Page 52 - Microtectonics

40 3 · Deformation Mechanisms
in the crystal lattice. The general term recovery is com- Fig. 3.24. TEM image of a small dislocation free amphibole grain in a ▼
monly used to cover these mechanisms of reducing dis- plastically deformed old amphibole grain with high dislocation den-
location density. sity (Cumbest et al. 1989). This microstructure is consistent with
static recrystallisation. Dynamically recrystallised grains would be
Dislocations in a crystal can be grouped into regular
deformed and contain some free dislocations. Senja, Norway. Recrys-
planar networks as a result of recovery (Figs. 3.13, 3.21; tallisation at 520–540 °C from amphibole plagioclase geothermo-
×Video 3.21a). These networks are known as subgrain walls metry. (Photograph courtesy Randy Cumbest and Martyn Drury)
or subgrain boundaries (Fig. 3.13, 3.21). Such boundaries
separate crystal fragments known as subgrains, which are
slightly misoriented with respect to their neighbour sub- vectors. Complex walls have an oblique rotation axis and
grains or to the host grain (Fig. 3.22, 3.23). The orienta- consist of networks of dislocations having two or more
tion of a subgrain boundary depends on the orientation different Burgers vectors.
of the slip system of the dislocations that accumulate in it Once formed, subgrain boundaries can migrate to
(Trepied et al. 1980). some extent (Means and Ree 1988) or evolve into grain
A subgrain boundary can be imagined as a plane sepa- boundaries by addition of more dislocations (Sect. 3.7.3).
rating two crystal fragments that have rotated slightly Box 3.6 lists evidence for recovery in thin section.
with respect to each other; such boundaries can there-
3.7 fore be classified according to the orientation of the ro- 3.7
tation axis. Subgrain boundaries with rotation axes par- Recrystallisation
allel and normal to the boundary are known as tiltwalls
and twistwalls respectively. A tiltwall is shown in Fig. 3.21 3.7.1
and consists of an array of edge dislocations with the Grain Boundary Mobility
same Burgers vector. A twistwall consists of two inter-
secting sets of screw dislocations with different Burgers Besides recovery another process, grain boundary mo-
bility, can contribute to the reduction of dislocation den-
Box 3.6 Evidence for recovery sity in deformed crystals (Poirier 1985; Gottstein and
Mecking 1985; Drury and Urai 1990; Jessell 1987). Imag-
In response to recovery, dislocations tend to concentrate in pla-
nar zones in the crystal, decreasing dislocation density in other ine two neighbouring deformed crystals, one with high
parts. In thin section, this results in the occurrence of zones in and one with low dislocation density (Figs. 3.24, 3.26a).
the crystal which have approximately uniform extinction, and Atoms along the grain boundary in the crystal with high
which grade over a small distance into other similar crystal dislocation density can be displaced slightly so that they
sectors with a slightly different orientation. These transition fit to the lattice of the crystal with low dislocation den-
zones are known as deformation bands (Fig. 3.21). They can be sity. This results in local displacement of the grain bound-
regarded as a transitional stage between undulose extinction
and subgrain boundaries (×Video 3.21b). ary and growth of the less deformed crystal at the cost of
Subgrains (Figs. 3.13, 3.22, 4.26, ×Video 3.22) can be recog- its more deformed neighbour (Figs. 3.24, 3.26a, inset).
nised as parts of a crystal which are separated from adjacent The process may increase the length of grain bounda-
parts by discrete, sharp, low relief boundaries. The crystal lat- ries and thereby increase the internal free energy of the
tice orientation changes slightly from one subgrain to the next, crystal aggregate involved, but the decrease in internal
usually less than 5° (Fitz Gerald et al. 1983; White and Mawer
1988). Subgrains can be equant or elongate (Fig. 4.26). In many free energy gained by removal of dislocations is greater.
cases, subgrain walls laterally merge into deformation bands As a result, new small grains may replace old grains. This
or high-angle grain boundaries (Fig. 3.28). reorganisation of material with a change in grain size,
It is also important to note that recovery in bent crystals as shape and orientation within the same mineral is known
described above is only one of the possible mechanisms to form as recrystallisation (Poirier and Guillopé 1979; Urai et al.
subgrains; alternative, though possibly less common mecha- 1986; Hirth and Tullis 1992). In solid solution minerals
nisms are sideways migration of kink band boundaries, the
reduction of misorientation of grain boundaries and impinge- such as feldspars, recrystallisation may be associated with
ment of migrating grain boundaries (Means and Ree 1988). changes in composition, which may be an additional driv-
Fracturing, rotation and sealing by growth from solution may ing force for the process (Sect. 3.12.4). There are three
also play a role in the development of some subgrains in quartz different mechanisms of recrystallisation that can oper-
(den Brok 1992). If crystals are separated into strongly undulose ate during deformation depending on temperature and/
subgrains of slightly different orientation but with fuzzy
boundaries, and if such crystals contain fractures, the ‘subgrain’ or flow stress. With increasing temperature and decreas-
structure may be due to submicroscopic cataclasis of the grains ing flow stress these are: bulging, subgrain rotation, and
(Tullis et al. 1990); such subgrain-like structures and even high temperature grain boundary migration recrystalli-
undulose extinction can form by dense networks of small frac- sation (Figs. 3.25, 3.26; Urai et al. 1986; Wu and Groshong
tures. Only TEM work can show the true nature of the struc- 1991a; Hirth and Tullis 1992; Dunlap et al. 1997; Stipp
ture in this case.
et al. 2002).