Solution and diffusion of helium and neon in tridymite and cristobalite

Abstract

The solution and trapping of helium and neon has been studied in cristobalite in the range 293–857°K and in tridymite between 293 and 838°K. These temperatures cover regions in which α- and β-cristobalite and α-, β₁- and β₂-tridymite are found. The solubilities are considerable, those in tridymite being close to solubilities in silica glass and those in cristobalite rather less. Heats of solution were evaluated from isotherms and had the same signs as those in silica glass. The results were discussed in terms of an oscillator model for dissolved atoms. For some of the systems approximate diffusion coefficients were obtained which could be represented by the Arrhenius equation: He in α-tridymite (236–336°K): D= 7·7×10^–4 exp –12000/RT cm² sec^–1, He in α-cristobalite (244–335°K): D= 2·0×10^–2 exp –13800/RT cm² sec^–1, Ne in α-tridymite (336–366°K): D= 6·7×10^–2 exp –22500/RT cm² sec^–1, Ne in β₁-tridymite (418–474°K): D= 9·9×10^–1 exp –24100/RT cm² sec^–1, Ne in β₂-tridymite (513–577°K): D= 4·5×10^–4 exp –19100/RT cm² sec^–1. Energies of activation are about twice their values in silica glass and diffusion coefficients are smaller.