Geology Reference
In-Depth Information
time‐consuming precautions are not necessary if thin
sections are made only for classifying the type of ice and
constructing the fabric diagram.
Oblique grain boundaries also generate viewing issues.
This is a serious problem for fine‐grained materials. It
generates technical problems in terms of usable section
thickness. For ice with grain diameters less than 1 mm,
the usable thickness is around 0.1-0.2 mm. To make such
thin sections with parallel surfaces is by no means trivial.
It is not only difficult to make such thin sections but
also the color under cross‐polarized lights is gray. Enhance-
ment of colors can be made using retardation plates as
described earlier, but still the finer details like areas
between very low‐angle boundaries such as those between
subgrains and other linear defects like deformation‐
induced tilt and small‐angle boundaries are not made vis-
ible. For sea ice, this raises some serious issues. Although
grains different in their crystallographic orientations are
recognizable under polarized light, the substructures
inside the grains cannot be evaluated except for the exam-
inations of the inclusions in the form of brine and air
pockets. The low‐angle boundaries, between platelets or
subgrains of pure ice, with very little lattice mismatch
(say less than 1°), are not delineated as clearly as desired
in polarized light. This lack of resolution is particularly
critical for oriented, S3 type (defined in section 4.3.3.5) of
FY sea ice for which the usual existence of grains with
distinct colors disappears, usually seen in freshwater ice.
Large areas are occupied by cellular structure consisting
of platelets or subgrains with their smallest dimensions
less than 1 mm. Such ice was named as “bottom ice” by
Peyton
[1966].
Figure 6.8
Double‐microtomed 5 mm wide horizontal thin
section of aircraft tire compacted snow at a Norwegian airport
under cross polarized light with a retardation plate to bring
the colors into the first order of interference (micrograph by
N. K. Sinha and A. Klein‐Paste, unpublished). (For color detail,
please see color plate section).
light beam has to be transmitted through four surfaces.
The quality of the two surfaces of the glass plate holding
the thin section and the top and the bottom surfaces
of the section affects the clarity of the views in transmitted
light. Scratch marks on the glass plates are the most
common problems and are often difficult to avoid. For
this reason, plates with scratches should be kept aside and
used for the preparation of the first surfaces of thin sec-
tions while using DMT. Scratch‐free glass plates should
then be used when transferring the specimen following
the preparation of the first surface. A common problem
is also caused by the condensation of moisture from
hands and breathing. Handling the glass plates during
and after completion of thin sectioning should never be
made with bare hands or even woollen gloves that could
be very porous. Leather gloves with no fur are best for
handling thin sections. One has to be careful also to avoid
nose drops (common when staying in cold environment
for long times) from falling on the sections or condensa-
tions coming from breaths, particularly during close
examinations with magnifiers or microscopes. Another
serious problem is caused by the condensation of water
vapor in the spaces between the bottom surface of the ice
specimens and the top of the glass surfaces. The water
vapor may solidify on the bottom surface of the speci-
men. Features created by this type of condensation may
not always be recognized as artefacts and may distort the
interpretations of micrographs. This is also the oppor-
tune moment to reemphasize the need for the use of
DMT. It is virtually impossible to avoid undesirable arte-
facts at the glass‐ice interface if the hot‐plate technique
is used for sectioning. However, these apparently
6.2.5. Optimum Thickness for Thin Sections
of Ice and Snow
It can be seen from equation 6.2 that a value of
R
λ
=
550 nm is obtained for a thickness, t =0.39 mm. This
amount of optical retardation, as presented graphically in
Figure 6.6, defines the limit of the first‐order interference
or the color of violet if white light is used to observe thin
sections between cross polarizers. An ice section with
thickness of 0.78 mm leads to the limit of the second order
of interference or the appearance of the second violet at
R
λ
=1100 nm. The interference colors in white light are the
best if the optical retardation is within the first two orders
preferably a little beyond the end of the first order. Higher
orders produce faded colors. Specimens cannot be too
thin either (except for snow and snow ice). Practically no
colors are produced for retardations less than about
250 nm or thickness less than about 0.2 mm. This is why
thickness for thin sections of polycrystalline ice (particu-
larly sea ice with brine inclusions) should be around 0.4-
0.8 mm for the best interference colors in white light.
