Unfrozen water and ice in ground

The phase composition of moisture, i.e. the content of vapour, unfrozen water and ice in the frozen ground, predetermines its specific physical and mechanical properties and the pattern of occurrence of cryogenic-geological processes.

The liquid phase of water in the frozen sediments can be in different energetic and structural states ranging from the state of free water to the substantially modified structure in the immediate vicinity of the surface of mineral particles.

A triangle is formed by the atoms of the free molecule of H20 with an angle H-O-H equalling about 104.5° (in ice 109°). The lengths O-H and H-H are equal to 0.096 and 0.154 nm, respectively (Fig. 7.2). Water, having a specific structure, is characterized by a number of anomalous properties. Thus, on melting of the solid phase the volume is reduced and not increased; the maximum density of water is at +4°C; the heat capacity on melting increases twofold and more - from 2.05 to 4.57kJ kg"1; the dielectric constant is rather high and varies from 88 at 0°C to 80 at +20°C, etc. The viscosity is about 1.8 x 10~2Pas at 0°C and diminishes with higher tem perature to 0.28 x 10 2 Pa s at 100°C. Surface tension at the air boundary with pressure equalling 105 Pa is 75.6 and 58.9mN m"1 at 0°C and 100°C, respectively. A sluggishness of structural transformations in water arising under the impact of external factors, is demonstrated by its properties. All the anomalous properties of water are associated with its structure, the distinctive feature of which is the availability of strong hydrogen bonds that form a quasicrystalline structure with triangular pyramid coordination of neighbouring molecules ('short-range order'). The energy of the hydrogen bond is 18.8 kJ mol"1 for the ice.

There are many models for the structures of water. The model elaborated by S.Ya. Samoylov is the one commonly accepted in Russia; it was confirmed by X-ray, spectroscopy and other methods of studying the structure and properties of water. According to Samoylov, in liquid water an ice-like lattice persists, somewhat disturbed by thermal movement, and its cavities are partially filled with monomer molecules. Transition of it into water occurs when heat is added at the rate of 0.334 kJ g"1 of ice. This takes place at breakage of 9-11 % of bonds as determined by the ratio of melting to sublimation heat units (0.334/2.834 kJ).

The ice is an important soil-forming mineral and monomineral rock in the permafrost regions. Its presence in the frozen ground takes the form of ice cement, ice inclusions and masses of concentrated ice. In general, ground ice occupies 2% of the total volume of ice of the cryosphere (about 0.5 million km3).

Ice is a specific mineral differing greatly by its composition and structure from other minerals and rocks. Some ten crystalline modifications of ice and amorphous ice are known. Under natural conditions a single ice form exists (ice 1), the crystals of which are arranged into a three-dimensional hexagonal grid and belong to the detriangular pyramid-like type of the triangular symmetry system. Such structure consists of six molecules of water forming a regular hexagonal cell with axis b = 0.9 nm (Fig. 7.3). Such a branched texture of the ice crystalline grid is dictated by the nature of hydrogen bonds that exist between its molecules. According to current hypothetical considerations tetrahedral molecules of water form the tetrahedronal aggregates of the ice structure. The coordination number of the ice structure is thus equal to four. The distance between the nearest centres of molecules is 0.267 nm. It was found that ice is built of discrete molecules of water connected by hydrogen bonds. The protons are arranged along bonds between the atoms of oxygen at a distance of 0.099 nm from the nucleus of one atom and 0.17nm from that of another. Around each molecule six centres of voids are arranged at a distance of 0.347 nm, while voids them-

Fig. 7.3. Ice structure: a - model (top view); b - fragment of ice structure showing immediate surroundings of an H20 molecule.

selves exceeding the sizes of molecules form the channels made by alternating hexagonal cells.

The molecules of water, and, consequently, of ice comprise stable isotopes 2H, 17O and lsO, apart from isotopes of'light' water XH2160. The content of stable isotopes of hydrogen and oxygen varies both spatially and in time, determined by different phase transitions (evaporation, condensation, thawing and freezing).

Ice in the frozen materials always contains admixtures in the form of solid, liquid and gaseous inclusions. According to origin, these are subdivided into primary (formed simultaneously with ice) and secondary, arising after the ice has been formed. The primary inclusions are authigenic (isolated from water or captured by ice during freezing) and xenogenic (formed of foreign admixtures in water). The secondary ones include hyper-genic (penetrated through open fractures and pores connected with the surface) and hypogenic inclusions (that fill cracks and pores isolated as a result of regeneration).

Different composition and formation conditions of underground ice lead to a variety of structure and texture types. Ice structure is determined by the shape, size, type of surface, quantitative ratio and the nature of interrelationship of structural elements. For ground ice these are ice crystals, air- and organo-mineral inclusions. The structure of ice is characterized by the relation of crystallographic orientation to the external shape of the crystals and the relation of the same orientation to the elements of occurrence of the ice rock, i.e. by the degree of ordering of the structure.

The following structures of ice are distinguished as dependent upon the shape and crystallographic orientation of grains: 1) prismatic-granular, where the ice crystals have a regular and ordered crystallographic orienta tion proper to them (main axes of symmetry are parallel); 2) allotriomor-phic-granular (irregular granular) with orderless crystallographic orientation; 3) hypidiomorphic-granular which is intermediate between the two aforementioned forms.

Ice texture is determined by the spatial arrangement of its components -crystals of different size and shape, air and mineral inclusions as well as by the degree of infilling. The most important attributes of the texture of ice are associated with the specific distribution of inclusions. In the absence of admixtures the ice texture is considered to be massive or glass-like, whereas with prevalence of gas in the ice volume it is called bubble-type; with intercalation of admixtures it is called laminated. Schistose texture is typical of ice composed of flat and prismatic crystals that form parallel layers. The structure and texture of ground ice reflect the conditions of crystal growth, availability of foreign admixtures in the form of insoluble inclusions and dissolved salts and gases as well as the thermodynamic conditions.

Ice crystals are characterized by anisotropy of mechanical, thermal-physical, optical, electrical and other properties, which are identified on measurement in different crystallographic orientations. These distinctions are associated with the crystallographic characteristics of ice in the spatial grid, in which the main role is played by epipolar planes that have a high reticular density of molecules.

Temperature regime changes cause thermal deformations of ice. Thermal deformations are measured by coefficients of linear (a) and volumetric (/?) expansion. These coefficients are computed as a relative change of length or olume of a body on a temperature change of 1°C. Coefficients a and /? increase rapidly towards the point of melting and become extremely low at low temperatures (Fig. 7.4).

Distribution of heat in ice is determined by its thermal conductivity (1). At an ice temperature of 0°C, k « 2.22(Wm"1 K~x). This value exceeds fourfold the thermal conductivity of pure water at 0°C. With lower temperatures the thermal conductivity of ice increases (see Fig. 7.4). It can be estimated from simple empiric relationships, for example, for fresh (nonsaline) ice k = 2.22 (l-0.0004t) where t is temperature (°C). With higher porosity and salinity, thermal conductivity diminishes. At a constant pressure the molecular heat capacity Cm = 37.7 J mol"1 K"1 at the melting point. The heat capacity diminishes with lower temperature (see Fig. 7.4). For nonsaline ice heat capacity C = 2.12 + 0.0078t. Heat capacity of ice is to a greater extent dependent on the amount of admixtures, especially at temperatures approaching the melting point.

The structural distinction existing between ice and water which is charac-

Cm or 106 k "1 J mole"1 K"1 XWm'K"1 60 - 12 -

0 0

Post a comment