Na-K interdiffusion in disordered potassium-rich alkali feldspar was studied experimentally using cation exchange between gem quality sanidine from the Eifel and alkali-halide melt at temperatures of 800°C to 1000°C and at close to ambient pressure. Sodium-potassium interdiffusion coefficient D NaK was determined for potassium mole fractions in the range 0.65 < X Or < 0.99. At 0.65 < X Or < 0.95 the sodium-potassium interdiffusion coefficient is largely independent of composition. At X Or > 0.95, it rises sharply with increasing potassium mole fraction. Diffusion perpendicular to (001) is about one order of magnitude faster and less strongly temperature dependent than perpendicular to (010). The parameters of the Arrhenius equation describing the temperature dependence of the sodium-potassium interdiffusion coefficient D NaK ‫؍‬ D 0 Exp (؊E A /RT) were estimated as D 0 Ќ(001) (0.92) ‫؍‬ 5.18 ⅐ 10 ؊8 m 2 /s, E A Ќ(001) (0.92) ‫؍‬ 179kJ/mole D 0 Ќ(001) (0.98) ‫؍‬ 1.82 ⅐ 10 ؊7 m 2 /s, E A Ќ(001) (0.98) ‫؍‬ 182kJ/mole D 0 Ќ(010) (0.92) ‫؍‬ 5.81 ⅐ 10 ؊5 m 2 /s, E A Ќ(010) (0.92) ‫؍‬ 272kJ/mole D 0 Ќ(010) (0.98) ‫؍‬ 1.34 ⅐ 10 ؊4 m 2 /s, E A Ќ(010) (0.98) ‫؍‬ 269kJ/moleThe results of our direct determinations are compared with theoretical calculations using the corresponding sodium-and potassium tracer-diffusion coefficients, and the processes underlying the observed composition-and temperature dependence of sodium-potassium interdiffusion are discussed.

Anisotropic diffusion is described by a tensor of diffusivities, D ␣␤ , which may be composition dependent leading to nonlinear anisotropic diffusion. In this work the practical problem of reconstructing such a tensor for Na-K interdiffusion in potassium-rich alkali feldspar in the composition range 0.85 < X Or < 1.00 is addressed. Gem quality sanidine with an initial composition of X Or ‫؍‬ 0.85 was exchanged with KCl salt melt at 850°C and Ϸ1 bar. The diffusivity tensor D ␣␤ (X Or ) and its composition dependence was reconstructed from composition profiles produced by cation exchange in six different crystallographic directions using a generalization of the Boltzmann approach. Na-K interdiffusion in potassium-rich alkali feldspar is considerably anisotropic and composition dependent. The principal axes of the diffusivity tensor representing the directions of highest and lowest diffusivity lie in the a-c plane with highest diffusivity parallel to the [101] direction, lowest diffusivity perpendicular to the (101 ) plane, and the direction with intermediate diffusivity parallel to the crystallographic b-axis. All diffusivities D ␣␤ (X Or ) increase as X Or tends to unity. Our main result is given in the form of numerical values for all components of the D ␣␤ (X Or ) tensor for 0.85 < X Or < 1.