Geology Reference
In-Depth Information
horizontal or the vertical plane in a floating ice sheet.
The characteristic of the crystalline orientations is known
as texture.
Columnar crystal
: Polycrystalline ice in nature can
be divided into two major types: columnar or granular.
Columnar materials have long crystals and the material
looks like a stack of pencils. Columnar ice can be subdi-
vided into two major groups depending upon the orienta-
tion of
c
axis of the constituent grains. Columnar‐grained
ice with the
c
axis of the grains parallel to the long axis
of the crystals is commonly seen in freshwater lakes and
is called S1 ice (see section 4.3.3.3 for details). If the crys-
tals have their crystallographic axes randomly oriented
in the plane normal to the long axis of the columns, then
it is called orthotropic or transversally isotropic. This is
often seen in lake ice and is classified as S2 ice (see
section 4.3.3.3).
Equiaxed crystal
: Granular material contains a con-
glomeration of grains of different shapes and sizes. If the
size of the grains does not vary significantly from each
other and the crystallographic axis (
c
axis) of the grains
are randomly oriented, then the material is called isotropic
and equiaxed. When snow saturated with water freezes, it
forms granular ice (see section 4.3.3.3).
Extinction position
: When the
c
axis (optic axis) of an
ice crystal is parallel (or when the basal plane is normal) to
the direction of propagation of light, the crystal appears
as black and independent of rotation when viewed through
cross polarizers. The crystal also appears as black in cross‐
polarized light when the direction of light propagation and
its optic axis is parallel to the pass direction of the polar-
izer or the analyzer as described in Section 6.1.1. These
two particular positions (under cross‐polarized light) are
known as the “standard photometric configuration”
according to
Martinez
[1958] when dealing with quartz.
This definition has also been adapted for ice.
Fabric diagram
: The stereographic projection for 2D
mapping (in the horizontal plane for floating ice sheet) of
a crystallographic axis (usually
c
axis for ice). The fabric
of polycrystalline ice is determined by plotting the pro-
jections of the
c
axis of the grains on a hemisphere with
its plane parallel to the surface of the ice cover. The dia-
gram shows the
c
‐axis directions of the crystals in a con-
venient and straightforward manner.
Grain
: In the general field of snow and ice mechanics,
grains are often used to describe particles, which may con-
tain one crystal or a number of crystals lumped together.
Thus the term grain is often loosely defined in the field
of glaciology including snow sciences. Unless clarified, a
grain may consist of a single crystal or several crystals dif-
fering from each other in the orientation of their crystal
lattice. However, the two terms are used interchangeably.
For example, the term polycrystalline material could
mean also polygranular material unless predefined. We
will use crystal and grain synonymously in this topic as
clarified next.
Grain and subgrain
: In polycrystalline materials,
different crystals (or grains) are separated by bounda-
ries where the lattice of one crystal does not match with
the lattice of the neighboring crystals. The mismatch
could be in the orientation of the
c
axis or the orienta-
tion of the
a
axis. More often than not, crystals within
a polycrystalline mass have substructures in the form of
cells and are known as subgrains.
Grain and subgrain boundaries
: The surfaces separat-
ing different crystals in a polycrystalline solid are defined
as crystal boundaries. However, historically speaking,
these crystal boundaries in metallurgy and most ceramics
are described as grain boundaries with the underlined
assumption that the constituent grains are single crystals.
If the differences in the orientation of the crystallographic
axes across the boundaries are large enough, say more than
about one degree then the boundaries may be called crystal
or grain boundaries. If the mismatch is small, say less
than about one degree, then the boundaries may be called
subgrain boundaries (see the schematics in Figures 2.24
and 2.25 and micrographs in Figures 2.26-2.28). However,
these are not strict definitions. Grains in sea ice are often
ill defined. Individual grains consist of families of sub-
grains and grain boundaries are difficult to distinguish
clearly. In sea ice small mismatches in the lattice develop
when the dendrites in the skeletal layer protrude in the
liquid (see the micrograph in Figure 6.17) and are sub-
jected to local disturbances. Since these dendrites are
delicate, their growth orientation may not align exactly
with each other. So, as the freezing front continues to
grow, the dendrites grow at slightly different orientations
and eventually merge with each other to form the grain.
The
c
axis mismatches within the grain and between the
initial dendrites mark the subgrain boundaries. When sea
ice is subjected to external forces, shearing occurs along
the subgrain boundaries. This leads to deformation and
localized cracking activities. Subgrain boundaries, there-
fore, play the most important roles in affecting all the
physical properties of sea ice. In fact, the concept of
grains is not of any importance in sea ice and this is
pointed out in many places throughout this topic.
Hexagonal crystals
: This includes all crystals in which
there is a single hexagonal zone and there are four axes of
reference. These include three interchangeable axes (
a
axes) in the basal plane and the fourth (
c
axis) normal to
the plane of the other three. The hexagonal characteris-
tics of ice in nature can be readily seen in snowflakes
formed by solidification from the vapor phase, as shown
in Figure 4.3. However, its unit cell is “not closed packed,”
and this aspect will be clarified later. The most important
metallic elements belong to the closed‐packed hexagonal
crystal structure are magnesium, zinc, alpha titanium
