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Lattice strain across Na-K interdiffusion fronts in alkali feldspar: an electron back-scatter diffraction study.

Schäffer AK, Jäpel T, Zaefferer S, Abart R, Rhede D - Phys Chem Miner (2014)

Bottom Line: Diffusion fronts developing in b-direction are very sharp, while diffusion fronts within the a-c-plane are comparatively broad.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090  Vienna, Austria.

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 in b-direction are very sharp, while diffusion fronts within the a-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 the a-c-plane, with a successive extension primarily in a-direction conferring to the composition strain in unstressed alkali feldspar. In contrast, lattice strain characterised by pronounced extension in b-direction is localised at the sharp diffusion fronts parallel to b, followed by a slight expansion in a-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.

No MeSH data available.


Related in: MedlinePlus

Relation between lattice direction and instrumental reference frame for samples with sharp diffusion fronts (left) and broad diffusion fonts (right)
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Fig4: Relation between lattice direction and instrumental reference frame for samples with sharp diffusion fronts (left) and broad diffusion fonts (right)

Mentions: In cases where the shifts towards more potassium-rich compositions do not induce fracturing, the lattice misfit across the diffusion fronts is accommodated by elastic strain. The associated lattice distortion was investigated by EBSD combined with the cross-correlation method. Lattice strain across wide and sharp diffusion fronts is treated separately. It must be noted that the samples were in different orientation with respect to the instrumental reference frame for the analysis of lattice strain across the sharp and broad fronts (Fig. 4). For the analysis of the sharp fronts, the longitudinal strain corresponds to the b-direction, and corresponds to the a direction. In contrast, for the analysis of broad fronts, the longitudinal strain corresponds to the a-direction, and corresponds to the b-direction.Fig. 4


Lattice strain across Na-K interdiffusion fronts in alkali feldspar: an electron back-scatter diffraction study.

Schäffer AK, Jäpel T, Zaefferer S, Abart R, Rhede D - Phys Chem Miner (2014)

Relation between lattice direction and instrumental reference frame for samples with sharp diffusion fronts (left) and broad diffusion fonts (right)
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC4509562&req=5

Fig4: Relation between lattice direction and instrumental reference frame for samples with sharp diffusion fronts (left) and broad diffusion fonts (right)
Mentions: In cases where the shifts towards more potassium-rich compositions do not induce fracturing, the lattice misfit across the diffusion fronts is accommodated by elastic strain. The associated lattice distortion was investigated by EBSD combined with the cross-correlation method. Lattice strain across wide and sharp diffusion fronts is treated separately. It must be noted that the samples were in different orientation with respect to the instrumental reference frame for the analysis of lattice strain across the sharp and broad fronts (Fig. 4). For the analysis of the sharp fronts, the longitudinal strain corresponds to the b-direction, and corresponds to the a direction. In contrast, for the analysis of broad fronts, the longitudinal strain corresponds to the a-direction, and corresponds to the b-direction.Fig. 4

Bottom Line: Diffusion fronts developing in b-direction are very sharp, while diffusion fronts within the a-c-plane are comparatively broad.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.

View Article: PubMed Central - PubMed

Affiliation: Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090  Vienna, Austria.

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 in b-direction are very sharp, while diffusion fronts within the a-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 the a-c-plane, with a successive extension primarily in a-direction conferring to the composition strain in unstressed alkali feldspar. In contrast, lattice strain characterised by pronounced extension in b-direction is localised at the sharp diffusion fronts parallel to b, followed by a slight expansion in a-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.

No MeSH data available.


Related in: MedlinePlus