Geology Reference
In-Depth Information
As can be seen in Figure 6.6, the first order of interfer-
ence for cross‐polarized white light corresponds to a
maximum thickness of about 0.4 mm. The corresponding
second order of interference is limited to a thickness of
around 0.8 mm. This provides an idea of the useful range
of ice thickness for thin sectioning purposes. If the thick-
ness is too thin, say less than about 0.3 mm, there may not
be much difference in the color of the crystals. The same
applies if the thickness is greater than about 0.8 mm.
Thus the goal for making thin sections with good colors
of the crystals is to obtain thicknesses somewhere between
0.4 and 0.8 mm.
The above calculations are made for a single crystal
of ice with its optic axis (or
c
axis) at a right angle to the
propagation of light. Pure single crystals are supposed to
have no grain or crystal boundaries, but that does not
apply to natural ice. This raises some interesting ques-
tions as to the choice of thickness for different types of
ice available in nature. Even the large grains of S1 type
of freshwater lake ice have subgrain structures with con-
voluted low‐angle boundaries tilted with respect to the
direction of transmitted light [
Sinha
, 2011]. On the other
hand, the grains in snow ice or frazil ice could be less than
1 mm with rather complex grain boundaries with large
amounts of inclusions at the boundaries. The useful
thickness has to be around 0.2 mm or less in these types
of fine‐grained ice.
easiest method is to thin the ice section by successive
melting from both the top and the bottom. The second
and more painstaking, yet the most popular, is the
method that employs the fixing of a thick section to glass
plates by melting followed by the removal of materials
from the top by microtoming as described in detail by
Weeks
[2010]. The third and the most rigorous cold‐plate
method is the solid‐state process, with absolutely no melt-
ing, called the double‐microtoming technique (DMT).
Historically, and therefore, the most popular method,
involves hot plates and glass plates that are warm to the
touch, and this should strictly be avoided for sea ice if the
microstructural features, such as the size and shape of
the air and the brine pockets, characteristics of intersub-
granular boundaries and intrasubgranular crystalline
defects, are to be examined. The best method (DMT)
uses neither any hot plates nor any warm glass plates and
is ideal for sea ice. Actually, DMT was developed and
used by the author [
Sinha
, 1977a] for the first time on sea
ice at −30C°. This was undertaken simply to preserve the
internal structure of FY Arctic sea ice sampled from
Strathcona Sound, Nunavut, Canada, during the dark
polar nights of late November when the air temperature
was about −30C°.
6.2.1. Hot‐ and Cold‐Plate Techniques
for Thin Sectioning of Ice
Preparation of petrographic and metallographic speci-
mens for microstructural investigations, to a large extent,
is an art. A number of steps are to be carefully followed
in cutting the specimens and preparing the flat surfaces.
This includes sawing, mounting the specimens on molds,
grinding the surfaces in steps from rough to fine grinding,
polishing the surfaces, and finally etching with chemicals
if required. Each of the steps requires a lot of practice to
control the quality of the finished surface.
In comparison to the above‐mentioned steps to follow
in preparing petrographic specimens of rocks, ceramics,
and metals, the preparation of usable thin sections of ice
by melting is relatively very simple. This simplicity is
believed to be responsible for the glaciologists to ignore
the fact that ice in nature is a high‐temperature material
and application of any heat is liable to change the micro-
structure. Of course, the changes may not be noticed by
unaided eyes or affect the coarse structure and texture
of large‐grained ice. Naturally, the practice of making
specimens of freshwater or glacial ice without any brine
inclusions will be continued in the future. In this aspect,
glaciologists dealing only with large‐grained ice have
great advantages over geologists and metallurgists. All
one needs for making a rudimentary ice section is a suf-
ficiently cold working area (below 0 °C, say −5 °C). The
procedures are simple and require only a band saw to cut
6.2. thin sectiOning techniques
fOR ice and snOw
Thin sections of sea ice are made from thicker sections
(1-2 cm thick) sliced from ice cores or blocks. The thin-
ning continues to bring the thickness down to less than
1 mm less. Taking advantage of the birefringence prop-
erty of ice crystals (as explained above), thin sections are
traditionally used to reveal individual crystals in poly-
crystyalline ice by different colors when observed through
cross‐polarized white light. The generated colors are
spectacular and provide the impetuous for making thin
sections in spite of the laborious (depending on methods
used) process that has to be completed in a cold labora-
tory (at temperature < −10 °C). The thinner the ice sec-
tion (in the range of about 0.4-0.8 mm) the better the
contrast between the colors of different crystals. It is,
therefore, desirable to make thin sections with thicknesses
in the range of 0.4-0.8 mm.
Thin sections of ice can be prepared by following essen-
tially two different types of methods. The first and cer-
tainly the most popular method, traditionally used by the
glaciologist in general, is to use warm to touch glass
plates, and this might be called the hot‐plate technique.
The hot‐plate technique may be subdivided further into
two different procedures—soft and hard. The soft or the
