NATROLITE GROUP ZEOLITES - CORRELATIONS OF OPTICAL-PROPERTIES AND CRYSTAL-CHEMISTRY

Framework aluminosilicates typically have low birefringence. Refractiv e indices and, thus, optical class, orientation, sign and 2V are very sensitive to slight changes in the crystal structure or composition. M any zeolites crystallize in more than one optical class, and optical p roperties are often reported but not interpreted structurally. In the natrolite group, both refractive indices and optical orientations can be correlated with the substitution of 2 Na double-line arrow pointing left and right Ca + H2O and the orientation of the H2O molecules with in the channels. From natrolite to mesolite to scolecite, the refracti ve indices (n) increase, with n's most nearly parallel to the a and b crystallographic axes, increasing at a rate greater than n most nearly parallel to c, explaining why the optical orientation for each minera l is different: natrolite length slow, mesolite length intermediate, a nd scolecite length fast. The refractive indices of natrolite decrease upon partial loss of channel H2O, but upon total dehydration, the str ucture collapses, increasing both density and refractive indices. The optical class of mesolite changes over the temperature range 20-70-deg rees-C. Temperature-induced changes in optical orientation or optical class can be accounted for by differing values of dn/dT for the princi pal refractive indices, which, in turn, are directly related to struct ural parameters. The reported optical properties of tetranatrolite and paranatrolite fit the overall trend for the natrolite group zeolites.