Radiotracer Experiments and Monte Carlo Simulations of Sodium Diffusion in Alkali Feldspar: Evidence against the Vacancy Mechanism

Abstract

Self-diffusion of sodium perpendicular to (001) in a potassium-rich alkali feldspar singlecrystal has been studied by self-diffusion experiments and by Monte Carlo simulations. Sodium diffusivitieswere measured with the radiotracer technique using the 22Na isotope in a temperature intervalfrom 773 K to 1173 K. It was found that self-diffusion coefficients follow a linear Arrhenius relationwith the pre-exponential factor of 1:2 10􀀀3 cm2/s and an activation enthalpy of 1:3 eV. To study correlationeffects in the monoclinic feldspar structure, a Monte Carlo method was applied assuming thatthe two cation species are randomly distributed on the common sublattice and are not influenced by thefixed sublattice of the silicate and aluminate anions. Correlation factors have been calculated assuminga vacancy mechanism and applying a developed four-frequency model for the nearest-neighborvacancy jumps on the alkali sublattice. Our findings strongly indicate that vacancy diffusion providesonly a minor contribution to sodium self-diffusion in potassium-rich feldspars.