Abstract
The lattice imaging technique of high resolution electron microscopy has been applied on anorthite (An96) and labradorite (An52) to examine their antiphase boundaries (APBs) and superstructure relations. The effects of the structure details and superstructure relations on the lattice images change with the thickness of the specimens. The lattice images of the areas with the usual thickness (>150 Å) generally represent not the structure details but the superstructure relations.
In anorthite the type b-APBs are rather sharp and narrow with the displacement vector of 1/2[110]An. The type c-APBs have the displacement vector of 1/2[111]An. The existence of the I \(\bar 1\) phase in the P \(\bar 1\) matrix is not observed for both types in this study.
In labradorite, the superstructure is coherent small-scale intergrowth of the anorthite-like and albite-like bands with the interval of 30 Å corresponding to the t vector of the e 1-e 1 pairs. The thickness of both bands in the lattice images indicates that the anorthite-like and albite-like regions have approximate compositions of An80 and An5, respectively. The ordered arrangement of the subcells in each anorthite-like band, due to the reversal arrangement of Si and Al in the tetrahedral sites, is in antiphase relation with the next anorthite-like band separated by an albite-like band, resulting in a periodic antiphase structure.
The type b-APBs in anorthite and the periodic APBs in labradorite are characterized with the displacement vector R=1/2[110]An=b 0.
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Nakajima, Y., Morimoto, N. & Kitamura, M. The superstructure of plagioclase feldspars. Phys Chem Minerals 1, 213–225 (1977). https://doi.org/10.1007/BF00307319
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DOI: https://doi.org/10.1007/BF00307319