Geology Reference
In-Depth Information
simply be defined as a naturally occurring inorganic
substance of definite chemical composition. It makes up
the rocks of Earth's crust or outer shell (and by the same
argument, ice). Naturally occurring minerals occur in
two essentially different conditions or states: (1) the crys-
talline and (2) the amorphous [
Rogers,
1937].
In the domain of crystalline structures, polycrystalline
material dwells between the two opposite ends of amor-
phous or noncrystalline and mono‐ or single‐crystal
materials. Amorphous solids, such as glass, fused silica,
and some gems, are materials that may lack long‐range
atomic or molecular order—the primary characteristic
of a crystal. In a mono‐ or single‐crystalline particle, on
the other hand, the entire body is composed of a single
continuous crystal strictly speaking with no subbounda-
ries. Monocrystalline materials are rare in nature because
nature usually favors the presence of some imperfections
in the atomic or molecular arrangements of solids such
as impurities, inclusions, or strain zones with lattice
imperfections such as point defects and line defects known
as dislocations. However, single crystals of ice and silicone
(used in computers) with very few imperfections can be
fabricated by zone refining techniques. Macroscopically
speaking, many of the physical properties, such as opti-
cal and mechanical (namely creep and fracture), vary
with the direction in a crystalline material, whereas these
properties are the same in all directions in an amorphous
substance.
Polycrystalline solids, which include almost all com-
mon metals and ceramics, refer to materials composed of
many crystallites of varying shape, size, and orientation.
While large single crystals of pure ice can be found in
temperate glaciers and can also be prepared in cold labo-
ratories, the normal icy objects found in nature or fabri-
cated for human consumption consists of an aggregate of
many crystals. Natural ice in all forms, including sea ice,
is therefore a
polycrystalline
material like most “normal”
metallic or ceramic materials. Here the term normal is
used to exclude the especially prepared amorphous or
mono‐ or single‐crystal materials. Polycrystalline natural
ice can be categorized on the basis of geometrical charac-
teristics of its constituent crystals. In many respects, how-
ever, sea ice is a very complex high‐temperature material.
The inner structure of sea ice can assume a variety of
shapes, sizes, orientation of the constituent crystals and
hence texture, and liquid, solid, and gaseous inclusions.
A few aspects of sea ice polycrystalline structure with illus-
trative examples that relate the structure to the formation
and growth history are addressed in some details in this
chapter.
Crystals in natural ice, like those in natural rocks or
man‐made ceramic and metallic materials are normally
referred to as its grains. Crystal or grain structures of
materials are usually examined and analyzed with the
Celsius
Kelvin
Homologous material
500
773
523
250
1.0
Rubber
120
293
1.0
Bitumen
0
773
Ice
1.0
0.78 (Ice)
0.54 (Bitumen)
0.41 (Rubber)
213
-60
Airport ice
temperature
range
-273
0
Figure 4.2
Comparison of the melting point and transforma-
tion temperature of some solid materials used in roads and
airports; “Airport Ice” indicates ice on runways or in‐flight ice
on aircraft wings and fuselage [
Norheim et al., 2001
].
temperatures only about 30% below the melting point.
Thus, the low temperature of −60 ° C (
≈
213 K) that the
people of Yakutsk, Russia, often face (and mentioned
earlier) is equivalent to a homologous ice temperature of
0.78, and this is only 22% below the melting point of ice.
Coincidentally, the maximum temperatures allowed for
the operations of man‐made nickel‐based superalloys,
used in gas turbine blades in jet engines and end casings
of rocket engines, are kept below 0.80
T
m
of these alloys.
For minerals and man‐made materials like ceramics and
complex metallic alloys, any temperature higher than about
0.4
T
m
is considered to be a very high temperature because
the constituent atoms and molecules in these materials
have high mobility at these temperatures. Materials, espe-
cially polycrystalline (metals, metallic alloys, and ceram-
ics) start to exhibit intergranular embrittlement as the
temperature rises above about 0.3
T
m
[
Garofalo,
1965;
Gittus,
1975;
Cadek,
1988;
Nabarro and de Villiers,
1995].
Temperatures greater than about 0.3
T
m
may, therefore, be
classified as high‐temperature regime. Thus, ice in nature
at temperatures higher than −27 °C or 0.9
T
m
is at extremely
high thermal states. Ice covers floating on their melts are,
therefore, always extremely hot, and this aspect of ice is of
utmost importance for understanding and demystifying
the descriptions found in the literature about ice as pecu-
liar, bewildering, confusing, puzzling, baffling, etc.
4.1.2. General Terms for Structural Aspects of Ice
The most significant aspect of any engineered or natu-
rally available minerals is its micro‐ and macrostructure in
terms of geometry of crystals and inclusions (solid, liquid,
and gaseous) and textural parameters (crystallographic
orientations of the crystals in a sample). A mineral may
