Geology Reference
In-Depth Information
applied to sea ice in which there are trapped brine pockets,
and the grains have substructures in the form of platelets
or subgrains separated by boundaries across which there
are mismatches of the crystalline lattice. Since the width
of the sea ice subgrains are usually less than 1 mm, the
surface does not allow sufficient space for the develop-
ment of well‐defined individual Higuchi pits
Higuchi
[1957, 1958], however, mistakenly assumed the
pits to form as a result of etching at the intersections of
dislocations (line defects) with the surface. It was neither
realized by him nor by numerous investigators who used
his technique that the pits were not due to any etching
effect at the points of emergence of lattice line defects. It
was
Kuroiwa and Hamilton
[1963] who showed that the
etch pits produced by the Higuchi method had no direct
correlation with crystal imperfections but were produced
by “evaporation” through tiny holes in the film. They
used the word “evaporation,” but the correct word for
the phenomenon is “sublimation.” It is a solid‐to‐vapor
transformation or sublimation. Therefore, the term “subli-
mation pits” is used in the rest of this text for the so‐called
Higuchi etch pits. Sublimation pits are very useful (a) if
the pits are replicated for scanning electron micrography
and (b) if dislocation etch (to be presented in Section
6.4.4) pits are also generated by chemical etching inside
the sublimation pits while replicating the surface features.
The physics and chemistry of the “dual processes of etch-
ing and replicating” has been described in details and
exemplified elaborately in
Sinha
[1977b, 1978a, 1987b].
Figure 6.22 illustrates an optical micrograph of a replica
showing two different types of sublimation pits, three of
them are hexagonal and the others are irregular. The hex-
agonal pits are formed on the grain with its basal plane par-
allel to the surface. Tiny features inside the sublimation pits
are the dislocation etch pits produced by chemical etching to
be presented in subsequent sections. An example of a family
of three sublimation pits and numerous dislocation etch pits
developed on a single crystal of ice (prepared by zone refin-
ing technique) is shown in Figure 6.23. It illustrates an SEM
of a replica of sublimation pits, generated first on a second
pyramidal surface of the single crystal, and dislocations
etch pits produced later by the dual processes of chemical
etching and replicating, or DPCER, as described in
Sinha
[1977b] and presented in Section 6.4.4. Figure. 6.24 shows
SEMs of a replica showing a sublimation pit for ice surface
with
c
axis inclined and
a
axis parallel to the surface and
numerous pyramidal dislocation etch pits with their cores
on the inclined basal face in the sublimation pit.
0.5 mm
Figure 6.22
Optical micrograph of a replica showing tiny dis-
location etch pits inside large hexagonal and irregular-shaped
sublimation pits on vertical sections on two columnar‐grained
ice crystal in S2 ice; the inclined grain boundary in this micro-
graph was actually in the vertical plane in the ice (micrograph
by N. K. Sinha, unpublished).
Figure 6.23
Scanning electron micrograph of a replica show-
ing three sublimation pits and numerous dislocations etch pits
on a second prismatic face {1120} of a single‐crystal ice;
arrows indicate
c
axis and
a
axis orientations [SEM by
N. K.
Sinha
, 1987b].
polishing operations are usually performed by making
use of the reflecting type of microscopes. Since the grain
boundaries have a thickness of the order of a few atomic
diameters, the surface grain structures in metallographic
specimens are not visible at the magnifications available
6.4.2. Etching Processes and Applications
Metals and most rocks and ceramics are not transpar-
ent like ice. Optical examinations of the surfaces of these
materials after the initial grinding followed by the final
