Interdiffusion of K and Na in alkali feldspars; homogenization experiments

Abstract

The rates of homogenization of coherent exsolution lamellae in cryptoperthites have been used to evaluate K-Na interdiffusion in alkali feldspars. An interdiffusion coefficient 1D) is between2 and 4 x l0-17 cm2/sec for a synthetic cryptoperthite at 600C in the composition range Or23-Or53, and between 6 and 12 x 10-16 cm2lsec for a natural cryptopertlite at 650'C in the composition range Or6Ors5. These values are for diffusion normal to (801) at I atmosphere pressure. K-Na interdiffusion coefficients calculated from published K and Na self-diffusion coefficients and thermochemical solution data for alkali feldspars depend strongly on composition. The minimum D's calculated are similar to the D's determined from the homogenization experiments. Finite-difference of the homogenization experiments using the calculated, composition-dependent interdiffusion coefficients are in good agreement with our experimental results. Homogenization experiments appear to be successful in determining average interdiffusion coefficients which are consistent with diffusion data obtained using other techniques. The method is especially useful for determining D's in the range of l0-16 to l0-le cm2/sec.