5 - The observation of crystal defects

Summary

In this chapter we discuss the origin and nature of the contrast arising from the main types of crystal defect. We also show how, in principle, the parameters characterizing these defects (such as the Burgers vector of a dislocation) can be determined from the images observed under different diffracting conditions. Some types of distortion of a perfect crystal that are frequently encountered in important rock-forming minerals (such as spinodal decomposition and exsolution) will not be discussed specifically here because we can easily understand their contrast in terms of the ideas to be developed for the main types of defect. This chapter is concerned with the principles of the observation of crystal defects; specific examples will be given in Chapters 8 and 9.

General ideas

We saw in Chapters 3 and 4 that contrast is observed in a perfect crystal if it is bent, giving rise to bend contours, or if it varies in thickness, giving rise to thickness fringes. The ways in which contrast from crystal defects arise are closely related to the origins of bend contours and thickness fringes. If the crystal planes in the immediate neighborhood of a crystal defect are bent, then the diffracted intensity from this distorted region will be different from that of the surrounding perfect crystal, and contrast will arise analogously to that from a bend contour.