In 1981, Chopin reported the first discovery of metamorphic coesite in a pyrope-quarzite from the Dora Maira Massif, Alps. The presence of relic coesite indicates that the metamorphic host rock was subducted to depths greater than 100 km, at peak metamorphic conditions of 3.7 GPa and 800 °C. The coesite occurs as inclusions in garnets and is surrounded by a rim of retrograde quartz, displaying usually a palisade structure.

The knowledge of the mechanical properties of coesite is important for understanding the rheology and deformation behaviour of lower crustal rocks and deeply subducted crustal slabs. Almost nothing is known however about the rheological laws and slip systems of coesite. Therefore, we conducted a transmission electron microscopy (TEM) study and characterized the dislocations in Dora Maira coesite. Furthermore, the TEM study was aimed to gather structural information on the back-transformation to quartz.

Fig. 3.4-4: Dark-field TEM image of dislocation nodes in Dora Maira coesite. Analysis of the node configuration reveals the reaction between a and a+b dislocations.

Coesite from Dora Maira exhibits very few signs of plastic deformation; dislocation densities are always less than 10¹¹ m^-2. Most dislocations are straight and presumably sessile but there are also few examples of dislocation loops, nodes and dipoles (Fig. 3.4-4). The Burgers vectors of dislocations in monoclinic coesite are [100], [001], [110] (i.e., a, c, and a+b) and other symmetrically almost identical vectors. In the hexagonal setting, these Burgers vectors correspond to a and a+c, just as in quartz. The (110) plane could be identified as a slip plane. Small prismatic dislocation loops with Burgers vector [010] (c in the hexagonal setting) are also observed and are possibly formed by water-related defects.

Fig. 3.4-5. Bright-field TEM image of Brazil twins parallel to planes in palisade quartz.

Retrograde quartz occurs at the margin of coesite with the palisade texture and as discrete veins within coesite. Palisade quartz exhibits a high dislocation density of dislocations pinned on bubbles and Brazil twins parallel to planes (Fig. 3.4-5). The twin planes are decorated with a large number of water-related bubbles. The quartz veins are aligned parallel to (100) and (021) composition planes of rotation twins in coesite. These observations indicate that back-transformation starts at grain and twin boundaries, and that coesite loses water to quartz during transformation. Altogether, the results show that water plays an important role in both formation of dislocations and back transformation to quartz.