0.077. This is the first structural information on chemically different paulingite crystals since introducing this natural zeolite and its complicated aluminosilicate crystal structure in 1966². Paulingite is a medium pore size zeolite constructed from 7 different cages (opr, grc, pau, plg, cpa, oto, gsm)³. The access to all cages is controlled by plane or deformed single eight rings (S8R). Preferred cation sites are the saddle shaped S8R openings from the open double eight rings (oto) to the pau cages (K1) and the compressed pau cages (cpa) (Ba1, K2). Two strong bonds to framework oxygen atoms and four to water molecules are possible at this site. The plg cages are stuffed by Ca cations coordinated by eight water molecules. An additional cation position occupied by K is located in the big channel of the structure in the S8R connecting the great rhombicuboctahedron (grc) with a double eight ring (opr). All other cages are stuffed with highly disordered and weakly bonded water molecules. The Si and Al atoms (ratio 2.9) are statistically distributed over 8 different T sites. The individual T-O distances of 1.64 Å (+/-0.01 Å) and (+/-0.02 Å), and the T-O-T angles of 146deg. (+/-3deg.) and 145deg. (+/-3deg.) respectively, show that the different cation populations in the pores do not affect the geometry of the framework of paulingite appreciably. The narrow distribution of the T-O-T angles shows that the framework is relaxed compared with the wide range of T-O-T angles observed in zeolites at large (116deg. to 180deg.)⁴.

(1) S.R. Hall, J.M. Stewart: Xtal3.0 (1990) Univ. Western Australia and Maryland. (2) E.K. Gordon, S. Samson, W.B. Kamb: Science 154 (1966) 1004. (3) J.V. Smith: Studies Surf. Sci. & Cat., 49A (1989) 29. (4) W.H. Baur, Proc. 2nd Polish-German Zeol. Coll. (1995) 171.