MINERALOGICAL COMPOSITION
The most important grain property of fine-grained soil materials is the mineralogical composition. If the soil particles are smaller than about 0.002 mm, the influence of the force of gravity on each particle is insignificant compared with that of the electrical forces acting at the surface of the particle. A material in which the influence of the surface charges is predominant is said to be in the colloidal state. The colloidal particles of soil consist primarily of clay minerals that were derived from rock minerals by weathering, but that have crystal structures differing from those of the parent minerals.
The most important grain property of fine-grained soil materials is the mineralogical composition. If the soil particles are smaller than about 0.002 mm, the influence of the force of gravity on each particle is insignificant compared with that of the electrical forces acting at the surface of the particle. A material in which the influence of the surface charges is predominant is said to be in the colloidal state. The colloidal particles of soil consist primarily of clay minerals that were derived from rock minerals by weathering, but that have crystal structures differing from those of the parent minerals.
All the clay minerals are crystalline hydrous aluminosilicates having a lattice structure in which the atoms are arranged in several layers, similar to the pages of a book. The arrangement and the chemical composition of these layers determine the type of clay mineral.
The basic building blocks of the clay minerals are the silica tetrahedron and the alumina octahedron. These blocks from tetrahedral and octahedral layers (Figure 1), different combinations of which produce a unit sheet of the various types of clays.
Figure 1: (a) Tetrahedral Layer, (b) Octahedral Layer (After Grim, 1968)
A single particle of clay may consist of many sheets of films piled one on another. Because each sheet or film has a definite thickness but is not limited in dimensions at right angles to its thickness, clay particles are likely to exhibit flat or curved terraced surfaces. The surfaces carry residual negative electrical charges, but the broken edges may carry either positive or negative charges in accordance with the environment.
In problems of interest to the civil engineer, clay particles are always in contact with water. The interactions among the clay particles, the water, and the various materials dissolved in the water are primarily responsible for the properties of the soil consisting of the particles.
CHARACTERISTICS OF PRINCIPAL CLAY MINERALS
Kaolinite is one of the most common clays minerals in sedimentary and residual soils (Grim 1968, Swindale 1975). A unit sheet of kaolinite, which is approximately 0.7 nm (nm = 10−9 m) thick, is composed of one aluminum octahedral layer and one silicon tetrahedral layer, joined together by shared oxygens. A typical particle of kaolinite consists of a stack sheets forming a stiff hexagonal plate with flat-faced edges. It is about 100 nm in thickness with a breadth/thickness of about 5 to 10, and a specif surface of 5 to 15 m2/g. The scanning electron microscope (SEM) photomicrograph in Figure 2 shows a range of particle sizes and shapes including terraced surfaces where packets of 0.7-nm sheets terminate.
Halloysite is one of the most common minerals in residual soils, particularly those derived from volcanic parent material. It is a member of the kaolin subgroup of clay minerals. A unit sheet of hydrated halloysite, including one molecular layer of water, is approximately 1 nm thick. A typical particle has the shape of a hollow tube or prism with the outside and inside diameters of 70 and 40 nm, respectively, and 300 to 500 nm long. The intersheet water in the hydrated halloysite is removed irreversibly starting 60° to 75° C.
Allophane, a major constituent of young residual soils formed from volcanic ash, is an amorphous hydrous aluminosilicate commonly associated with halloysite (Grim 1968, Fieldes and Claridge 1975, Wesley 1973). In some residual soils, the transition to halloysite from allophane, which forms at early stages of weathering, is not well defined. Allophane consists of very loosely packed chains of silica tetrahedra and alumina octahedra, cross-linked at a relatively small number of points. In the natural state, it exists as microaggregates of extremely fine particles of the order of several nanometers and specific surface areas of 250 to 800 m2/g. The allophane aggregates or clusters are relatively incompressible. They are sometimes cemented by iron or aluminum oxides, and they enclose a large amount of water. Since the aggregates are susceptible to structural breakdown upon mechanical manipulation, as by heavy equipment, a deposit of allophane may change from a granular material to a plastic sticky mass that cannot be handled easily. The natural aggregates do not readily lose water; however, when water is removed and the fabric shrinks, the process cannot be reserved. The resulting soil, with silt- and sand sized particles that are quite hard, is practically nonplastic.
