NA AND CA TRACER DIFFUSION IN PLAGIOCLASE GLASSES AND SUPERCOOLED MELTS

The diffusion of Na and Ca was investigated in various glasses of plagioclase compositions by the residual activity method in the temperature range 400-1000-degrees-C at normal pressure. An anomalous diffusion was observed for both Na and Ca diffusion in high-calcic glasses. This behaviour is attributed to a partially interrupted contact between the sample and the activated surface layer. Na diffusivity decreases strongly with increasing CaAl2Si2O8 (An) content of the glass, especially for An-rich glasses. By contrast, the compositional dependence of Ca diffusion is less pronounced. For example, at 800-degrees-C the diffusivity of Ca in a glass containing 60 mole% An is lower only by a factor of 5 compared to pure NaAlSi3O8 (Ab) glass whereas it is by a factor of 60 for Na. Arrhenius plots of Na diffusivities in the An50 and An70 glasses showed marked changes of slope, probably caused by the transformation from a glass to a supercooled melt. Apparent transformation temperatures obtained from these plots are lower than those obtained calorimetrically. This indicates, that relaxation processes occur in these glasses at temperatures below the onset of non-linear changes in macroscopic properties.