Geology Reference
In-Depth Information
Fig. 6.14 a Subgrains revealed in optical microscopy in plane-polarized light in naturally
deformed olivine (scale: the long edge is 1 mm) (supplied by Dr. J. D. Fitz Gerald). b Dislocation
structure marking the boundaries of a subgrain in experimentally deformed quartz (copy of Fig.
3c in Fitz Gerald et al.
1991
)
Fig. 6.15 Deformation
lamellae in synthetic quartz
revealed in plane-polarized
light with x10 objective; the
trace of (1010) is marked
(copy of Fig. 4a from
McLaren et al.
1970
)
in order to obtain an image of the dislocation configuration as it actually exists
while the crystal is under internal stress, as will be discussed further later in this
subsection.
Passing from the individual dislocation line to the spatial distribution of the
dislocation assemblage, the primary observation is that there is commonly a
marked heterogeneity in this distribution, which may take different forms. On the
one hand, there may be a variation in dislocation density without obvious variation
in the organization of the dislocations relative to each other, giving rise to a cell
structure such as illustrated in Fig.
6.12
b, in which the walls defining the cells
consist of tangles of dislocations. On the other hand, the dislocations may be
organized into walls which consist of orderly arrays containing one or more sets of
parallel dislocation lines of given sign, forming what are called subgrain bound-
aries, because of the small change in orientation across them (Fig.
6.13
). Subgrain