Illite is the most common clay mineral in stiff clays and shales as well as in postglacial marine and lacustrine soft clay and silt deposits (Grim 1968, Radoslovic 1975, Reichenbach and Rich 1975). It is often present, sometimes interstratified with other sheet silicates, in sedimentary and residual soils, except in residual soils derived from amorphous volcanic material. Illite is also reffered to as fine-grained mica and weathered mica. The crystal structure of illite is similar to that of muscovite mica in the macroscopic form. However, in microscopic illite particles the stacking of the sheets is not so regular as in well-crystallized micas, and weathering may remove intersheet K+ from the edges of the plates. The resulting illite particles, with terraced surfaces where one or more unit sheets terminate, have frayed and tattered edges, are flexible and elastic, are 10 to 30 nm in thickness, have a breadth/thickness of 15 to 30, and have a specific surface of 80 to 100 m2/g. A SEM photomicrograph of illite particle is shown in Figure 3.
Chlorite is a clay mineral commonly associated with micas and illite, but is usually a minor component (Grim 1968, Bailey 1975). A unit sheet of chlorite, which is about 1.4 nm thick, consists of one biotite mica sheet in which all octahedral sites are occupied by magnesium and one brucite sheet, an octrahedral layer in which magnesium atoms are in octahedral coordination with hydroxyls. The biotite mica and brucite sheets are strongly bonded together. The unit sheets are stacked and are connected to each other by hydrogen bonding of surface oxygens of the tetrahedral layer of mica and surface hydroxyls of brucite. The size and platyness of chlorite particles are similar to those of illite. However, in contrast to illite, in which the unit sheets are bonded by potassium, the hydrogen bonding between chlorite sheets results in pseudohexagonal or euhedral-shaped platelets that are flexible but inelastic.
Montmorillonite, the most common member of a group of clay minerals known as smectites, is the dominant clay mineral in some clays and shales and in some residual soils derived from volcanic ash (Grim 1968, Mering 1975). Relatively pure seams of montmorillonite are found in some deposits, Wyoming bentonite being the best-known example. A unit sheet of montmorillonite is similar to that of the micas. In montmorillonite, octahedral Al is partially replaced by Mg atoms. Each isomorphous replacement produces a unit negative charge at the location of the substituted atom, which is balanced by exchangeable cations, such as Ca2+ and Na+ situated at the exterior of the sheets. In a packet of montmorillonite in the anhydrous state, where as many as 10 unit sheets are in contact, the stacking of the sheets is disordered in the sense that the hexagonal cavities of the adjacent surfaces of two neighboring sheets are not matched face-to-face. In a hydrous environment, water molecules penetrate between the sheets and separate them by 1 nm (i.e., four molecular layers of water).
In sodium montmorillonite, the exchange capacity is satisfied by Na cations. If the electrolyte concentration in water is less tahn 0.3 N, further separation of unit sheets takes place to more than 3nm by diffuse doublelayer repulsion. The resulting particles of sodium montmorillonite in water are thin films 1 nm thick, with breadth-to-thickness ratios in excess of 100 and specific surfaces as much as 800 m2/g. In calcium montmorillonite, the electrostatic attraction of the Ca cations links the successive sheets together and prevents seperation beyond 1 nm. The resulting domains of 8 to 10 unit sheets, which are separated from each other by up to four molecular layers of water, experience minimal swelling. Among clay minerals, sodium montmorillonite has the smallest and most filmy particles, as shown by the SEM photomicrograph in Figure 4.
Attapulgite is a fibrous clay mineral composed of silica chains linked together by oxygens along their longitudinal edges to form single laths or bundles of laths (grim 1968, Henin and Caillere 1975). Water molecules fill the interstices between the chains. The particles of attapulgite are relatively rigid, 5 to 10 nm thick, 10 to 20 nm wide, and 0.1 to 1µm in length, as shown in the SEM, Figure 5. The specific surface of the bundles is about 150 m2/g, but it can be much higher of dispersed single laths. Attapulgite, frequently associated with carbonate rocks, is not a very common clay mineral in soil deposits. However, when present, it results in unusual physical properties such as a very high plastic limit, a very high plasticity index, and high frictional resistance.
MIXED-LAYER CLAY MINERALS
Particles in some soils are composed of regularly or randomly interstratified unit sheets of two or more types of clay minerals (MacEwan and Ruiz-Amil 1975). For example, a marine or lacustrine clay deposit may include regularly alternating montmorillonite-illite particles, and a volcanic ash residual soil may contain randomly interstratified halloysite and hydrated halloysite. These mixed-layer clay minerals often represent an intermediate phase in the transformation, by weathering or through a diagenetic process, of one mineral to another mineral, such as that of mica to montmorillonite or vice versa. However, the physical properties of interstratified clay minerals are not well defined, inasmuch as a wide range of combinations is possible. Mixed-layer clay mineral particles possess properties generally representative of the constituent minerals.
