Inter-diffusion of K and Na in alkali feldspar was studied at 5 and 15 kbar confining pressure using diffusion couples annealed at 900 degrees and 1000 degrees C. Concentration profiles were measured using an electron microprobe and results were obtained for inter-diffusion in four crystallographic directions at 1000 degrees C. Inter-diffusion coefficients (D) were calculated as a function of composition from Or 10 to Or 80 . 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 D between 5 and 15 kbar. There is agreement between the D values determined experimentally and those calculated from published self-diffusion coefficients. The nonideality of the alkali feldspar binary at 1000 degrees C has a significant effect on D, and this effect becomes increasingly important at lower temperature.--Modified journal abstract.

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