overall effect of the passage of a dislocation through a lattice is that the
crystal will change shape. Clearly edge dislocations are much more efficient
than screw dislocations as a whole part of the lattice can be displaced.
Dislocations can move along a number of different lattice planes. If several
dislocations become entangled then movement may cease. This is the cause of
strain hardening in materials. Such tangles can be removed if vacancies are
allowed to jump over the entanglement. This effect is enhanced at higher
temperatures.
The unfulfilled chemical bonds associated with lattice defects have a higher
energy than other bonds in the lattice. Hence, the energy of a crystal can be
minimised by a reduction in the number density of the defects. This is termed
recovery. If the defects migrate to the edge of the crystal then the grain size is
unchanged. However, it is generally more energetically favourable to accumu-
late the defects along surfaces within the crystals and hence form new, smaller
subgrains. Each subgrain will have a slightly different orientation from its
neighbours. The overall result of deformation and recovery is the subdivision
of larger grains into a mosaic of smaller grains. There is some evidence that the
CSDs of grain size reduced rocks may be fractal, that is they have a power-law
distribution (e.g. Armienti & Tarquini, 2002).
A much more limited amount of deformation can be accommodated by
twinning (Karato & Wenk, 2002). This effect is commonly seen in plagioclase
and calcite, but can occur in a number of other minerals. Crystals can also
deform by migration of lattice vacancies. This process is more important at
higher temperatures.
3.2.5.2 Crystal fragmentation (cataclasis)
Fragmentation or cataclasis is the mechanical breakage of crystals in response
to strain. It occurs if a crystal cannot deform fast enough by the production
and migration of lattice defects. Hence, it occurs in rocks that are strained
rapidly, or at lower temperatures. The exact threshold values for rapid strain
are variable: some minerals, such as calcite are very weak and plastically
deform easily. Other minerals, such as diamond, are strong, but may be broken
if the emplacement process is sufficiently violent.
Strain can have an external or internal origin. External strain results from
the deformation of the matrix, most commonly by other solid phases that are
in contact with the grain, but it is also possible that rapid deformation of a very
viscous liquid could induce fragmentation. Internal strain comes from changes
in the volume of different parts of the crystal in response to changes in pressure
or temperature. The most common source of internal strain is the effect of
pressure on large fluid inclusions. The liquid and/or gas in inclusions has a very
70 Grain and crystal sizes