Effects of Al2O3, B2O3, Na2O, and SiO2 on nepheline formation in borosilicate glasses: chemical and physical correlations

Raman spectroscopy was applied to complex sodium alumino-borosilicate glasses that precipitate nepheline as their primary phase. The main focus was on the Raman band at 850 cm−1 (wavenumber) observed in quenched glasses. Based on the literature, the 850 cm−1 band was assigned to AlIV–O–Si units in which tetrahedral AlIV are substituted for Si in the network, creating [AlO4/2]− anions that are charge-balanced by Na+ cations. The same glasses with various temperature histories were examined with optical microscopy, scanning electron microcopy and transmission electronic microscopy. The results indicated that the 850 cm−1 Raman band was associated with nanocrystals that formed in the melts and were preserved in the quenched samples. The chemistry of these nanocrystals resembled nepheline. Models relating the intensity of the Raman band at 850 cm−1 and the liquidus temperature (TL) to glass composition revealed that a relationship exists between these two properties. An increase in B2O3 and SiO2 decreased the Raman band intensity and the TL, whereas an increase in Al2O3 and Na2O increased both properties. These effects were attributed to the influence of various oxides on the Na+–[AlO4/2]− activity in the melt.