(010) DISORDER, PARTIAL SI,AL ORDERING, AND CA DISTRIBUTION IN TRICLINIC (C1) EPISTILBITE

The crystal structure of epistilbite, Na0.3K0.2Ca2.7Al6.2Si17.9O48. 16 H2O, from Gibelsbach, Fiesch (Valais, Switzerland) was refined from X -ray single-crystal data at 100 K and 293 K in space group C1 (100 K: a = 9.051(3), b = 17.689(6), c = 10.152(7) Angstrom, alpha = 89.93(4), beta = 124.35(4), gamma = 89.95(3)degrees; 293 K: a = 9.083(1), b = 1 7.738(3), c = 10.209(1) Angstrom, alpha = 89.95(1), beta = 124.58(1), gamma = 90.00(1)degrees). In contrast to previous studies where the sp ace groups C2 or C2/m were assumed, reflection intensities clearly sug gest triclinic symmetry. The reason for the symmetry lowering from mon oclinic C2 to triclinic C1 is partial Si,Al ordering accompanied by a preferred distribution of the channel cations close to the underbonded oxygen atoms of the tetrahedral framework. The hitherto enigmatic occ urrence of two domain variants (A, B) formed by different tetrahedral configurations could be modelled by (010) disorder caused by a local m irror plane.