Geology Reference
In-Depth Information
Ordinary ice Ih
: This is the ice found in nature on Earth
with crystallographic structure belonging to the hexagonal
system (see section 4.2.1).
Platelets
: Dendrites develop at the growing front or
the interface of ice and seawater. This leads to a cellular
or platelet‐shaped substructure in sea ice. Each platelet
is a single crystal and is also called subgrains. Platelets
and hence subgrains are often separated by layers of
brine pockets.
Polariscope
: A device that has two polarizing filters,
a universal stage and a light source. It is used to examine
the crystallographic structure of ice and determine a few
properties such as the fabric diagram. Polariscopes are sim-
ple in design (Sections 6.1.1 and 6.3.1) and should not be
confused with polarising microscopes involving complex
optical components.
Polarizing filters
: Traditionally, Nicol prisms are used
in polarizing microscopes for obtaining linearly polar-
ized light. These prisms are small and the view is limited
to areas less than15 mm in diameter. Grain diameters in
polycrystalline ice are significantly larger than most
minerals. Therefore, polarizing microscopes are not very
useful due to their limited viewing areas. Large sheets
of linearly or circularly polarizing Polaroid filters are
commercially available products and are very conveni-
ent for making large field‐of‐view polariscopes required
for ice.
Polygonization
: Deformation processes induced by
external forces may break up the subgrains in sea ice
into even smaller units (see micrograph in Figure 2.31).
Under cross‐polarized light, polygonization in ice may
be recognized as areas of fine‐grained ice and may,
therefore, be wrongly identified as granular snow ice.
Polygonized areas are often called damaged zones in the
field of ice engineering (see cross‐polarized image in
Figure 2.32).
Recrystallization and polygonization
: Both terms have
been used in glaciology as if they are synonymous. Strictly
speaking, they are not the same and these two terms are,
therefore, confusing. Recovery processes in a crystal
following moderate to severe deformation at high tem-
peratures produce a structure that may be significantly
different from the original structure. The process usually
leads to smaller crystals and hence the name polygoniza-
tion. However, recrystallization occurs similar to a pro-
cess of nucleation and growth. It is related to the changes
in substructure when a metal is cold worked and then
annealed at higher temperatures. Ice is always at high
temperatures. So, stress concentrations at brine‐air pock-
ets in sea ice leads to tilt boundaries and eventual cell
formations [Figures 2.30 and 2.31]. Therefore, the term
polygonization is applicable, although historically the
term recrystallization has also been used. This is primar-
ily due to the fact that ice has never been thought of as a
(a)
(b)
c
-axis
Figure 4.3
(a) Solidification from the vapor phase, such as a
simple snowflake exhibits the hexagonal crystal structure of ice
I
h
photographed near North Pole by N. K. Sinha (International
Glaciological Society,
News Bulletin
, ICE, 2009) and (b) sche-
matic diagram of the basal plane of hexagonal ice, Ih, showing
the locations of the oxygen atoms (open and closed circles in
two slightly different planes, 9.23 nm apart,
c
axis or optic axis
perpendicular to the page) and one of the three
a
axes; broken
line joining two oxygen atoms in the same plane provides a
measure of the distance (45.23 nm).
(below 882 °C), zirconium (below 863 °C), berylium, and
cadmium. Interestingly, these metallic elements are also
the basis of complex high‐temperature alloys used in
components of jet and rocket engines.
Ice crystal types
: At least eight different solid forms of
water are known to exist at pressures greater than about
200 MPa or at very low temperatures. Under usual temper-
atures and pressures encountered on Earth's surface and
atmosphere, natural ice belongs to the hexagonal crystal
system (Ih) (see definition of ordinary Ih in section 4.2.1).
Large‐angle grain boundaries
: In a polycrystalline
solid, if the lattice orientations of crystals across the
boundaries separating them are large (loosely say—more
than one degree), the boundaries are called large angled
or simply grain boundaries.
Lattice
: The alignment of atoms in a crystal (in 3D).
Low‐angle grain boundaries
: If the mismatch in the
lattice orientations of the neighboring crystals is small
(say, around 1° or less), the boundaries between them are
called by various names, such as low‐angle, small‐angle,
subboundaries, or simply subgrain boundaries.
Melting point or ice point
: The melting point of ice,
T
m
,
is defined as the temperature at which ice is in equilibrium
with the surrounding water under the existing pressure.
Temperature
T
m
depends on hydrostatic pressure and the
purity of water. For pure ice and for water under unit of
atmospheric pressure (1 atm = 1.01325 bars = 103.32 kPa),
it is fair to assume that
T
m
= 0 °C (273.16 K). This is also
known as the ice point.
Optic axis
: The axis of optical symmetry coincides
with the
c
axis. These two terms (optic axis and
c
axis) are
therefore interchangeable. This will be clarified later while
presenting the birefringence of ice in section 6.1.2.
