Cryolite, chiolite and cryolithionite: optical data redetermined

Abstract

The optical data for cryolite from Ivigtut, South Greenland, have been determined using the immersion method and the Abbe refractometer to bracket the refractive indices, Tobi's method to establish 2V and the Berek compensator for the determination of the birefringences. The dispersion has been determined with the Abbe refractometer. The refractive indices and the dispersion of chiolite and cryolithionite have also been determined using the Abbe refractometer. The obtained results differ from those to be found in the literature where, moreover, several different values are reported. The extremely low refringence and birefringence of cryolite and its numerous twin lamellae impart difficulties for the determination but it appears that erroneous values have slipped into the literature. As the main sources are more than 50 years old, instrumental inaccuracies may likewise play a role.