Equations which describe incongruent nucleation and subsequent crystal growth are derived. A ZrF4-BaF2-LaF3-AlF3-NaF (ZBLAN) glass was used to test the validity of these equations. Nucleation rate measurements were fitted to theory and some growth rate measurements were in reasonable agreement with theor. predictions. Both nucleation theory and crystal growth theory were used for computer simulations of the crystn. behavior during heat treatments. Some heat treatments were performed in a DSC app. to verify the theories. The exptl. results were in good agreement with the numerical data. Using these theor. results it is possible to est. fiber scattering losses due to crystn. Depending on drawing temp., estd. losses can vary from 0.014 (310 Deg) to >=25 decibel/km (320 Deg). [on SciFinder (R)]