CALCULATIONS, OBSERVATIONS, AND APPLICATIONS OF BIREFRINGENCE OF NATURAL AND CATION-EXCHANGED ZEOLITES

Retardation and thickness of single grains can be easily observed in cross-polarized light with the aid of a polarizing light microscope equipped with a spindle stage. Precise numerical birefringences based on observed retardations and thicknesses are relatively easy to obtain compared to measuring precise refractive indices, 2V, or optical orientation. Correct interpretation of retardation can be used to determine chemical composition in mineral solid solution series, observe zoning in minerals, and calculate diffusion coefficients in cation-exchanged zeolites, along with industrial applications such as controlling the thickness of extruded plastic films by real-time monitoring of retardation. While the Gladstone-Dale method provides a method to calculate the mean refractive index of a material, no method is currently widely used to calculate the birefringence of a material.

Computational methods, based on point-dipole theory, developed by Abbott (1993, 1994) were used to calculate the optical properties of natural and cation-exchanged zeolites, with an emphasis on the numerical birefringences. The calculated birefringences are consistently proportional to changes in cation proportions. The calculated refractive indices generally do not correspond as well with the observed refractive indices as the calculated birefringences correspond to the observed birefringences. Therefore, once a set of electronic polarizabilities was optimized for a specific zeolite, it was possible to model variation of numerical birefringence with changes in composition. This allows the prediction of the effects of natural ionic substitution or artificial cation exchange. This method was applied to the natrolite-mesolite-scolecite group using data obtained by Gunter and Ribbe (1993) and Na-exchanged and Pb-exchanged heulandites described by Gunter et al. (1994).