HIGH TEMPERATURE POWDER DIFFRACTION OF POLLUCITE UP TO 1073K. J. Schneider, H. L. Meyerheim, Institut fuer Kristallographie, Universitaet Muenchen, Theresienstrasse 41, D-80333 Muenchen, Germany

Framework silicates such as pollucite (CsAlSi2O6.nH2O), leucite (KAlSi2O6) or analcite (NaAlSi2O6.H2O) are characterized by four- and six-membered rings of corner linked (Si,Al)O4 tetrahedra. In the cubic polymorphs this leads to formation of non-intersecting channels parallel to {100} - and {111} -directions, housing Cs- and Na-cations in the case of pollucite.

Powder diffraction of a sample from Bikita, Simbabwe (K. Rank, Bergakademie, TU Freiberg) was performed on a STOE diffractometer (MoK[[daggerdbl]]1 radiation) and a STOE stove using a rotating sample capillary. Rietveld analysis yields a distinct contraction of cell dimensions in the range between 473K and 873K, which may be ascribed to irreversible loss of crystal water. The structure model of R. M. Beger (Z.Kristallogr.129(1968)280-302, space group Ia3d), which takes into account a significant amount of analcite, could be confirmed by refining the occupancies of the Cs,O(16b) and Na(24c) positions. Special emphasis was put onto the refinement of the temperature factors, which reach rather high values, i.e. at T=673K: B(Cs)=5A2, B(Na)=10A2. While B(Cs) shows a linear temperature dependence, B(Na) displays a change of slope at about 470K. Complementary single crystal measurements permitting anharmonic temperature factor analysis will be presented.