ROLE OF ISOMORPHIC SUBSTITUTION
Isomorphic substitution is the replacement of a cation in the mineral structure by another cation of lower electrovalence. The difference in the valences leads to a negative charge, and the difference in size of the cations produces a distortion of the mineral structure. Both effects tend to decrease the resistance of a mineral structure to chemical and mechanical weathering. Quartz is a space-lattice silicate composed of silica tetrahedrons, (SiO4)-4 linked together by primary valence bonds to form a three-dimensional network with the formula SiO2. There is no isomorphous substitution in quartz, and each silica tetrahedron is firmly and equally braced in all directions. As a result, quartz has no planes of weakness and is very hard and highly resistant to mechanical and chemical weathering. Quartz is not only the most common mineral in sand-and silt-sized particles of soils, but quartz or amorphous silica is frequently present in colloidal (1 to 100 nm) and molecular (< 1 nm) dimensions (Mitchell 1975). In some young soils of volcanic origin, clay-sized particles may consist essentially of silica, an appreciable proportion of which is amorphous. The boundary between quartz and amorphous silica, however, is not distinct, inasmuch as physical processes such as grinding may render crystalline quartz amorphous.
In feldspar, some of the silicon are replaced by aluminum. This results in a negative charge and in distortion of the crystal structure, because Al atoms are larger than Si atoms. The negative charge is balanced by taking in cations such as K+, Na+, Ca2+ in orthoclase, albite, and anorthite feldspars, respectively. The distortion of the lattice and the inclusion of the cations cause cleavage planes that reduce the resistance of feldspars to mechanical and chemical weathering. For these reasons, feldspars are not so common as quartz in the sand-, silt-, and clay-sized fractions of soils, even though feldspars are the most common constituent of the earth's crust.
Common micas such as muscovite and biotite are often present in the silt- and sand-sized fractions of soils. In a unit sheet of mica, which is 1 nm thick, two tetrahedral layers are linked together with one octahedral layer. In muscovite, only two of every three octahedral sites are occupied by aluminum cations, whereas in biotite all sites are occupied by magnesium. In well-crystallized micas one fourth of the tetrahedral Si4+ are replaced by Al3+. The resulting negative charge in common micas is balanced by intersheet potassiums. In a face-to-face stacking of sheets to form mica plates, the hexagonal holes on opposing tetrahedral surfaces are matched to enclose the intersheet potassium. Perfect cleavage occurs along the K-plane, however, as the intersheet bond strength of (KO12)-23 is only 1/12 of the bond strength of (SiO4)-4 inside the sheets.
This is very little isomorphous substitution in kaolinite, the particles of which are formed by hydrogen bonding of unit sheets. The edge charge is important, because the particles are thick and the edge surface area is significant. The edge charge, which results from broken bonds and exposed oxygens at the edges of the particles, and give the edge a net positive charge. When the pH is high, H+ ions disassociate from the edge, which then becomes negatively charged. In mica and illite a major part of the negative charge resulting from isomorphous substitution is neutralized by K atoms which also connect the successive unit sheets to each other to form particles.
Figure 2: Photomicrograph of Kaolinite
Halloysite is one of the most common minerals in residual soils, particularly those derived from volcanic parent material. It is a member of the kaolin subgroup of clay minerals. A unit sheet of hydrated halloysite, including one molecular layer of water, is approximately 1 nm thick. A typical particle has the shape of a hollow tube or prism with the outside and inside diameters of 70 and 40 nm, respectively, and 300 to 500 nm long. The intersheet water in the hydrated halloysite is removed irreversibly starting 60° to 75° C.
Allophane, a major constituent of young residual soils formed from volcanic ash, is an amorphous hydrous aluminosilicate commonly associated with halloysite (Grim 1968, Fieldes and Claridge 1975, Wesley 1973). In some residual soils, the transition to halloysite from allophane, which forms at early stages of weathering, is not well defined. Allophane consists of very loosely packed chains of silica tetrahedra and alumina octahedra, cross-linked at a relatively small number of points. In the natural state, it exists as microaggregates of extremely fine particles of the order of several nanometers and specific surface areas of 250 to 800 m2/g. The allophane aggregates or clusters are relatively incompressible. They are sometimes cemented by iron or aluminum oxides, and they enclose a large amount of water. Since the aggregates are susceptible to structural breakdown upon mechanical manipulation, as by heavy equipment, a deposit of allophane may change from a granular material to a plastic sticky mass that cannot be handled easily. The natural aggregates do not readily lose water; however, when water is removed and the fabric shrinks, the process cannot be reserved. The resulting soil, with silt- and sand sized particles that are quite hard, is practically nonplastic.
