Myrmekite: constraints on the available models and a new hypothesis for its formation
Hippertt, João F.; Valarelli, José V.
European Journal of Mineralogy Volume 10 Number 2 (1998), p. 317 - 332
published: Mar 31, 1998
manuscript accepted: Nov 17, 1997
manuscript received: Mar 20, 1996
ArtNo. ESP147051002011, Price: 29.00 €
Abstract Based on a myrmekite occurrence around K-feldspar megacrysts (porphyroblasts) in a mylonitic augen gneiss from southeast Brazil, a critical discussion on myrmekite genesis highlights some constraints on the available myrmekite models, mainly with regard to the replacement direction in the K-feldspar/myrmekite interface. In the investigated augen gneisses, chemical and microstructural data indicate that myrmekite does not replace Kfeldspar (as generally inferred in other myrmekite occurrences), but in fact is replaced by it. Alternatively, myrmekite is suggested to form by SiO2 infiltration of pre-existing plagioclase grains during K-feldspar porphyroblastesis. The local stress induced by volume change during plagioclase replacement by K-feldspar enhances deformation around the growing porphyroblast, enabling SiO2 infiltration to occur along the discontinuities (microcracks and kink boundaries) of deformed plagioclase grains. The development of quartz vermicules in the K-feldspar vicinity accounts for the access of SiO2 necessary for plagioclase replacement by K-feldspar, enabling the advance of the K-feldspar porphyroblast.