Polycrystalline Ice Structure - Sea Ice: Physics and Remote Sensing - page 147

cooperative hydration. These two factors determine the
degree of branching in the ice crystal. Normally, hexago-
nal ice crystals tend to grow rapidly in the basal plane.
Growth may also take place in any primary or secondary
prismatic faces [ Hobbs, 1974 ; Pounder, 1965]. Growth
rate in any primary or secondary prismatic face is more
or less the same, but growth from any of the secondary
prismatic faces such as the 1120 turns it into primary pris-
matic face [ Nada and Furukawa, 2005].
This information confirms the most fundamental
knowledge that ice grows relatively faster and more easily
in the basal plane or normal to the c axis unless there is
a constraint [ Hillig, 1958]. These differential growth rates
were used by Perey and Pounder [1958] and Pounder [1965]
to explain the preferred growth of crystals with inclined
optic axis, resulting in gradual extinction of vertically ori-
ented crystals. From a laboratory study, Shaw et al. [2005]
concluded that when nuclei exist near a supercooled
water surface, ice growth parallel to the surface (i.e., in
the basal plane with vertical c axis) advances by a rate
orders of magnitude greater than that when nuclei origi-
nate in the bulk water. However, when ice growth in the
basal plane is constrained, then ice has to continue its
growth along a prismatic plane.

in pure water. This aspect of density change has been
emphasized repeatedly in the ice literature.
The density of air‐saturated pure melt (i.e., water) increases
from about 999.83 kg/m 3 as the temperature rises from 0 °C
until the maximum density of about 999.96 kg/m -3 reaches
at 4 °C [ Pounder, 1965]. This increased amount to about
0.013%. As the temperature increases higher than this
critical temperature of 4 °C, the density of air‐saturated
water decreases as expected from thermal expansion.
Increase in thermal energy and the consequent thermal
expansion certainly plays a role in decreasing the density
as the temperature rises beyond 4 °C, but this mechanism
contradicts the trend between 0 and 4 °C. One simple
and probable explanation is the concept that the open
structure of solid Ih does not disappear altogether at the
melting point, but some ice‐like structure (lower density)
is retained within the clusters of water molecules. These
clusters or groups of molecules diffuse away with the
increase in temperature and thereby increasing the den-
sity up to 4 °C beyond which thermal expansion takes the
dominant role [ Pounder, 1965]. The temperature of 4 °C is
equivalent to the homologous temperature of 1.0147 T m .
The ice‐water system may appear to exhibit a somewhat
strange behavior, but then, how many melts of crystalline
materials have been examined at these high homologous
temperatures? Also, can the accuracy of density measure-
ments be reached, as was achieved for ice‐water, anywhere
close to the melting temperatures for commonly known
metals (e.g., 3273 K for tantalum and 3683 K for tungsten)
including those exhibiting a hexagonal crystallographic
structure?

4.2.4. Ice Density in Relation to Crystalline Structure

A final note about the density of ice compared to water
is worth mentioning. It is well known that ice covers in
lakes, rivers, and oceans, floats on its own melt (i.e., water).
Ice is one of very few substances for which the solid is
lighter than its melt. In this respect ice certainly falls in the
category of strange materials.
Ordinary ice is lighter than water, primarily because
the structure of ordinary hexagonal ice (Ih) is an open
hexagonal structure. Each oxygen (O) atom is at the
center of a tetrahedron with four other O atoms at the
apices. The O atoms are concentrated close to a series
of parallel planes (these are the basal planes that are
9.23 nm apart from each other) as can be imagined from
Figure 4.3. On an atomic scale, the whole structure looks
much like a beehive, composed of layers of slightly crum-
pled hexagons. The web of O atoms is held together by
hydrogen bonds. The H atoms lie along these bonds. It
is the length of the hydrogen bond that creates the open
structure of Ih. Upon melting, some of the bonds are
broken, causing a disordered structure with a higher den-
sity. Nonetheless, some short‐range order remains even
in liquid water, with a few water molecules retaining the
crystal‐like bonded structure very similar to amorphous
materials in general. The short‐range order tends to be
destroyed by thermally induced motions. As the temper-
ature drops and approaches the solidification point, the
changes in the structure cause “strange density” behavior

4.3. sTrucTure‐ and TexTure‐Based
classificaTion of naTural ice

Depending upon the scale of observations, there are two
major aspects of classifying freshwater and seawater ice:
1. Micro‐ to macroscale based on grain structure and
texture
2. Mesoscale based on age, history, size, and thickness
of the ice cover
Aspect 1 is relatively simpler to discuss and present by
making inferences to the accepted type and the classifi-
cation based on freshwater ice given in Table 4.1 and
adapting to sea ice. This aspect of sea ice classification
is one of the most important factors for ice engineering
related to the design of near‐ and offshore structures
and ice-strengthened ships. In fact, most of the struc-
ture‐ and texture‐dependent studies of sea ice mechan-
ics reported in the literature are concerned with this
aspect of sea ice.
Aspect 2 is important for navigation and, therefore, it
is commonly addressed in remote sensing and will be
described in Chapter 10. This aspect is also important for