Geology Reference
In-Depth Information
microscope [
Sinha
, 1977a]. Most brine pockets are irregu-
lar, and, when precipitation occurs, the salt crystals are
loosely packed in the cavities in a random manner.
Replications should, however, be performed as soon as
possible after the completion of the DMT procedures in
order to avoid complications due to the surface grooves
formed by thermal etching during storage.
For saline‐free or brackish water ice, thermal etching has
been found to be very effective at temperatures at or
below about −10 °C. For sea ice it is better to apply it, as
mentioned before, for temperatures below −20 °C. In
polycrystalline ice, apart from defects such as grain and
subgrain boundaries, there are point defects such as inter-
stitials and line defects called dislocations (see definitions
in section 4.1.3). Points of intersections of line defects and
the surfaces cannot be usually revealed by thermal etching.
Chemical etching is extremely powerful and very appropri-
ate for delineating dislocations in ice in the form of etch pits
at surfaces as well as nanoscale deep holes along the cores
of dislocations. Consequently, mechanically or thermally
induced mobility of dislocations or network of these line
defects can be detected without any ambiguities and have
been exploited exhaustively since
Sinha
[1977b, 1978a].
of sublimation etch pits developed by the Higuchi
method. He showed that for lake ice the densities of etch
pits in these small areas (6.5 × 10
6
/cm
2
) were an order of
magnitude less than the densities (6.5 × 10
7
/cm
2
) on the
original surface prepared by polishing with emery paper.
It was considered by
Muguruma
[1961] that polishing
generated many dislocations. He used this assumption as
an argument to establish a correspondence between the
etch pits and dislocations. The increase in dislocation
density by mechanical polishing was the reason that led
Kuroiwa and Hamilton
[1963],
Levi et al.
[1965] and
Kuroiwa
[1969] to limit their experiments to dendritic
crystals, fresh cleavage and fracture surfaces, and sur-
faces produced by evaporation etch pits.
Krausz and Gold
[1967] prepared their surfaces by melting briefly on a
brass plate kept at a temperature slightly above 0 °C and
then polishing with soft leather or by chemical dissolu-
tion using alcohol. The surface melting method, however,
cannot be used for the detection of dislocations produced
in previously deformed ice if the ice is to be maintained
at a constant temperature. Moreover, surface melting
could introduce certain undesirable morphological
changes [
Krausz and Gold
], 1967. In the case of sea ice, in
addition to the loss of brine during hot plating, the mor-
phological changes involve modifications of brine pock-
ets, geometry, and characteristics of subgrains and the
subgrain boundaries.
The solid‐state DMT method was developed in order to
avoid all the critical issues related to sea ice and has been
described earlier. The details of the procedure to follow for
the dual process of etching/replicating are described below.
After preparing a thin section using the DMT method,
the freshly prepared specimen is kept in a horizontal posi-
tion (making certain that it is perfectly leveled) inside a
transparent plastic or glass box containing crushed ice (as
described in Figure 6.26). As discussed earlier, the repli-
cating solution is made by dissolving Formvar (polyvinyl
Formvar) in ethylene dichloride. Its concentration in the
replicating solution is varied as required. Formvar solu-
tion is placed slowly on the prepared surface with an eye-
dropper preferably with a long stem and the surface
coated with the desired thickness of etchant. Since the
solution spreads slowly over the surface due to surface
tension, the desired thickness has to be built up by adding
more drops until the solution becomes uniformly distrib-
uted over the entire surface. The overflowing of the solu-
tion is prevented by building a wall of ice with water
around the edges of the specimen. The coated surface is
allowed to dry while etching the surface simultaneously at
the same time. The solvent evaporates slowly depending
upon the vapor pressure, leaving a coat of thin plastic
film (the replica) of polyvinyl Formvar. The duration of
the etching time depends on the drying time and hence on
the vapor pressure above the surface. Vapor pressure
6.4.4. Chemical Etching and Replicating Ice Surfaces
Chemical etching in case of metals and ceramics is
achieved by the application of a solution that allows
absorption of high‐energy surface atoms and molecules,
leaving their spots vacant in the lattice structure. As
explained earlier, etched surface features can be identified
and examined by using optical microscopes or SEM.
Carbon replicas, for transmission electron microscopy
(TEM), can be made by the vapor deposition of subli-
mated carbon on surfaces. Replicas can also be made by
applying replicating solutions consisting of some kind of
plastics dissolved in solvents. Replication is a technique
that produces imprints of the surface of the specimen.
The techniques of replicating normal (e.g., fracture sur-
faces) or polished and etched surfaces of metals and
ceramics do not involve (or not supposed to encompass)
any additional etching processes that may alter the char-
acteristics of the surface features.
Due primarily to the unavoidable fact that ice exists at
temperatures very close to its melting point, replicating
ice surface are subjected to very complex processes of
continuously evolving surface features if surfaces are
exposed to air. Sublimation and consequent thermal
etching are almost impossible to avoid. These issues have,
in fact, hindered progress in extending the application of
the etching and replicating technique to ice. Surface
preparation prior to etching is another challenge.
Muguruma
[1961] could only use his method on very
small areas, less than 0.5 mm in diameter, at the bottom
