# Potassium self-diffusion in a K-rich single-crystal alkali feldspar

## TL;DR

This study measures potassium self-diffusion in alkali feldspar, revealing a high activation energy and low diffusivity, and suggests correlated K-Na motion via an interstitialcy mechanism based on experimental and simulation data.

## Contribution

It provides new diffusion coefficients for potassium in alkali feldspar and proposes a correlated interstitialcy diffusion mechanism supported by experimental and simulation evidence.

## Key findings

- Potassium diffusion coefficients follow Arrhenius behavior with E_a=2.4 eV.
- Potassium diffusivity is over three orders of magnitude lower than sodium in the same mineral.
- Diffusion likely occurs via a correlated interstitialcy mechanism involving K and Na.

## Abstract

The paper reports potassium diffusion measurements performed on gem-quality single-crystal alkali feldspar in the temperature range from $1169$ to $1021 \, \mbox{K}$. Natural sanidine from Volkesfeld, Germany was implanted with $\mbox{}^{43}\mbox{K}$ at the ISOLDE/CERN radioactive ion-beam facility normal to the (001) crystallographic plane. Diffusion coefficients are well described by the Arrhenius equation with an activation energy of $2.4 \, \mbox{eV}$ and a pre-exponential factor of $5\times10^{-6} \, \mbox{m}^{2}/\mbox{s}$, which is more than three orders of magnitude lower than the $\mbox{}^{22}\mbox{Na}$ diffusivity in the same feldspar and the same crystallographic direction. State-of-the-art considerations including ionic conductivity data on the same crystal and Monte Carlo simulations of diffusion in random binary alloy structures point to a correlated motion of K and Na through the interstitialcy mechanism.