Influence of Boric Oxide on Vitrification of Amorphous and Crystalline forms of Silica

Phase transformations of silica in mixtures containing boric oxide and three forms of silica, namely, amorphous silica, quartz and ordered cristobalite, were studied by the X-ray and DTA methods. With 5, 7.5 and 10 wt per cent of boric oxide, amorphous silica readily and completely vitrified at about 1480°C, 1300°C and 1040°C, respectively but not the crystalline forms, even after heating for periods upto 50 hours. The results suggest that with increasing boric oxide content, larger proportions of amorphous silica were directly converted to SiO2-B2O3 glass before any cristobalite could appear.Boric oxide accelerates the formation of tridymite from quartz and cristobalite, but does not permit conversion of quartz to cristobalite. At higher temperatures, quartz was probably transformed to an intermediate amorphous form and then to SiO2-B2O3 glass before any cristobalite could be formed. Starting with either ordered cristobalite or amorphous silica only a particular disordered form of cristobalite, showing α-β transformation at 265°C, was obtained in the presence of boric oxide.