Geology Reference
In-Depth Information
(a)
(b)
a
a
a
Grain
c
c
c
a
axis
ϕ
Subgrain
c
axis
c
axis
S1 type
Figure 4.11
Schematic diagram of columnar‐grained, type‐S1 ice with
c
axis in the (a) vertical plane and subgrains
inside grains due to small mismatches in (b) both
c
axis and
a
axis [
Sinha,
1989]
with addition of subgrains in (b)
.
mismatches in both the
a
axes and
c
axis within the grains.
Subgrains are formed as a result. Slight variations in
colors and very small changes in the shades can be noticed
inside the large grains of this ice under polarized light.
The subgrains in S1 ice are areas separated by small‐angle
boundaries similar to those in sea ice, though not in scale.
Slight instability, induced thermally as well as chemically,
at the growing front, changes the planar front to wavy or
undulating ice‐water interface. This mechanism is directly
related to the formation of dendrites during solidification
of seawater when concentrated brine is pushed to the ice‐
water interface, as described in section 2.3.2. However, the
amount of dissolved impurities in lake and river waters
are considerably less than that in seawater. Naturally, the
subgrains in S1 ice could be one or two orders of magni-
tude larger than those in sea ice. They are separated by
low‐angle boundaries, and the angles may be large enough
to be seen under cross polarized ice. If not, they can be
seen clearly by the application of etching and replicating
technique to bring out the dislocation etch pits, described
in Chapter 6. Dislocation etch pits on the basal planes, at
the points of intersections of nonbasal lattice dislocations,
are hexagonal pyramidal. These pits can be developed on
the surface of horizontal (or normal to the length of the
crystals) thin sections of S1 ice. An example is shown in
Figure 4.13. The orientations of the
a
axis can be seen
clearly from the hexagonal cross sections of the etch pits.
Note the complex patterns of these pits with mismatches
between both
a
axes and
c
axis. The proof for the one‐to‐
one correspondence between the pyramidal etch pits and
nonbasal dislocations (line defects with cores of disloca-
tions), intersecting the basal plane, was presented by
Sinha
[1977b] and briefly discussed in section 6.4.2.
While presenting S1 ice type, it is informative to report
some interesting results from a laboratory study on one‐
directional freezing of distilled water under still conditions
10 mm
Figure 4.12
Horizontal thin section of freshwater columnar‐
grained S1 ice showing a single large grain with incursion of
another crystal [
Sinha et al.,
1996].
Figure 4.13
Optical micrograph of a replica of basal surface,
normal to a columnar grain of freshwater S1 ice, exhibiting
hexagonal pyramidal dislocation etch pits and complex net-
work inside “one grain” (identified by observation under cross‐
polarized light); the mismatch of
a
axis and slight tilting of
c
axis can be seen from the shape of the pits (N. K. Sinha,
unpublished).
