Radiotracer experiments and Monte Carlo simulations of sodium diffusion in alkali feldspar: Evidence against the vacancy mechanism

Wilangowski F., Abart R., Divinski S., Stolwijk N.

Abstract

Self-diffusion of sodium perpendicular to (001) in a potassium-rich alkali feldspar single crystal has been studied by self-diffusion experiments and by Monte Carlo simulations. Sodium diffusivities were measured with the radiotracer technique using the 22Na isotope in a temperature interval from 773 K to 1173 K. It was found that self-diffusion coefficients follow a linear Arrhenius relation with the pre-exponential factor of 1.2×10-3 cm2/s and an activation enthalpy of 1.3 eV. To study correlation effects in the monoclinic feldspar structure, a Monte Carlo method was applied assuming that the two cation species are randomly distributed on the common sublattice and are not influenced by the fixed sublattice of the silicate and aluminate anions. Correlation factors have been calculated assuming a vacancy mechanism and applying a developed four-frequency model for the nearest-neighbor vacancy jumps on the alkali sublattice. Our findings strongly indicate that vacancy diffusion provides only a minor contribution to sodium self-diffusion in potassium-rich feldspars.