| 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 DT A 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 Si02-B20 3 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 Si02-B20 3
glass before any cristobalite could be formed. Starting with either ordered cristobalite
or amorphous silica only a particular disordered form of cristobalite, showing tl(.-{3
transformation at 265°C, was obtained in the presence of boric oxide. |
