Geology Reference
In-Depth Information
directions in a rather straightforward manner [
Reed‐Hill
and Abbaschian,
1992].
Two types of projections are used: the Wulff net and
Schmidt net. The Wulff net is a meridional stereographic
net and is a projection of latitude and longitude lines so
that the north‐south axis is parallel to the plane of the
paper. These lines serve essentially the same function as
the corresponding lines on usual geographical maps.
However, the major difference is that the measured angle
of the principal axis (such as
c
axis for ice) is stereograph-
ically projected instead of distances in geographical
maps. The Schmidt net is an equal area net and provides
another way for stereographic projection. In this projec-
tion, a unit area in any position on the net corresponds to
a unit area on the spherical projection (lower hemisphere
of the spherical projection is normally used) from which
the net was developed. This projection has been used
extensively for ice that belongs to the family of hexagonal
uniaxial crystal with one major axis—the optic or
c
axis
(<0001>). It is relatively simple to determine the orienta-
tion of the
c
axis of individual crystals of ice in thin
sections on the basis of extinction in cross‐polarized
light. For this purpose, thin sections are mounted on a
universal, Rigsby‐type stage between two polarizers in
a polariscope as will be seen later (section 6.3.1). Measured
data on the orientation of the
c
axis of the grains are plotted
on the net as points. The projection is called “orientation
fabric,” “fabric diagram,” or simply “fabric.” A minimum
of 200 points are actually required for making a statisti-
cally meaningful fabric diagram, but often that is not pos-
sible for many types of glacial ice samples due to the large
size of the constituent grains. Most manageable sized thin
sections, i.e., prepared from 100 mm diameter ice cores,
may not contain a sufficient number of grains. However,
this is not considered a major problem for highly oriented
columnar‐grained ice of the type S1‐or S3. In case of S1‐
type freshwater ice, the entire thin section may contain
only a fraction of one grain. This may also apply for S3
type of sea ice for most of the bulk ice away from the top
surfaces. S2 type of sea ice is rarely seen in oceans because
of the water current or tidal actions.
The two major requirements for making the meas-
urements for fabric diagram are the availability of thin
sections, about 0.5 mm thick, with parallel surfaces.
The surfaces must be very smooth and undistorted. The
hot plate technique or its slight variations, described in
section 6.2.1, that applies surface warming and melting
for mounting, thinning, and polishing thin sections is
commonly used for the fabrication of usable specimens of
freshwater and glacier ice. The necessary background for
making relatively good thin sections of saline‐free glacial
ice, performing the petrographic measurements and plot-
ting the data for microstructural investigations of glacial
ice with identifiable large crystals has been presented by
Langway
[1958]. The basic principle developed in the field
of structural petrology and metallurgy to measure the
crystallographic orientations and plotting the data has
essentially been adapted for ice.
The method was also extended to sea ice, in a practical
way, for ascertaining the fabric of columnar‐grained ice
with large cross‐sectional grain sizes [
Weeks and Gow,
1978, 1980
; Nakawo and Sinha,
1981]. Recently,
Weeks
[2010
,
Appendix E] has provided an excellent description
for procedures to be followed for sampling sea ice and pre-
paring thin sections using the “hot‐plate” (using warm-to-
touch glass plates) technique. He has also explained the
use of Rigsby universal stage and described in detail the
procedures originally developed by
Langway
(1958) for
general grain‐based petrographic analysis for glacier ice.
The present authors would like to recommend this highly.
However, it should be pointed out here that the proce-
dures allow one to determine “an average
c
‐axis orienta-
tion for grains” in case of large‐grained sea ice. Unlike
crystals in glacier ice or freshwater lake and river ice,
grains in sea ice are actually ill defined. Each grain con-
sists of innumerable subgrains with slightly differing
c
‐
axis orientations. Moreover, there are entrapped brine
pockets that are smeared on the surfaces of thin sec-
tions prepared by warm‐to-touch glass plates. The cold‐
state DMT for preparing thin sections with parallel and
smooth surfaces, with surface roughness of fractions of
1
μ
m, developed by
Sinha
[1977a], offers the best option
for sea ice with substructures and trapped inclusions.
This technique, along with the hot‐plate method is pre-
sented in detail in section 6.2. The DMT prepared thin
sections can be used not only for the preparation of the
grain‐based fabric diagrams but also for examining the
c
‐axis orientations of individual subgrains, as will be seen
in section 6.4.3.
Measurement type such as polar or equatorial can be
obtained for ice crystals using a Rigsby universal stage
installed in between the polarizer and the analyzer in a
polariscope as seen in Figure 6.11.
Rigsby
[1953] made
the necessary modifications and designed the universal
stage suitable for glacier ice. Further improvements in the
design of the stage were also made by him and had been
described by
Langway
[1958] in detail. However, using
this stage in polarizing microscopes are not fully satisfac-
tory for ice samples simply because of the limitations
imposed on the size of the specimens that can be used.
The size of grain diameters in natural ice bodies is rela-
tively large. Therefore, to accommodate a statistically
meaningful number of grains, say 200, the size of thin
sections has to be as large as 100-150 mm in diameter.
The universal stages, with three axes of rotation, used in
petrographic microscopes had to be modified for using
relatively large samples of ice specimens. The Rigsby
stage, which actually permits four axes of rotation to
