CRYSTAL-STRUCTURE AND OPTICAL-PROPERTIES OF NA-EXCHANGED AND PB-EXCHANGED HEULANDITE-GROUP ZEOLITES

Cation exchange experiments were conducted on three heulandite-group z eolites from Fiesch, Switzerland (Ca3.6K0.8Al8.8Si27.4O72.26.1H2O); Su ccor Creek, Oregon (Ca2.1Mg0.3Na2.5 K0.3Al8.0Si28.2O72.25.5H2O); and P oona, India (Ca3.7Na1.3K0.1Al8.9Si27.1O72.21.4H2O). Only the Poona sam ple 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 (Na8.8Al8.9Si27.1O72.19.2H2O), and Pb-exchanged (Pb4.4Al8.9Si27.1O72. 16.4H2O) samples were performed with resultant R values of 5.2, 5.7, a nd 8.1% in space groups C2/m, C2/m, and Cm, respectively. There is onl y one metal position in the B channel for the Pb sample, whereas this position is split for the non-exchanged and Na-exchanged samples. This , along with channel H2O locations, causes the reduction in symmetry f or 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 va ry dramatically, ranging from 0.001 for the nonexchanged sample to 0.0 145 for the Pb-exchanged sample, whereas the mean refractive indices r anged from 1.488 for the Na-exchanged sample to 1.573 for the Pb-excha nged sample.