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

Framework aluminosilicates typically have low birefringence. Refractive indices and, thus, optical class, orientation, sign and 2V are very sensitive to slight changes in the crystal structure or composition. Many zeolites crystallize in more than one optical class, and optical properties 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 within the channels. From natrolite to mesolite to scolecite, the refractive 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 mineral is different: natrolite length slow, mesolite length intermediate, and scolecite length fast. The refractive indices of natrolite decrease upon partial loss of channel H2O, but upon total dehydration, the structure collapses, increasing both density and refractive indices. The optical class of mesolite changes over the temperature range 20-70-degrees-C. Temperature-induced changes in optical orientation or optical class can be accounted for by differing values of dn/dT for the principal refractive indices, which, in turn, are directly related to structural parameters. The reported optical properties of tetranatrolite and paranatrolite fit the overall trend for the natrolite group zeolites.