Cathodoluminescence microscopy of myrmekite

Rim, wartlike, and isolated myrmekite are the three types of myrmekite that occur in the Rubidoux Mountain leucogranite. Combined cathodoluminescence, scanning electron microscopy, and electron microprobe investigations suggest that myrmekite is the product of K-feldspar and plagioclase replacement. Clusters of optically continuous quartz vermicules are interconnected, exhibiting a dendroid shape. The surrounding myrmekitic plagioclase is chemically undistinguishable from the host bluish-luminescing plagioclase with a mean composition of Ab_0.81An_0.18Kf_0.01. Only the fractured cores of some plagioclase grains, which display yellowish luminescence, represent a plagioclase with a higher An content (An_0.59Ab_0.39Kf_0.2). Petrographic evidence shows that myrmekite is a complex reaction, where the main phase formed prior to exsolution of albite from K-feldspar and crystallization of albitic rims. The formation of myrmekite is therefore a reaction occurring during late-stage magmatic crystallization.