Geology Reference
In-Depth Information
Pit on basal plane
c-axis inclined to the surface
a-axis parallel to the surface
c-axis inclined to the surface
a-axis normal to the surface
a-axis
c-axis
c-axis parallel to the surface
a-axis parallel to the surface
c-axis parallel to the surface
a-axis normal to the surface
Figure 6.21
Sublimation pits in ice; note esp
ec
ially the shape of the pits for the first prismatic faces {
10 10
} at the
lower left and the second prismatic faces {
1120
} at the lower right (N. K. Sinha, unpublished).
homologous temperatures, this is not a serious problem.
However, in the case of ice at very high homologous tem-
peratures, even miniscule separations between the film
and the surface allow the ice to sublimate locally and pro-
duce sublimation pits. The formation of sublimation pits
depends on the temperature and humidity of the cold
chamber. These pits may take a day to develop at −10 °C
and can be seen if viewed through the transparent film.
The pits can also be seen for short times after the removal
of the film, but the rapid sublimation of the exposed sur-
face ruins the details of the pits by rounding the edges.
The sublimation pits can be generated in significantly less
time (in a few minutes or fraction of one hour) under thin
layers of dried film when thin layers of less concentrated
solutions are used inside cold rooms at very low humidity.
Thin layers of solutions dry quickly and may warp and/or
produce holes as the films dry. If the holes are large, then
the ice sublimates quickly, producing pits or depressed
areas on the surface that may not be interesting to look at.
A special case of pitting is due to the sublimation
through tiny pin holes in wrappers placed tightly on bod-
ies of single or large‐grained polycrystalline pure ice.
The process produces pits that have geometric shapes
depending on the location of the holes with respect to
the crystallographic planes of the grains under the holes.
Such pits develop very quickly if a dilute (1%) replicat-
ing soution (say Formvar in ethylene dichloride) is
applied to ice. The rapid drying produces, as mentioned
above, pin holes in the film through which ice sublimates
and pits are developed. The pits can have geometric
shapes like hexagon, triangle, rectangle, etc.
Higuchi
[1957, 1958] did not realize that they were sublimation
pits, but he showed how the orientation of both the
c
axis and
a
axis of the crystals associated with the pits
can be determined from the shape of these pits. Using
1% solution, he noticed that at −24 °C, mature etch pits
can develop within a few minutes after formation of the
plastic film. Since then, these pits are known as Higuchi
pits, and his petrographic technique has been used exten-
sively for glacier and freshwater ice.
A schematic of a few sublimation of Higuchi pits in
relationship with the crystallographic planes of a hexago-
nal crystal of ice is shown in Figure 6.21. As can be seen,
the shape of the sublimation pits depends on crystallo-
graphic orientation of the sublimated surface.
The technique of developing sublimation pits was a
breakthrough in the history of ice physics. The orientation
of the
c
axis could be determined with polarized light,
but determination of the a‐axis orientation required elabo-
rate methods like
x
‐ray diffraction techniques. Naturally,
Higuchi's etch‐pitting method was simple and has been
widely used for quick petrographic analysis of large‐
grained ice with no substructures, such as ice from gla-
ciers, lakes, and rivers. This method was not successfully
