Geology Reference
In-Depth Information
crystallographic analysis for ice and described the design
of an automated instrument called a multiple axis pho-
tometer (MAP) using an HeNe gas laser emitting red
light with a wavelength of 632 nm and three solid‐state
phototransistors to measure intensities of three beams
with their diameters reduced by a condenser lens to less
than 0.2 mm within the thin section. The technique
involves thin sections with thicknesses less than 0.22 mm
of ice. This means that the section should be thin enough
to produce optical retardation less than about 200 nm or
interference colors not beyond the first‐order gray (as can
be seen in Figure 6.6) when examined through cross
polarizers using white light. The optical rather than geo-
metric thickness determines the practical suitability of
the thin sections for analysis with MAP. This automatic
system was successfully applied to ice from Cape Folger
borehole core at a depth of 300 m with grain diameters in
the range of about 9 mm [
Lile
, 1977].
The ice used to demonstrate the applicability of MAP
was large grained (9 mm) and free of any brine pockets.
Required thin sections with thicknesses less than 0.22 mm
could be made for this large‐grained pure ice using the
hot‐plate technique of fixing thick sections on warm
glass plates and then thinning down to the suitable thick-
ness. Making such thin sections is highly improbable for
sea ice because of small sizes of subgrains and every pos-
sibilities of brine draining to the bottom or smeared on
the top surfaces. Consequently, analysis with MAP was
never tried on sea ice—according to the knowledge of the
current authors.
The solid‐state DMT technique eliminates the above‐
mentioned limitation imposed on making very thin sec-
tions required for MAP. However, the main obstacles for
using the MAP for sea ice investigations are the presence
of subgrains with widths less than 1 mm and their inclined
boundaries and hence their surfaces tilted within the
depth of thin sections. More importantly because the size
of the brine pockets (often with air bubbles) is compara-
ble to the light beam diameter of about 0.2 mm, the inclu-
sions are the sources of undesirable and disturbing
scattered light.
At temperatures below 0 °C, ordinary ice sublimes
without the intervention of the liquid phase. One should
be careful in making such a broad statement because the
surface atoms may not follow the regularities of the lattice
structure and may be in quasi‐liquid states. The liquid‐like
layers, however, are confined within the depth of a few lat-
tice distances. In any case for gross surfaces, sublimation
continues until reaching a metastable equilibrium pressure,
known as the vapor pressure, between the ice and the
vapor. Complications from the localized thermal gradients
produce convective currents in the environment. In princi-
ple, as long as the equilibrium condition is not reached, ice
continues to sublimate. This is why clothes can be dried
simply by hanging them after washing in an open atmos-
phere at temperatures below 0 °C, allowing the soaked
water in them to freeze and then sublimate. This is also the
process that causes loss in moisture in frozen foods, such as
meat and vegetables, leading to conditions known as
freezer burn. Freezer burning can be delayed by storing the
foods in tightly sealed containers in deep freezers at −30 °C
to slow down the sublimation/condensation processes, but
eventually grain growth will occur and ruin the natural
texture; after all −30 °C is 0.89
T
m
only 11% below the
melting point
T
m
of 273 K and therefore a rather high tem-
perature indeed. Nonetheless, the wrappers should be free
from holes and wrapping should be very tight. Otherwise
localized sublimation may occur through the openings or
in the spaces above the surface, producing characteristic
zones of depletion (pits) depending upon the size of the
apertures. The craters or the pits that form in the depleted
zones under the orifices in clear ice are known to have geo-
metric features that are complex. The shape and surface
features of the craters depend strongly on the type of ice.
Seemingly unwanted pits may develop when a common
petrographic (metallographic) technique of replicating
surface features are applied to ice. Ideally, a replica is
expected to copy the features of the surface without affect-
ing any changes. A common method of the replication of
surface features of metals, rocks, or ceramics involves the
application of a solution of a plastic dissolved in a solvent
on the surfaces and allowing the plastic film to dry. In the
case of metallographic specimens at room temperatures,
about 25 °C or 298 K (equivalent to very low homologous
temperatures of about 0.2
T
m
for some metals), surface
features can be replicated as long as the solvent is not cor-
rosive. One common practice is to use 5% or 10% solution
of polyvinyl formar (registered trade name is Formvar) in
ethylene dichloride (or similar solutions). Polyvinyl for-
mar belongs to the family of polymers (synthetic plas-
tics). It has found diverse applications in wire insulation,
coatings for musical instruments, and preservation of
woods. The film may warp during drying and may sepa-
rate from the surface, producing localized spaces between
the surface and the film. For metals, etc., at very low
6.4. etching techniques
6.4.1. Sublimation and Sublimation (Higuchi)
Etch Pits in Ice
One of the manifestations of the high thermal state of
ice is the phenomenon of solid‐state removal of surface
atoms or molecules to the environment adjacent to the
surfaces. The dissolution of surface molecules by this
process of solid‐to‐vapor transformations is called subli-
mation, whereas the liquid‐to‐vapor transformation is
called evaporation.
