Geology Reference
In-Depth Information
glaciologists interested mostly in large‐scale aspects of
glaciers. No chemical etchings are necessary to bring out
the grain structures in ice after the completion of thin
sectioning. However, for detailed microstructural studies,
and especially for sea ice with entrapped pockets of brine,
thin sections are to be made carefully to preserve the
structures at both surfaces. The solid‐state DMT method
was, therefore, developed for preparing both the surfaces
of thin sections to mirror finishes and has ben presented
earlier. However, making thin sections to the desired thick-
ness of 0.4 to 0.8 mm by DMT is not simple and requires a
lot of practice and patience. It is not necessary to make
thin sections with both the top and the bottom surfaces
microtomed to the mirror finish if petrographic analyses
for fabric diagrams are not required. Thicker specimens,
say thickness of 5 mm, with reasonable parallel surfaces
can be used. Preparation of only the top surface as des-
cribed earlier is sufficient for the application of a powerful
technique called thermal etching to be described below.
Thermal etching can reveal information about micro-
features of crystalline structures at the prepared surfaces
of ice. By definition, thermal etching refers to the removal
of parts of the surface using thermally activated processes.
In case of ceramics, such as single‐crystal silicon wafers,
short exposures of polished surfaces to very high tempera-
tures (say 1200 °C) are routinely performed to remove the
surface layers disturbed during polishing. This is known
as purging. In case of polycrystalline materials like ice
it means removal of atoms from the surface at points
of high-energy levels. These are points of mismatch within
the lattice structure such as grain boundaries, subbounda-
ries, point defects and line defects such as dislocations, tilt
boundaries, etc. (definitions are provided in section 4.1.3).
Grain boundary grooves that are seen to form readily in
ice are generated because ice is always at extremely high
temperatures. They form because of thermal etching.
The technique of thermal etching of ice is the process
of exposing ice surfaces from freshly prepared thin sec-
tions to high‐humidity (near saturation) sublimation.
Schaefer
[1950] reported that a polished ice surface could
be etched simply by exposing it to air not saturated with
respect to ice. The method was used by
Krausz and gold
[1967] for revealing grain boundaries and by
Gold
[1972]
for bringing out the deformation‐induced defects like
low‐angle boundaries in freshwater ice by storing thin
sections inside boxes under subsaturated conditions.
However, they prepared their thin sections by using hot-
plate technique which invariably introduced artifacts due
to thermally induced strains.
The process of exposing ice to a subsaturated environ-
ment decelerates the sublimation of atoms and molecules,
which retain higher energy levels. Atoms and molecules
at any open surface (as opposed to molecules that exist
inside the material) or along crystal boundaries and lines
of lattice defects are at a significantly higher energy state
than those inside the bulk of the material. They can dis-
sociate from the surface and subsurface of a thin section
to the surrounding area more rapidly than atoms and
molecules in the lattice spaces in the bulk. The rate will
be particularly higher in the case of ice because it exists
in nature at temperatures close to its melting point, i.e.,
thermodynamically speaking it exists at extremely high
temperatures. Actually if thin sections of ice samples are
placed in open space, sublimation will continue to take
place until the specimen disappears. For this reason and
for preserving the microtomed surfaces, freshly prepared
thin sections must be stored inside boxes or sealed zip-
locked bags until they are photographed. Storing in con-
fined space slows down the processes of sublimation by
orders of magnitude. In fact, thin sections can be pre-
served for several months in sealed zip‐locked bags for up
to a year without any damage, provided the storage tem-
peratures are lower than −20 °C and preferably at −30 °C.
Another way is to keep it in a box containing crushed ice
so that the ice with its huge surface area sublimates
quickly and saturates the environment inside the boxes.
For very low angle grain boundaries, the use of thin
sections under cross‐polarized light alone does not delin-
eate the separating walls. This is especially applicable to
the subgrain boundaries in sea ice and also for the intra-
granular substructures in undeformed S1 ice. For those
cases, thermal etching can be very effective in bringing
out the features of microtomed surfaces to delineate the
large‐ as well as the small‐angle boundaries. This is
achieved by storing thin sections in closed boxes, as
shown in Figure 6.26 at −10 °C (0.96
T
m
) or lower for
freshwater ice and preferably at about −22 °C (~0.92
T
m
)
or below for sea ice. The closed environment of the etch-
ing box enhances the vapor pressure and slows down the
processes of sublimation from the exposed top surfaces
as compared to the open environment of cold chambers
used for thin sectioning. For a given vapor pressure, the
rate of sublimation of the water molecules at the surfaces
depends on their thermally activated energies together
with the additional energies due to the mismatch of the
lattices of the neighboring grains. All the boundaries of
lattice mismatch are etched. Grain boundaries with larger
lattice mismatch and hence higher energies are etched
more than the subgrain boundaries with relatively lower
lattice mismatches and lower energy levels. This way, all
the boundaries exposed to the environment are thermally
etched by the diffusion of water molecules from the sur-
face to the surrounding environment. The temperature
range of 0.92
T
m
to 0.96
T
m
is high, but low enough in
slowing down the processes of sublimation, and the water
vapor present in the environment of the closed chamber
modifies the rate of relative sublimation with respect to
the boundary energies. The moderating effect can be
