Studies of crystallization process of silica and germania glasses at high pressure

The process of crystallization of SiO2 and GeO2 glasses was studied using s amples retrieved after heat treatment at high pressures up to 12 GPa. Two d ifferent samples of fused quartz and silica gel were studied in order to co mpare SiO2 glass structure. Upon heating beyond 400 degrees C at fixed pres sures under which stishovite. rutile-type phase, is thermodynamically stabl e, the SiO2 glasses underwent crystallization into coesite and stishovite, without quartz. In the course of heating GeO2 glass in the stability field of rutile-type phase, low-quartz-type phase appeared as an intermediate at pressures below 7 GPa, whereas only rutile-type phase was observed at 12 GP a. The crystallization sequences are discussed in terms of coordination num bers in the glass and crystalline states.