(010) disorder, partial Si, Al ordering, and Ca distribution in triclinic (C1) epistilbite

A crystal of epistilbite, Na (sub 0.3) K (sub 0.2) Ca (sub 2.7) Al (sub 6.2) Si (sub 17.9) O (sub 4816H) (sub 2) O, from Gibelsbach, Fiesch (Valais, Switzerland) was refined at 100 and 293 K in space group C1 (273 K: a 9.083, b 17.738, c 10.209 Aa, alpha 89.95, beta 124.58, Gamma 90.00 degrees ); reflection intensities clearly suggest triclinic symmetry. The symmetry is lowered from monoclinic C2 to triclinic C1 due to partial Si,Al ordering accompanied by a preferred distribution of the channel cations to the underbonded O atoms of the tetrahedral framework. The hitherto enigmatic occurrence of two domain variants (A,B) formed by different tetrahedral configurations could be modelled by (010) disorder caused by a local mirror plane.