Inter-diffusion of K and Na in alkali feldspars; diffusion couple experiments

Inter-diffusion of K and Na in alkali feldspar was studied at 5 and 15 kbar confining pressure using diffusion couples annealed at 900° and 1000°C. Concentration profiles were measured using an electron microprobe and results were obtained for inter-diffusion in four crystallographic directions at 1000°C. Inter-diffusion coefficients (D̅) were calculated as a function of composition from Or10 to Or80. Alkali inter-diffusion is about 10 times faster normal to (001) than normal to (010), and the rate is intermediate parallel to [100] and [011]. A trace amount of water appears to have no influence on the inter-diffusion rate, and within experimental error there is no difference D̅ between 5 and 15 kbar. There is agreement between the D̅ values determined experimentally and those calculated from published self-diffusion coefficients. The nonideality of the alkali feldspar binary at 1000°C has a significant effect on D̅, and this effect becomes increasingly important at lower temperature.—Modified journal abstract.