Fully Cd-exchanged heulandite was obtained from a Na-exchanged sample by tr
eatment with 1 M Cd acetate solution at 373 K. Subsequent electron micropro
be analyses revealed 4.0 Cd2+ per formula unit (pfu), that was in good agre
ement with 4.11 Cd pfu refined from single-crystal X-ray data collected at
293 K. Crystal structure refinements were performed in the space groups C2/
m, C2, Cm, C (1) over bar, and C1, of which only the Cm model yielded reaso
nable agreement with the observed diffraction data. Cadmium preferentially
occupied the center of the A channel, where it formed a distorted octahedra
l Cd2+(H2O)(6) complex, and a central position in the B channel that was se
venfold coordinated by oxygen atoms of the framework and channel H2O molecu
les. Seven additional low-populated Cd sites were located in the channels.
Cd-exchanged heulandite was refined in Cm symmetry in contrast to most othe
r natural and cation-exchanged heulandites that commonly show C2/m symmetry
. Symmetry lowering is attributed to Si,AI ordering in the tetrahedral fram
ework as well as to the asymmetrical distribution of Cd2+ ions due to prefe
rred Cd bonding of oxygen atoms sharing Al-enriched tetrahedra and cation-c
ation repulsion. (C) 2000 Elsevier Science B.V. All rights reserved.