Jf. Hippertt et Jv. Valarelli, MYRMEKITE - CONSTRAINTS ON THE AVAILABLE MODELS AND A NEW HYPOTHESIS FOR ITS FORMATION, European journal of mineralogy, 10(2), 1998, pp. 317-331
Based on a myrmekite occurrence around K-feldspar megacrysts (porphyro
blasts) in a mylonitic augen gneiss from southeast Brazil, a critical
discussion on myrmekite genesis highlights some constraints on the ava
ilable myrmekite models, mainly with regard to the replacement directi
on in the K-feldspar/myrmekite interface. In the investigated augen gn
eisses, chemical and microstructural data indicate that myrmekite does
not replace K-feldspar (as generally inferred in other myrmekite occu
rrences), but in fact is replaced by it. Alternatively, myrmekite is s
uggested to form by SiO2 infiltration of pre-existing plagioclase grai
ns during K-feldspar porphyroblastesis. The local stress induced by vo
lume change during plagioclase replacement by K-feldspar enhances defo
rmation around the growing porphyroblast, enabling SiO2 infiltration t
o 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-f
eldspar porphyroblast.