Stability of Glass in the Geologic Environment: Some Evidence from Studies of Natural Silicate Glasses

The geologic literature contains numerous studies of the stability of natural silicate glasses under a variety of environmental conditions. The processes of glass hydration, devitrification, and dissolution are described and their rates are estimated from field and laboratory observations. These processes can also affect any glass proposed as a waste solidification medium. Observations of natural silicate glasses are particularly informative if waste disposal glasses are similarly reactive. Review of the data for natural silicate glasses suggests that

1. Breakdown of glass in the total absence of water is too slow to be of concern for the periods of time required for safe waste disposal.
2. Contact between glass and a hot aqueous phase greatly accelerates all processes of glass destruction. Glass hydration and dissolution rates become fast enough that significant alteration of glass can occur in the period required for hazardous radionuclides to decay.
3. Other environmental parameters that can influence the rate of glass/water interaction, but to a lesser extent than temperature, include solution pH (highly acid or alkaline), solid surface area and composition, solution Eh, salinity, and the presence of complexing agents.