Geology Reference
In-Depth Information
core, including at kinks and jogs. It may result from the imbalance of positive and
negative ions along the boundary of the ''half-slab'' that terminates in the core of a
dislocation with an edge component in a pure crystal (intrinsic charging) or from
the presence of impurity ions of different charge (extrinsic charging). Straight
dislocations of orientations that would involve intrinsic charging will tend not to
occur because of the high associated electrostatic energy (leading to bowing-out
instability) but intrinsic charging at jogs, kinks, and points of emergence may still
arise in dislocations that would be intrinsically neutral if perfectly straight (for a
detailed discussion of intrinsic charging of dislocations in the NaCl structure see
Amelinckx
1979
, p. 379). Extrinsic charging, at least in alkali halides, often
appears to arise by a ''sweeping up'' of charge during the movement of the dis-
location, in addition to any initial charge from bound point defects (Whitworth
1975
). In the alkali halides, in which there tends to be a preponderance of cation
vacancies due to their formation energy being lower than that of anion vacancies,
the dislocation charge is generally negative in both nominally pure crystals and in
those doped with divalent cations, but positive charge can arise in other cases and
it is possible that an isoelectric point may exist at which there is a changeover from
negative to positive charge with increase in temperature.
The presence of charge on dislocations can give rise to various effects in the
electric properties of a crystal, such as transient changes in electric conductivity
during plastic deformation because of charge transport. Also the formation of a
compensating charge cloud around a charged dislocation will tend to pin the
dislocation and lead to hardening but, at least in alkali halides, the magnitude of
this effect in the flow stress is thought to be negligible (Whitworth
1975
). Very
little is known about charge on dislocations in ionic crystals other than the alkali
and silver halides, but charging may be expected to be a widespread effect.
6.2.8 Dislocations in Minerals
Since chemical bonding in minerals is generally of a character intermediate
between ionic and covalent, it is to dislocations in ionic and covalent compounds
that one looks for guidance on the likely characteristics of dislocations in minerals,
rather than to metals, except for those properties for which the character of the
bonding is not of importance. The latter situation can be expected to hold where
the long-range elastic strain energy is the principal determining factor in the
behavior of the dislocations, but the nature of the bonding tends to enter impor-
tantly where the core properties are playing a determining role.
The following factors tend to be of importance in characterizing the disloca-
tions in minerals (Nabarro
1984a
; Paterson
1985
):
1. Commonly the unit cell is fairly large and contains many atoms of several
kinds, resulting in large Burgers vectors (0.5-1 nm typically) and complex
structure of the dislocation cores, with dissociation or zonal spreading expected
to be a common feature.
