  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.

Materials Science