Myrmekite; constraints on the available models and a new hypothesis for its formation

Based on a myrmekite occurrence around K-feldspar magacrysts in a mylonitic augen gneiss from SE Brazil, a critical discussion on myrmekite genesis highlights some constraints on available models, mainly with respect to the replacement direction at the myrmekite/K-feldspar interface. In the augen gneisses studied, chemical and microstructural data indicate that myrmekite does not replace K-feldspar, but is replaced by it. Alternatively, it may be that it forms by SiO (sub 2) infiltration of pre-existing plagioclase grains during K-feldspar porphyroblastesis. Local stress induced by volume change during plagioclase replacement by K-feldspar enhances deformation around the growing porphyroblast, enabling SiO (sub 2) infiltration to occur along the discontinuities (microcracks and kink boundaries) of deformed grains of plagioclase. The development of vermicular quartz in the vicinity of K-feldspar accounts for the excess of SiO (sub 2) needed for plagioclase replacement by K-feldspar, allowing the advance of the K-feldspar porphyroblasts.