Chloritisation of hornblende and biotite: a HRTEM study

The coarse textures produced by hydration of hornblende and biotite during retrograde metamorphism of the rocks of the Klodzko-Zloty Stok area were identified easily by both optical microscopy and scanning electron microscopy (SEM). However, the micro-textures that are a few unit cell in dimensions were identified through high resolution transmission electron microscopy (HRTEM) only. For the alteration of biotite to chlorite the replacement of potassium in the interlayer by a brucite-like layer and the subsequent increase in volume was observed. There were, however, some missing brucite-like layers within the chlorite units. Moreover, both topotactic and low angle replacement of biotite to chlorite were identified. For the direct hydration of hornblende to chlorite, HRTEM and selected area electron diffraction pattern (SAED) showed that the c* and a* of chlorite were parallel to the b* and a* of hornblende, respectively. Nevertheless, other orientation relationships were also observed by optical microscopy.