Sodium diffusion in alkali feldspars: towards the impact of Al/Si ordering

ABSTRACT Tracer diffusion of Na in natural alkali feldspars including sanidine, adularia and orthoclase with different Na:K ratios, minor element contents and different degrees of Al/Si ordering is investigated using the radiotracer technique employing ${}^{22}$22Na as the radioisotope. In adularia, the tracer diffusion measurements along the crystallographic directions ⊥(001) and ⊥(010) revealed a slight (within a factor of two to three) anisotropy of Na diffusion, with the faster diffusion rates along the ⊥(001) direction. The full diffusivity tensor for Na diffusion in one orthoclase is determined and a strong, temperature-dependent anisotropy of diffusion in the (010) plane is found. The state of Al/Si ordering is inferred to have a significant influence on the Na diffusion rates in alkali feldspar, while the presence of spurious alkaline impurities and major element composition plays a less important role.