An electron-microscopic and X-ray study of complex exsolution textures in a cryptoperthitic alkali feldspar
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The configurations and geometrical relations of the exsolution lamellae from a cryptoperthite from the Wausau, Wisconsin syenite have been studied by transmission electron microscopic and X-ray methods. By using bright and dark field techniques it has been possible to relate the lattice geometries, the configurations of the exsolved regions and their composition planes. Two major configurations were found of alternating sodic and potassic lamellae: (a) albite-twinned low albite and maximum microcline in diagonal configuration with the boundary plane near \((\bar 8\bar 61)\): (b) pericline-twinned low albite and finely twinned M-type microcline with the boundary plane near \((\bar 801)\). The second configuration appears to be more abundant. The two configurations do not appear to alternate periodically and the numbers of lamellae in each configuration appears to vary from grain to grain. The twins in the low-albite lamellae are periodic and give rise to satellite spots.
The lattice angles appear to be nearly normal for the various regions, whereas the cell edges are strained. The diffraction spots are joined by streaks in some cases. The boundaries are thus essentially coherent. The b*axis for low albite in configuration (a) is parallel to b in configuration (b), although they occur in different areas.
KeywordsDark Field Boundary Plane Diffraction Spot Cell Edge Geometrical Relation
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