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Mg₂SiO₄ ringwoodite was synthesised from a forsterite powder loaded in a rhenium capsule fitted in a 10/4 multianvil assembly and annealed for 240 min at 22 GPa and 1500-1700°C. After the run, the specimen was recovered and put in a second high-pressure (10/4) assembly designed to induce plastic deformation. Two deformation experiments have been performed: the first one run for 120 min at 22 GPa and 1000°C, the second one for 60 min at 22 GPa and 1400°C.

Fig. 3.1-6: Sample deformed at 22 GPa, 1000°C. Dislocations in glide configuration. Transmission electron micrograph. Weak beam dark field with g : 022.

Both samples show clear evidences for dislocation activity (Fig. 3.1-6). Dense deformation bands are more frequent in the sample deformed at 1000°C. In both cases, dislocations are in glide configurations. They show no evidence of climb structures nor dissociation. These microstructures are very similar to those observed in MgAl₂O₄ deformed at lower temperatures and lower strains.

The slip direction in the spinel structure is always observed to be parallel to <110>, i.e. 1/2<110> is the Burgers vector of the perfect dislocations. In our samples, the dislocations are found to glide in several planes: {111} and {110} (whatever the temperature of the experiment).