An electron optical study of muscovite breakdown in pelitic xenoliths during pyrometamorphism

M 13 9PL ABSTRACT. Transmission electron microscopy and analytical electron microscopy have been used to study the breakdown reaction of muscovite during pyrometa-morphism. The transformation has been shown to be topotactic n nature, with the orientations ofthe product biotite and alkali feldspar being strongly controlled by the precursor phase. Microprobe investigations i dicate that the reaction is probably isochemical with no detectable interaction with adjacent phases. Chemical nalyses ofthe products carried out by AEM have enabled a possible balanced reaction to be calculated. It is proposed that the spatial distribution of the product minerals can be interpreted in terms of diffusion of species from one domain in the crystal to another over distances of5 to 10 #m as a result of chemical potential gradients which develop during the reaction. The reaction probably occurred under conditions of declining temperature between 900 and 750 ~ over a period of 4 5 days. This time period was insufficient toenable the reaction to reach completion as indicated by the presence of relics of muscovite coexisting with K-feldspar, corundum, biotite, hercynite, and mullite. K E Y W O R D S " muscovite breakdown, electron microscopy, pelitic xenoliths, pyrometamorphism. I N recent years transmission electron microscopy (TEM) has been increasingly used to study the mechanisms of mineral transformations on the microstructural level. Almost without exception these studies have been restricted to phenomena which occurred during the extended cooling histories of most igneous and metamorphic rocks, for example, exsolution, order-disorder trans-formations and reactions involving rehydration