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

TL;DR

This study measures sodium tracer diffusion in natural alkali feldspars, revealing anisotropic diffusion behavior and suggesting that Al/Si ordering significantly influences Na diffusion rates.

Contribution

It provides the first detailed diffusion tensor measurements in alkali feldspars and links Al/Si ordering to diffusion behavior.

Findings

Na diffusion is anisotropic with faster rates perpendicular to (001)

Diffusion tensor determined for orthoclase shows directional dependence

Al/Si ordering significantly affects Na diffusion rates

Abstract

Tracer diffusion of Na in natural alkali feldspars including sanidine, adularia and orthoclase with different Na:K ratios is measured using the radiotracer technique and applying the 22Na radioisotope. The tracer diffusion measurements along the crystallographic directions perpendicular to (001) and (010) in alularia feldspar reveled a slight (within a factor of two to three) anisotropy of Na diffusion with the faster diffusion rates along the perpendicular to (001) directions, while this difference was less prominent in orthoclase. The anisotropy of Na diffusion is specifically addressed for orthoclase and the full diffusion tensor is determined. The impact of temperature and composition on diffusion in natural alkali feldspars is discussed with respect to impurities and intrinsic defects. The state of Al/Si ordering is proposed to have a significant influence on the Na diffusion rates in alkali feldspars.