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Myrmekite formed by Na- and Ca-metasomatism of K-feldspar

Myrmekite, an intergrowth of plagioclase and quartz vermicules, often occurs in felsic-to-intermediate, calc-alkalic plutonic and gneissic rocks. Several hypotheses for myrmekite genesis are briefly reviewed and discussed. The proportional relationship between the volume of quartz vermicules and the An value of plagioclase is consistent with the hypothesis or replacement, solid state exsolution, and recrystallization of primary plagioclase. (1) Perthitic albite and K-feldspar relicts in myrmekite and (2) swapped myrmekite between two differently oriented K-feldspar crystals obviously indicate that myrmekite is formed by nibble replacement of K-feldspar by myrmekitic plagioclase because of modification by Ca-bearing sodic emanation or fluid. Myrmekite of different morphology and spatial distribution are of the same metasomatic origin. The myrmekite that was produced during deformation should be later than that formed prior to deformation.

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