Diffusion-exchange kinetics were determined at 320 to 425K for Na+ in single crystals of natural heulandite, and for K+, Rb+, and Cs+ for Na+ in Na-exchanged heulandite. Cation diffusion was measured on (010) cleavage plates with the aid of a polarizing microscope; exploiting the fact that the optical properties, and especially the extinction angle, varied with the chemical composition of the channel occupants. This method was restricted to platy zeolites of monoclinic or triclinic symmetry where the channels were parallel to the crystal plate. No indication of anisotropic diffusion was observed on (010) plates for any of the exchanged cation pairs. The diffusion coefficients (tables 21 and 22) showed that the diffusion rate of Na+ → Ca2+ was much slower than those of K+ → Na+, Rb+ → Na+ and Cs+ → Na+ because of a strong Coulombic interaction of Ca2+ with the tetrahedral framework; compared with an exchange of monovalent species. The activation energy and the pre-exponential factor were deduced. Comparable values of activation energy for K+ → Na+ and Rb+ → Na+ suggested the operation of a similar diffusion process, while a lower value of activation energy, combined with relatively low diffusion rates for Cs+ → Na+ implied a different diffusion mechanism for Cs+.

Na, K, Rb, and Cs Exchange in Heulandite Single Crystals: Diffusion Kinetics. P.Yang, J.Stolz, T.Armbruster, M.E.Gunter: American Mineralogist, 1997, 82, 517-25

Table 21

Exchange Diffusion of Cations in Heulandite