Page 49 - Microtectonics

3.5 · Twinning and Kinking 37

Box 3.4 Slip system terminology 3.5 3.5
Twinning and Kinking
A slip system in a crystal is defined by a slip plane and a direc-
tion of slip (the Burgers vector) within this plane. These elements
are usually indicated by Miller indices of the plane, followed by Some minerals can deform by deformation twinning (or
the indices of the slip direction vector, e.g. (001)[010]. Instead of mechanical twinning) in addition to dislocation creep
indices, standard abbreviation letter symbols are used in some and glide (Fig. 3.19; Jensen and Starkey 1985; Smith
cases. Notice the shape of the brackets used; if a specific plane and Brown 1988; Burkhard 1993; Egydio-Silva and Main-
and direction are indicated this is done as (plane)[direction]. A price 1999). Twinning can accommodate only a limited
set of symmetrically equivalent slip systems is indicated as
{planes}<directions>. <f ∩ r> indicates the intersection line of amount of strain and always operates in specific crystal-
f- and r-planes. If the Burgers vector does not correspond to a lographic directions, so that additional pressure solution,
unit cell length, the length can be indicated with the indices, e.g. dislocation creep or recrystallisation (see below) is
as {110}1/2<–110>. needed to accommodate large strains. In general, twin-
ning occurs in the lower temperature range of defor-
mation (Sect. 3.12). Twinning is most common in pla-
on the orientation and magnitude of the stress field in the gioclase and calcite, but also occurs in dolomite, kya-
grain and on the critical resolved shear stress (CRSS) τ for nite, microcline (Eggleton and Buseck 1980; White and
c
that slip system; τ must be exceeded on the slip system to Barnett 1990), biotite (Goodwin and Wenk 1990), quartz
c
make the dislocation move. The magnitude of τ depends (Dauphiné twinning; Barber and Wenk 1991; Lloyd 2004),
c
strongly on temperature, and to a minor extent on other diopside (Raleigh 1965; Raleigh and Talbot 1967) and
factors such as strain rate, differential stress and the chemi- jadeite (Ferrill et al. 2001). Deformation twins are com-
cal activity of certain components such as water that may monly wedge-shaped or tabular and can propagate by
influence the strength of specific bonds in a crystal. For each movement of the twin tip, or by movement of the twin
slip system this dependence is different. As a result, the types boundary into the untwinned material, where the twin
of dominant slip system that are active in a crystal change boundary remains straight. At elevated temperatures,
with metamorphic and stress conditions (Sect. 3.12). twin boundaries can bulge into the untwinned crystal,
When different slip systems intersect in a crystal, mi- except where they are pinned by grain boundaries or
grating dislocations can become entangled and their fur- other, crosscutting twins (Sect. 9.9; Fig. 9.7). This proc-
ther movement is obstructed. Dislocations may also become ess of twin boundary migration recrystallisation (Vernon
pinned by secondary phases in the crystal lattice (Fig. 3.14). 1981; Rutter 1995; Figs. 3.20, 9.7) can completely sweep
Such dislocation ‘tangles’ can inhibit movement of other the untwinned parts of grains. In this sense, it resembles
newly formed dislocations, which pile up behind the blocked other recrystallisation mechanisms but it only occurs
ones. The crystal becomes difficult to deform and hardens within grains; twin boundary migration does not cause
(Fig. 3.24). This process is referred to as strain hardening. grain growth, since grain boundaries are not affected.
If we twist a piece of steel wire, it is difficult to bend it Kinking resembles twinning but is not so strictly lim-
back into its original shape, and the wire becomes harder ited to specific crystallographic planes and directions.
to deform upon renewed bending. Eventually the wire may Kinking is common in crystals with a single slip system
snap; by bending the wire, we have caused migration and such as micas but also occurs in quartz, feldspar, amphi-
entanglement of dislocations in the lattice of the metal crys- bole, kyanite and pyroxenes at low temperature (Sect. 3.12;
tals. Strain hardening occurs also in rocks, and can enhance Bell et al. 1986; Nishikawa and Takeshita 1999, 2000; Wu
brittle failure. There are, however, mechanisms that work and Groshong 1991a). Box 3.5 lists evidence for deforma-
against strain hardening and allow ductile deformation to tion twinning in thin section.
continue. One important mechanism that allows dislocations
to pass obstruction sites is the migration of vacancies to dis-
Box 3.5 Evidence for deformation twinning
location lines (Fig. 3.16); this effectively displaces the dislo-
cation, and allows it to ‘climb’ over a blocked site. The mecha- Deformation twins can commonly be distinguished from
nism of dislocation glide with climb of dislocations is known growth twins by their shape; deformation twins are commonly
tapered, while growth twins are commonly straight and stepped
as dislocation creep. The term crystal plastic deformation is (Figs. 3.19, 9.7). Twins may be restricted to certain parts of a
used to describe deformation by dislocation creep. crystal. Growth twins are commonly bounded by zoning, while
An important effect of intracrystalline deformation is deformation twins can be concentrated at high strain sites such
the development of a lattice-preferred orientation (LPO). as the rim of crystals or sites where two crystals touch each
Since dislocations move only in specific lattice planes, a rock other. In plagioclase, growth and deformation twins occur. De-
deforming by movement of dislocations may develop a pre- formation twins commonly taper towards the crystal centre
(Fig. 3.19; Sect. 3.12.4). In calcite, most twins are deformation
ferred orientation of the grains that make up the rock. The
twins that tend to taper towards the grain boundary (Sects. 9.5.1,
development and interpretation of lattice preferred orien- 9.6.3; Fig. 9.7a).
tation is discussed in Sect. 4.4.