The structure of dislocations in quartz under electron irradiation

It has been observed that, using the weak-beam technique, the images of a-type dislocations in quartz split into two peaks under electron beam irradiation at 100 KV, the peak separation increasing to about 250 A. This result was interpreted as the dissociation of the dislocations into two partials, which are able to reach their equilibrium separation as a result of enhanced mobility promoted by ionisation damage. To follow up these experiments specimens of naturally-deformed coarse-grained quartz from shear zones in the Cap de Creus area in N.E. Spain were examined. Experiments were carried out at room temperature and at elevated temperatures above the α-β transition at 5730C in 100 KV and 200 KV TEMs and at 800 KV in an HVEM. Radiation damage was minimised by using low levels of illumination and by setting up diffraction conditions away from the area of interest. Typical weak-beam conditions for g = brace 1 0 bar1 0 brace at 100 KV were (g, 9g) giving deviation parameters ssub(g) approximately 8 x 10-3 A-1 and w approximately 13. The conditions were not quite so favourable in the HVEM, but even so dislocation images approximately 20 A wide could be obtained and dynamic observations made using video recording and an image intensifier. The results are discussed. (author)