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Glasses of the 6Na2O-(24-х)B2O3-70SiO2-хFe2O3 system with x ranging from 2 to 10 mol % are studied by dilatometry and X-ray powder diffractometry. The density of the glass is measured and the molar volume is calculated. The heat treatment of the glass is carried out at 550 and 700°C for 96 and 2 h, respectively. Magnetite is formed in the heat-treated glasses and its content increases with the growth of the Fe2O3 concentration. In the glasses containing from 4 to 8 mol. % of Fe2O3, cristobalite is formed after treatment at 700°C for 2 h. An increase in the density and the glass transition temperature and a decrease in the molar volume is observed with the growth of the iron oxide content. [ABSTRACT FROM AUTHOR]

Glasses of the 6Na2O · хB2O3 · (86 – x)SiO2 · 8Fe2O3 compositions, where x varies from 16 to 31 mol %, are studied by X-ray powder diffractometry and dilatometry. The glasses were preliminarily held at a temperature of 550°C for 8–144 h. It is determined that there are crystalline phases of magnetite and hematite in the studied glasses in addition to the amorphous component. The hematite content gradually decreases as the amount of SiO2 increases and it completely disappears when the SiO2 concentration is higher than 65 mol %. As the duration of the heat treatment increases, the amount of magnetite does not change. At the maximum duration of the heat treatment (144 h), crystallization of the silica phases—tridymite and cristobalite—is observed for all the studied glasses. The stability of the glass transition temperature of all glasses with an increase of the duration of the heat treatment indicates that the coexisting phases of phase-separated glass reach their equilibrium compositions after 8 hours. The crystallization of silica in the glasses is accompanied by a decrease in the glass transition temperature of the low-viscosity phase. [ABSTRACT FROM AUTHOR]