The crystal structure of heulandite-Sr from Campegli (Liguria, Italy)

Abstract

The crystal structure of heulandite-Sr, a natural zeolite from Campegli (Liguria), was refined in the Cm space group. The average chemical analysis [1] is Ca 2.20Sr 1.90Ba 0.10Na 0.53K 0.29 [Al 9.35Si 26.68O 72]·19.74H 2O. Six sites for the extra-framework cations were found, two of which, Ca2A and Ca2, related by symmetry in the C2 and C2/m space group, show a very high difference in their occupancy factors. (An A is added to the name of atoms related by symmetry in the C2/m space group). As the topological symmetry is C2/m, the lowering of the symmetry is related to the order of the extra-framework cations, induced by the Si/Al order in the tetrahedral (T and TA) sites. T and TA sites are not equivalents: their Al contents are in the order T2A > T1A > T4A = T2. It is interesting to note that T2A and T2 have the maximum Al values of the two groups. Sr1 and Sr1A are occupied by strontium, Ca2A and Ca2 by calcium, while K, Ba and perhaps Na are in the K3 and K3A sites. Heulandite-Sr was compared with Sr-exchanged heulandite in Döbelin and Armbruster [2] ( Cm space group) and with heulandite-Ba published by Larsen et al. [3] ( C2/m space group). The reduction of symmetry in our heulandite and in the Sr-exchanged form is related to greater order in the Si/Al distribution, which is confirmed by the higher value of the “S” order coefficient introduced by Alberti [4].