Geology Reference
In-Depth Information
help of microscopes. The inner structures, beyond the
recognition of normal human eyes are, therefore, known
as
microstructures
. Naturally occurring ice often have
large grains in comparison to most minerals and the cor-
responding grain‐scale structural analysis can be made
without the help of any microscopes. In such cases, one
may use the term macrostructure. For simplicity, we will
use the term microstructure for any grain‐scale or finer
structural aspects of ice.
In addition to the high thermal states, natural ice
shows a multitude of structural features and a great
deal of trapped impurities depending upon the purity of
water or the melt from which the ice grew. It is, therefore,
a complex composite composed primarily of pure ice
crystals (or grains with subgrains) in various states of
shape and sizes along with brine pockets and isolated air
bubbles or air bubbles trapped inside the pockets of brine.
Understanding the formation, growth, and decay processes
of ice in oceans is important for persons involved in
a wide range of disciplines dealing with ice‐rich water
bodies. This includes, for example, ice engineers dealing
with problems of interactions of ice and structures,
climate scientists for interpreting impacts of climate
change on polar ice, and remote sensing specialists for
developing methods for the retrieval of ice parameters
from observations.
The morphology of the sea ice microstructure at cen-
timeter, millimeter, and nanometer scales influences all
aspects of thermal, mechanical, optical, electrical, and
radiative properties of sea ice. Moreover, the results of
investigations on microstructure also reveal information
about the oceanic and atmospheric conditions under
which the investigated ice was formed and grown. These
include the following:
• The state of the ocean surface during the early ice
formation (whether calm or turbulent),
• The dominant direction of the ocean current
• Oceanic activities occurring underneath the ice sheet
(e.g., transportation and deposit of frazil at the underside
of the ice sheet)
• Rate of atmospheric cooling
• Amount of snow on the surface
• Biological activities at the ice‐water interface
looks at these axes) with each other as can be visualized
from Figure 4.3 and illustrated later in Figure 4.5. Because
of the symmetry in the basal plane, the three
a
axes are not
distinguishable from each other.
Amorphous
: Any noncrystalline solid in which the atoms
and molecules are not organized in a definite lattice pattern
over a long range. Examples are glass, plastic, and gel.
Basal plane
: The plane of closest packing of atoms
in a close‐packed hexagonal crystal. It contains all the
three
a
axes. This plane is also known as the (0001) plane
according to Miller indices (Figure 4.5).
Brine layer or platelet spacing
: This is the spacing
between layers of brine pockets or rows of brine pockets,
illustrated in Figures 2.21a, that can be seen inside a grain
in a cross section normal to the length of the columns in
columnar‐grained sea ice. Usually, the average value of the
spacing is used as a measure. Since the columnar grains in
sea ice also consist of columnar subgrains, in the shape of
plates (as illustrated in Figures 2.21b, 2.22, and 2.24), brine
layer spacing is also known as platelet spacing. The former
is preferred and is used in this topic.
c axis
: The
c
axis is the unique axis of symmetry in
hexagonal crystals like hexagonal ice, I
h
(or Ih). It is normal
to the basal plane (i.e., to the three
a
axes) and known as
<0001 > axis on the basis of Miller indices. The
c
axis is also
the optic axis of the crystal (Figure 4.5). For illustrations,
it is also indicated simply by <c>.
Crystal
:
Macroscopically speaking, a crystal is a solid
particle such as quartz, one of the most common minerals
on Earth, with a regular shape in which plane faces inter-
sect at definite angles. Inside a crystal, a large number
of atoms are arranged in a regular array (lattice) forming
a periodic structure with a long range of order within
the boundaries of the particle. Thus the simplest defini-
tion would be “a crystal is a body inside which there is an
orderly array of atoms in space.” A large number of crystal
types exist in nature. Some of them are very complicated
[
Rogers
, 1937]. In case of metals, however, there are three
common types: the face‐centered cubic, the body‐centered
cubic, and the close‐packed hexagonal.
Crystal defects
: Crystals are rarely perfect in nature,
i.e., not all the atoms inside a crystal are perfectly aligned
(i.e., free of defects). The lattice of a crystal may have
point defects (vacancies of interstitials), line defects
(dislocations), or planar defects (stacking faults). The
vacancies are missing atoms in the lattice and the inter-
stitials are extra atoms in the lattice. Dislocations are
rows of vacancies. Stacking faults are defects within the
planes where lattice planes are not stacked in the expected
geometrical order.
Crystalline texture
: In polycrystalline solids, the ori-
entation of the crystals (or grains clarified and defined
later) may be random or oriented, such as a preferred
orientation in a two‐dimensional plane, particularly the
4.1.3. Basic Terms and Definitions
A number of basic terms, definitions, and necessary
clarifications related to the structure of polycrystalline
ice used in this topic are introduced here. The list does
not cover a wide range of items but it may serve as a
quick reference. Some items are defined in terms of other
items that are also on the list.
a axis
: The basal plane in ice contains three
a
axes
making an angle of 120° (or 60° depending on how one
