Interdiffusion of K and Na in Alkali Feldspars: Homogenization Experiment

John B. Brady, Smith College
Richard A. Yund, Brown University

Abstract

The rates of homogenization of coherent exsolution lamellaein cryptoperthites have been used to evaluate K-Na interdiffusion in alkali feldspars.An "average" interdiffusion coefficient (Ď) is between 2 and 4 x l0^-17 cm²/sec for a synthetic cryptoperthite at 600°C in the composition range Or₂₃-Or₅₃, and between 6 and 12 x 10^-16 cm²/sec for a natural cryptopertlite at 650°C in the composition range Or₀-Or₈₅. These values are for diffusion (801) at 1 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 Ď's calculated are similar to the "average" Ď's determined from the homogenization experiments. Finite-difference models 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 Ď's in the range of l0^-16 to l0^-19 cm²/sec.