Anomalous birefringence in nucleated Li2O center dot 2SiO(2) and Na2O center dot 2CaO center dot 3SiO(2) glass melts

Anomalous birefringent glasses were obtained by thermomechanical treatment of quenched or pre-nucleated Li2O . 2SiO(2) (LS2) and Na2O . 2CaO . 3SiO(2) (NC2S3) glasses, Cylindrical glass samples were compressed in the viscoelastic temperature range above T-g under various loads and cooled under these loads slowly enough to avoid thermal stresses. In contrast to this, normal stress optical behavior of these glasses is observed at room temperature. The frozen-in anomalous birefringence may be correlated to a distribution birefringence, indicating order phenomena in the stoichiometric LS2 and NC2S3 glass melts. Results are compared with multicomponent di-and metasilicate glasses which show normal frozen-in birefringence. Microstructural heterogeneities are discussed to affect volume nucleation in the LS2 and NC2S3 glass melts. (C) 1997 Elsevier Science B.V.