Sodium and potassium transport in alkali feldspar: tracer diffusion, ion conductivity and Monte Carlo simulation

The rock-forming alkali feldspars belong to the most abundant minerals in the Earth’s crust and are formed as a solid solution between the sodium (NaAlSi₃O₈, albite) and potassium (KAlSi₃O₈, orthoclase) end-member compositions. Well-founded knowledge of self-diffusion data in alkali feldspar is a prerequisite for interpreting existing interdiffusion data that, in turn control re-equilibration features in alkali feldspar that pertain to evolution and dynamics of the crust. Sodium diffusivities in a potassium-rich alkali feldspar single crystal originating from Volkesfeld, Germany, were measured with the radiotracer technique using the ²²Na isotope. It was found that the diffusion coefficients follow a linear Arrhenius relation with an activation enthalpy of 1.3 eV and a pre-exponential factor of 1.2 × 10^-3 cm²/s. The same feldspar was implanted with ⁴³K at the ISOLDE/CERN radioactive ion-beam facility normal to the (001) crystallographic plane. Potassium diffusion coefficients are described by an activation energy of 2.4 eV and a pre-exponential factor of 5 × 10^-6 cm²/s, which is more than three orders of magnitude lower than the ²²Na diffusivity and rules out a vacancy-controlled diffusion mechanism for alkali diffusion in alkali feldspar. State-of-the-art considerations including ionic conductivity data on the same crystal type [3] and Monte Carlo simulations of diffusion in random binary alloy structures [4] reveal a predominance of indirect interstitial jumps (I-S/S-I) over direct interstitial jumps (I-I), pointing towards correlated motion of K and Na through the interstitialcy mechanism.

[1] F. Wilangowski et al., Defect and Diffusion Forum 363 (2015) 79-84

[2] F. Hergemöller et al., Phys. Chem. Minerals, DOI 10.1007/s00269-016-0862-1

[3] H. El Maanaoui et al., Phys. Chem. Minerals 45 (2016) 327-340

[4] F. Wilangowski and N. A. Stolwijk, Phil. Mag. 97 (2017) 108-127