Transmission electron microscopy of coesite inclusions in the Dora Maira high-pressure metamorphic pyrope-quartzite

Inclusions composed of relict coesite and retrograde quartz in garnet of a pyrope-quartzite from the Dora Maira massif were examined in transmission electron microscopy. Although coesite has suffered very little plastic deformation, the Burgers vectors of dislocations could be identified. They are [100], [001], [110] (i.e., a, c, and a+b), and other symmetrically almost identical vectors. The (110) plane could be identified as a slip plane. The dislocations are very straight, pointing to a high Peierls stress. Some small prismatic dislocation loops with Burgers vector [010] are observed, possibly formed by water-related defects. Retrograde quartz occurs at the margin of coesite with a palisade texture and as veins within coesite. Palisade quartz exhibits a high density of dislocations pinned on bubbles, and Brazil twins parallel to {1011} planes. The quartz veins are aligned parallel to the (100) and (021) composition planes of twins in coesite crystals. These observations strongly suggest that back-transformation of coesite into quartz starts at grain and twin boundaries, and that coesite loses water to quartz during transformation.