Crystal structure of a natural and a partially silver-exchanged heulandite

Abstract

The crystal structure of a natural calcium heulandite (Ca)heu and a partially silver-exchanged heulandite (Ag,Ca)heu is reported. The monoclinic crystals, space group C2/m, have cell parameters a= 17.672(6), b= 17.930(6), c= 7.443(4)Å, β= 116.5(1)°, Z= 1, and formula Ca_3.1Na_1.1K_0.1Al_7.6Si_28.5O₇₂·24H₂O (from electron microprobe analysis) for (Ca)heu. For (Ag,Ca)heu a= 17.741(6), b= 17.974(6), c= 7.408(4)Å, β= 116.4(1)°, Z= 1, and the formula is Ca_3.1Na_0.1Ag_1.3Al_7.8Si_28.2O₇₂·25H₂O (from electron microprobe analysis). The final R index is 0.041 for (Ca)heu and 0.051 for (Ag,Ca)heu using 1 863 and 1 414 independent reflections respectively in the least-squares refinement. Chemical analysis and X-ray results show that Ag⁺ ions have exchanged only univalent cations which are all located in one of the two occupied cationic sites, the other being populated by only Ca²⁺ ions. These results may be interpreted on the basis of the chemical and structural features of heulandite.