title: Lattice strain across Na-K interdiffusion fronts in alkali feldspar: an electron back-scatter diffraction study.
abstract: Cation exchange experiments between gem quality sanidine [Formula: see text] and KCl melt produced chemical alteration of alkali feldspar starting at the grain surface and propagating inwards by highly anisotropic Na-K interdiffusion on the alkali sublattice. Diffusion fronts developing inb-direction are very sharp, while diffusion fronts within thea-c-plane are comparatively broad. Due to the composition dependence of the lattice parameters of alkali feldspar, the diffusion induced compositional heterogeneity induces coherency stress and elastic strain. Electron back-scatter diffraction combined with the cross-correlation technique was employed to determine the lattice strain distribution across the Na-K interdiffusion fronts in partially exchanged single crystals of alkali feldspar. The strain changes gradually across the broad fronts within thea-c-plane, with a successive extension primarily ina-direction conferring to the composition strain in unstressed alkali feldspar. In contrast, lattice strain characterised by pronounced extension inb-direction is localised at the sharp diffusion fronts parallel tob, followed by a slight expansion ina-direction in the orthoclase-rich rim. This strain pattern does not confer with the composition induced lattice strain in a stress-free alkali feldspar. It may rather be explained by the mechanical coupling of the exchanged surface layer and the mechanically strong substratum. The lattice distortion localised at the sharp diffusion front may have an influence on the diffusion process and appears to produce a self-sharpening feedback, leading to a local reduction of component mobilities.
authors.affiliation: Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090  Vienna, Austria.
authors.forename: Anne-Kathrin
authors.initials: AK
authors.lastname: Schäffer
authors.affiliation: Max-Planck Institute for Iron Research, Max-Planck-Strasse 1, 40237 Düsseldorf, Germany.
authors.forename: Tom
authors.initials: T
authors.lastname: Jäpel
authors.affiliation: Max-Planck Institute for Iron Research, Max-Planck-Strasse 1, 40237 Düsseldorf, Germany.
authors.forename: Stefan
authors.initials: S
authors.lastname: Zaefferer
authors.affiliation: Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090  Vienna, Austria.
authors.forename: Rainer
authors.initials: R
authors.lastname: Abart
authors.affiliation: Helmholtzzentrum Potsdam, Deutsches GeoForschungsZentrum, Telegrafenberg, 14473 Potsdam, Germany.
authors.forename: Dieter
authors.initials: D
authors.lastname: Rhede
keywords: Alkali feldspar; Coherency strain; EBSD; Interdiffusion