Structure and Properties of Lake Ice - Freezing of Lakes and the Evolution of their Ice Cover - page 52

3.1 Ice Ih: The Solid Phase of Water on Earth

3.1.1 Ice Crystal Structure

'

In the Earth is nature, water is the only substance, which occurs in all three phases: gas,

liquid, and solid. Phase transitions take place continuously. The water molecule consists of
one oxygen atom and two hydrogen atoms connected to the oxygen atom by covalent bonds
(Fig. 3.1 a), which are strong and keep the water molecule stable. The hydrogen atoms are
not symmetrically located but the bonds H

that makes the
water molecule an electric dipole. The center of each hydrogen atom is approximately
0.0957 nm from the center of the oxygen atom. The oxygen pole is negatively charged and
the hydrogen pole is positively charged. The dipolar structure is the cause of several
exceptional properties of water; e.g., liquid water is an excellent solvent.
Due to their dipolar structure, the water molecules are connected with each other by
hydrogen bonds into linear chains in liquid water. The oxygen atom of a water molecule
and a hydrogen atom of a neighbouring water molecule form the hydrogen bond, with the
length of 0.117 nm. These bonds are much weaker than covalent bonds, and they form and
break continuously. The hydrogen bonds are the reason of relatively high freezing and
boiling points of water, as compared with similar substances.
There are several possible crystal lattice structures for the solid phase of water (Pe-
trenko and Whitworth 1999). About ten are known. But in the natural pressure and
temperature conditions within 10 km up or down from the Earth

O H form an angle of 104.5 °

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s surface, all ice has
hexagonal crystal structure, named as Ice Ih (Fig. 3.1 b). Cubic ice (Ice Ic) occasionally
occurs in the upper atmosphere at low temperatures and low pressures. In this topic only
the hexagonal ice is considered.
The crystal lattice of Ice Ih (from now on, just

'

) is tetrahedral where each ice
molecule is joined by hydrogen bonds to four other ice molecules. Then electron pairs are
evenly spaced in their orbits and the covalent bonds H

' ice '

,
which is very close to the angles in a perfect tetrahedron. The distance between neigh-
bouring molecules is 0.2760 nm, and the distance between oxygen and hydrogen atoms in
one molecyle is 0.095 nm. The lattice follows the Bernal

O H form an angle of 109.5 °

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Fowler rules (e.g., Petrenko and
Whitworth 1999): (1) There are two hydrogen bonds connected to each oxygen atom, and
(2) There is only one hydrogen atom in a bond. Exceptions to these rules are defects in the
crystal lattice and are concerned with ordering of the water molecules. The concentration
of foreign molecules in ice crystal lattice is extremely small and can be ignored in lake ice
investigations.
It is seen (Fig. 3.1 b) that the ice crystal tetrahedra possess hexagonal symmetry, which
is often visible, e.g.,

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owers. The
arrangement of molecules is, however, anisotropic in the crystal lattice. They are con-
centrated in a series of parallel planes, which are called basal planes. Normal to the basal
plane is the crystal c-axis or the optical axis. The basal plane is isotropic but there is

in the morphology of snow crystals and frost fl