Structure and crystallization of boron-containing sodium fluorphlogopite NaMg2.5(B,Si)4O10F2 glasses

Abstract

Transparent glasses based on a stoichiometric boron-containing sodium fluorphlogopite composition, NaMg2.5(B, Si)4O10F2, were prepared. Additions of boron in glasses lower the melting temperature and strongly affect the nucleation and crystallization temperatures. The crystallization process was studied by means of DTA, XRD, and SEM. Thermal treatment of glasses at 800 °C for 5 or 20 h did not show any crystallization of sodium fluorphlogopite, whereas at 500 °C/2 h and 750 °C/2 h crystallization of sodium fluorphlogopite took place. However, magnesium fluorrichterite, fluoborite, suanite and small amounts of enstatite were detected in the crystallized glasses. Increase of crystal growth rate was observed in the microstructure with the increase of boron oxide in the glass-ceramic samples. The incorporation of boron oxide changed the infrared spectra of the parent glasses and the corresponding glass-ceramics and bands referred to triangular (BO3) and tetrahedral (BO4) groups were observed. However, crystallization of the sodium fluorphlogopite led to intensification of the bands related to BO4, which means that boron was substituted for silicon in the sodium fluorphlogopite structure. In the case of the samples with the highest boron oxide content or borate-rich phases, i.e. suanite and fluoborite phases, the BO3 group was the dominant one.