Page 45 - Microtectonics

3.4 · Intracrystalline Deformation 33

Box 3.3 Evidence for intracrystalline deformation
Individual dislocations cannot be observed with an optical micro- and Tuttle 1945; Carter 1971; Christie and Ardell 1974; Drury 1993;
scope. However, the effect of the presence of dislocations in a crystal Figs. 3.10, 3.18, ×Video 3.18) also known as Fairbairn lamellae
lattice may be visible. A crystal lattice which contains a large number (Groshong 1988; Wu and Groshong 1991a). Deformation lamellae
of similar dislocations can be slightly bent; as a result, the crystal consist of dislocation tangles, small elongate subgrains (Blenkinsop
does not extinguish homogeneously as observed with crossed polars; and Drury 1988; McLaren 1991; Trepmann and Stöckhert 2003),
this effect is known as undulose extinction (Figs. 3.10, 3.17, and arrays of very small solid or fluid inclusions that are only vis-
×Video 3.17). Undulose extinction can be ‘sweeping’ when it occurs ible by TEM. Deformation lamellae are particularly common in
as large-scale, regular bending of the crystal due to the presence of quartz, where they usually have a sub-basal orientation. How de-
dislocations, but can also be patchy and irregular, when it is associ- formation lamellae actually develop and how they should be inter-
ated with (microscopically invisible) small fractures and kinks be- preted is only partly understood.
sides dislocation tangles (Hirth and Tullis 1992). Microkinks occur as Finally, the presence of a lattice preferred orientation has been
small isolated structures in quartz and feldspars. They are probably suggested as evidence for deformation by dislocation creep, al-
associated with cataclastic failure at sites of dislocation tangles (Tullis though in some minerals (calcite) it can also form by deformation
and Yund 1987) and are therefore indicative of dislocation glide. twinning. At elevated temperature, intracrystalline microstructures
Another effect that is commonly observed in crystals deformed such as undulose extinction and deformation lamellae may be ab-
at low temperature by intracrystalline deformation are lamellae with sent due to recovery or recrystallisation (see below). In this case,
a high optical relief which usually have a distinct preferred orien- the presence of a strong lattice preferred orientation can be taken
tation, known as deformation lamellae (Fairbairn 1941; Ingerson as evidence for dislocation creep.

Fig. 3.11.
a Lattice with two types of point
defects. b Edge dislocation de-
fined by the edge of a half-plane
in a distorted crystal lattice.
c Screw dislocation defined by a
twisted lattice. d Dislocation
with edge and screw dislocation
regions in a crystal. A square
itinerary of closed arrows around
the dislocation is used to find
the Burgers vector of the dislo-
cation, indicated by open arrows

Fig. 3.12.
a The principle of movement of a vacancy. b Horizontal
shortening of a crystal by displacement of vacancies
from right side of the crystal to the top. Black arrows
indicate movement of vacancies