5. Myrmekite Formed by Exsolution?

At the first site, the Weinsberg (Moldanubian) granite and an older quartzmonzonite facies of the South Bohemian pluton in Austria both contain orthopyroxene and clinopyroxene and must have crystallized from magmas at high temperatures. The granite contains orthoclase megacrysts (as much as 10 to 20 cm long), which (1) are chemically zoned with Ba-rich cores (1 to 2 % mol Cs; and locally as much as 6 to 8 % Cs) and (2) are mineralogically zoned with aligned inclusions of plagioclase whose subhedral to euhedral faces are parallel to potential faces of the orthoclase (from illustrations at poster session, III Hutton Symposium, Koller and Kloetzli, 1995; personal communication, Koller, 1996). The megacrysts are perthitic and locally are bordered by small bulbous myrmekite with tiny quartz vermicules. Plagioclase lamellae are absent in the orthoclase adjacent to the myrmekite. The myrmekite is also associated with symplectic biotite which Koller and Kloetzli (1995) suggest is formed by the breakdown reaction orthopyroxene + K-feldspar + water = biotite + quartz.