Die Albit/Mikroklin-Orientierungs-Beziehungen in Mikroklinperthiten und deren genetische Deutung

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Abstract
The orientation of the albite component relative to the K feldspar host was determined by x-ray methods in macro-, micro- and cryptoperthites from 27 different localities. Two main types (with intermediate orientations) were found: Type I , The orientation of the albite indicates separation from a “monoclinic” K feldspar (macro-, micro- and partly cryptoperthite). This orientation was observed when the K feldspar appeared optically “monoclinic” or when the microcline twin structure appeared diffuse and without sharp boundaries. Type II , The orientation shows no relation to an original monoclinic state; on the contrary the albite shows such relations to the microcline that it must be considered to have separated from triclinic microcline (cryptoperthite in larger microcline lamellae which show sharp boundaries and cryptoperthite in large microcline areas which may already be viewed as single crystals). On the basis of the x-ray results the following processes are mainly responsible for the formation of the perthites examined (mostly of pegmatitic character): 1. Formation of relatively coarse “vein perthite” (A ndersen ) through unmixing from a sanidine state; 2. Formation of “film perthite” (A ndersen ) and “monoclinically oriented” cryptoperthite through unmixing from an “orthoclase” state; 3. Formation of “triclinically oriented” cryptoperthite through unmixing from a microcline state; 4. Recrystallization of twinned microcline and of perthitic albite to “single crystal” microcline with albite inclusions, the orientation of which is such that the Al/Si distribution in the microcline and in the albite inclusions is “topologically coherent” (“plate perthite”). The cleavelandites with random orientation to the K feldspar are considered as a relatively late albitization coming from outside.

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