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Stishovite is likely to be present in subducted oceanic crust from a depth of about 300 km throughout the mantle transition zone and into the lower mantle. Transformation from the low-pressure polymorphs of silica to stishovite involves a change in the coordination of Si from four-fold to six-fold. Stishovite is therefore an ideal prototype structure to study the effect of coordination and bonding changes on the plasticity of a material. This information is critical for understanding the rheology of the Earth's mantle.

We have first determined the nature of dislocations that are stable in stishovite. Defects present in a sample synthesized at 15 GPa and 1200°C have been characterized by transmission electron microscopy and Large Angle Convergent Beam Electron Diffraction (LACBED). The latter technique enables the Burgers vectors of dislocations in beam sensitive materials like stishovite to be determined precisely and unambiguously. We find that <100>, <001>, <110> and <101> dislocations occur in stishovite (an example of a Burgers vector determination is shown in Fig. 3.1-12.
 

Fig. 3.1-12: Example of a Burgers vector determination perfomed in stishovite by Large Angle Convergent Beam Electron Diffraction. Case of a dislocation crossed with 150, and Bragg lines.

In the course of this study, we have synthesized coarse-grained stishovite with the aim of studying plastic deformation involving dislocation creep. Corning grade G silica glass was used as a starting material. This glass is relatively wet with an OH content (determined by FTIR spectroscopy) of 2850±40 ppm H/Si. Glass cylinders were contained in a Re capsule and annealed in the multianvil apparatus at 14 GPa and 1300°C for 10 hours. The recovered material consists of large needle-shaped grains (Fig. 3.1-13). FTIR spectroscopy shows that,
 

Fig. 3.1-13: SEM image showing the morphology of a stishovite single crystal synthesized at 14 GPa and 1300°C from wet silica glass. The needle is elongated along [001], with the lateral faces parallel to {110}.

despite the water content of the starting material, no water-related species are incorporated in the stishovite. Following synthesis, the recovered samples were deformed in the multianvil apparatus at 1400°C over a range of temperatures. A sample assembly incorporating hard Al₂O₃ pistons was used in order to generate high deviatoric stresses.

Preliminary investigations of a sample deformed at 1300°C for 1 hour show that two deformation mechanisms are competing. Optical examination reveals that some grains exhibit undulatory extinction, suggesting that dislocation creep has been activated. This is confirmed by TEM which shows dislocations in glide configuration (Fig. 3.1-14). The second
 

Fig. 3.1-14: TEM bright field micrograph showing <001> dislocations gliding in a {110} plane and a subgrain boundary parallel to (001). g: 002.

deformation mechanism involves twinning on (101) (Fig. 3.1-15). This is the first observation of deformation twinning in stishovite. Further TEM investigations of the twins and dislocation structures are being undertaken to provide precise information on the deformation mechanisms.
 

Fig. 3.1-15: Optical micrograph (crossed nicols) showing deformation twins parallel to (101).