Crystal structure and optical properties of Na- and Pb-exchanged heulandite-group zeolites

Cation exchange experiments were conducted on three heulandite-group zeolites from Fiesch, Switzerland (Ca_3.6K_0.8Al_8.8Si_27.4O₇₂ · 26.1H₂O); Succor Creek, Oregon (Ca_2.1Mg_0.3Na_2.5K_0.3Al_8.0Si_28.2O₇₂ · 25.5H₂O); and Poona, India (Ca_3.7Na_1.3K_0.1Al_8.9Si_27.1O₇₂ · 21.4H₂O). Only the Poona sample was completely Na and, in turn, Pb exchanged. Diffusion gradients for the other two samples were observed by both backscattered electron images and optical retardation on (010). For the Poona sample, single-crystal X-ray structure refinements of the nonexchanged, Na-exchanged (Na_8.8Al_8.9Si_27.1O₇₂ · 19.2H₂O), and Pb-exchanged (Pb_4.4Al_8.9Si_27.1O₇₂ · 16.4H₂O) samples were performed with resultant R values of 5.2, 5.7, and 8.1% in space groups C2/m, C2/m, and Cm, respectively. There is only one metal position in the B channel for the Pb sample, whereas this position is split for the nonexchanged and Na-exchanged samples. This, along with channel H₂O locations, causes the reduction in symmetry for the Pb-exchanged sample. The framework topology remains essentially unchanged for the three samples.

The optical properties of the Poona sample were determined, and the birefringence in (010) was found to vary dramatically, ranging from 0.001 for the nonexchanged sample to 0.0145 for the Pb-exchanged sample, whereas the mean refractive indices ranged from 1.488 for the Na-exchanged sample to 1.573 for the Pb-exchanged sample.