Electron irradiation induced vitrification at dislocations in quartz
Recent observations using the ‘weak beam’ electron microscopy technique of split images of dislocations in quartz have been interpreted as arising either from dissociation of the dislocations into partials1, or from a glassy (metamict) phase at the core2, resulting from irradiation damage. In this letter we describe observations of lattice images of near end-on dislocations in quartz, which enable us to distinguish between these two models.
Dissociated ‘a’ dislocations in quartz
We have fully characterized by TEM the dislocations left by low temperature deformation in quartz. They are a dislocations in the basal plane with a strong preferential orientation at 60° to the Bürgers vector. Weak beam observations reveal that these dislocations are dissociated; the width of the ribbon increases under beam irradiation and reaches an equilibrium value of 250 Å. Comparisons with twins in conjunction with crystallographic and physical considerations enable a dissociation model to be proposed in terms of zonal dislocations. A slight modification to the Griggs model of hydrolytic weakening of quartz is put forward.
Dislocation structure of the deformation lamellae in synthetic quartz; a study by electron and optical microscopy
Single crystals of experimentally deformed synthetic quartz showing optical deformation lamellae were examined by transmission electron microscopy. Dislocations are distributed fairly uniformly throughout the crystal. However, parallel to the trace of the deformation lamellae, which may be irrational, there are walls of tangled dislocations whose characteristics suggest that they are directly associated with the lamellae. The nature and formation of the optical image is discussed in detail.