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Line defects are called dislocations. As lines, they possess extent in one direction and are therefore one‐dimensional defects. Dislocations commonly result from shearing stresses produced in crystals during deformation. These stresses cause atomic planes to shift position, producing distortions in the crystal lattice that can be represented by a line called a dislocation line. Two major types of dislocations are recognized: edge dislocations (Figure 4.33a) and screw dislocations (Figure 4.33b).

Figure 4.32 (a) Perfect crystal lattice; (b) substitution defect; (c) interstitial defect; (d) omission defect.

Figure 4.33 (a) Edge dislocation with an extra half plane of atoms; this is a line defect because the base of the half plane can be represented by a dislocation line (⊥). (b) Screw dislocation, where a plane of atoms has been rotated relative to the adjacent plane.

Source: Klein and Hurlbut (1985). © John Wiley & Sons.

Dislocations are extremely important in the plastic deformation of crystalline materials that leads to changes in rock shape and volume without macroscopic fracturing. Dislocations permit rocks to flow plastically at very slow rates over long periods of time. Detailed discussions are available in many books on mineralogy (e.g., Wenk and Bulakh 2016) and structural geology (e.g., van der Pluijm and Marshak 2004; Davis et al. 2011). Figure 4.34 shows how an edge dislocation can migrate through a crystal by breaking a single bond at a time. The result of dislocation migration is a crystal shape change accomplished without rupture. Dislocations are important plastic deformation processes allowing rocks to change shape without visible rupture (Chapter 16).

Figure 4.34 Two‐dimensional depiction of how an edge dislocation created by slip due to shear can migrate through a crystal by breaking one bond at a time, so that no fractures develop as the crystal changes shape during deformation (steps 1–6).

Source: Adapted from Hobbs et al. (1976). © John Wiley & Sons.

Figure 4.35 Three types of planar defect (shown in two dimensions): (a) intergranular grain boundary between two different crystals; (b) intragranular mechanical twin boundary resulting from mechanical slip; (c) intragranular subgrain boundary within a crystal, separated by a wall of dislocations. Imagine each extending in a second dimension perpendicular to the page and note how (b) and (c) accommodate changes in crystal shape.