Cordierite microstructure and texture in a moldanubian gneiss

The microstructure and texture in cordierites of a moldanubian gneiss from the Bohemian Massif has been analysed by transmission electron microscopy (TEM) and universal stage in order to get information on the deformation mechanisms and textural development of this rock-forming mineral. Deformation may have taken place at temperatures between about 500° C and 630° C and pressures smaller than about 3 kb. The elongated cordierite xenoblasts show a typical dislocation creep microstructure consisting of subgrain boundaries and free dislocations. The dislocations have 001], 010] and 1/2<110> Burgers vectors. 001] dislocations often have pure screw and edge character the latter type being climb-dissociated on (001). Among the dislocations reactions are common. The main subgrain boundaries observed are (010)001], {110}001] and (001)010] tilt boundaries. Burgers vectors and dislocation line directions reveal (100)001], (010)001], (100)010], {110} 1/2<110> and (001)1/2<110> as activated slip systems. The crystallographic preferred orientation (here referred to as texture) consists of a 001] maximum in the foliation parallel to the mineral lineation. 100] and 010] maxima are perpendicular to it within and normal to the foliation, respectively, with a girdle tendency normal to the lineation. The texture may be explained by simple shear deformation on the {hkO}001] slip systems with preference of (010)001].