Illite is the most common clay mineral in stiff clays and shales as well as in postglacial marine and lacustrine soft clay and silt deposits (Grim 1968, Radoslovic 1975, Reichenbach and Rich 1975). It is often present, sometimes interstratified with other sheet silicates, in sedimentary and residual soils, except in residual soils derived from amorphous volcanic material. Illite is also reffered to as fine-grained mica and weathered mica. The crystal structure of illite is similar to that of muscovite mica in the macroscopic form. However, in microscopic illite particles the stacking of the sheets is not so regular as in well-crystallized micas, and weathering may remove intersheet K+ from the edges of the plates. The resulting illite particles, with terraced surfaces where one or more unit sheets terminate, have frayed and tattered edges, are flexible and elastic, are 10 to 30 nm in thickness, have a breadth/thickness of 15 to 30, and have a specific surface of 80 to 100 m2/g. A SEM photomicrograph of illite particle is shown in Figure 3.
Figure 3: Photomicrograph of Illite
Chlorite is a clay mineral commonly associated with micas and illite, but is usually a minor component (Grim 1968, Bailey 1975). A unit sheet of chlorite, which is about 1.4 nm thick, consists of one biotite mica sheet in which all octahedral sites are occupied by magnesium and one brucite sheet, an octrahedral layer in which magnesium atoms are in octahedral coordination with hydroxyls. The biotite mica and brucite sheets are strongly bonded together. The unit sheets are stacked and are connected to each other by hydrogen bonding of surface oxygens of the tetrahedral layer of mica and surface hydroxyls of brucite. The size and platyness of chlorite particles are similar to those of illite. However, in contrast to illite, in which the unit sheets are bonded by potassium, the hydrogen bonding between chlorite sheets results in pseudohexagonal or euhedral-shaped platelets that are flexible but inelastic.
Montmorillonite, the most common member of a group of clay minerals known as smectites, is the dominant clay mineral in some clays and shales and in some residual soils derived from volcanic ash (Grim 1968, Mering 1975). Relatively pure seams of montmorillonite are found in some deposits, Wyoming bentonite being the best-known example. A unit sheet of montmorillonite is similar to that of the micas. In montmorillonite, octahedral Al is partially replaced by Mg atoms. Each isomorphous replacement produces a unit negative charge at the location of the substituted atom, which is balanced by exchangeable cations, such as Ca2+ and Na+ situated at the exterior of the sheets. In a packet of montmorillonite in the anhydrous state, where as many as 10 unit sheets are in contact, the stacking of the sheets is disordered in the sense that the hexagonal cavities of the adjacent surfaces of two neighboring sheets are not matched face-to-face. In a hydrous environment, water molecules penetrate between the sheets and separate them by 1 nm (i.e., four molecular layers of water).
In sodium montmorillonite, the exchange capacity is satisfied by Na cations. If the electrolyte concentration in water is less tahn 0.3 N, further separation of unit sheets takes place to more than 3nm by diffuse doublelayer repulsion. The resulting particles of sodium montmorillonite in water are thin films 1 nm thick, with breadth-to-thickness ratios in excess of 100 and specific surfaces as much as 800 m2/g. In calcium montmorillonite, the electrostatic attraction of the Ca cations links the successive sheets together and prevents seperation beyond 1 nm. The resulting domains of 8 to 10 unit sheets, which are separated from each other by up to four molecular layers of water, experience minimal swelling. Among clay minerals, sodium montmorillonite has the smallest and most filmy particles, as shown by the SEM photomicrograph in Figure 4.
Figure 4: Photomicrograph of Montmorillonite
Attapulgite is a fibrous clay mineral composed of silica chains linked together by oxygens along their longitudinal edges to form single laths or bundles of laths (grim 1968, Henin and Caillere 1975). Water molecules fill the interstices between the chains. The particles of attapulgite are relatively rigid, 5 to 10 nm thick, 10 to 20 nm wide, and 0.1 to 1µm in length, as shown in the SEM, Figure 5. The specific surface of the bundles is about 150 m2/g, but it can be much higher of dispersed single laths. Attapulgite, frequently associated with carbonate rocks, is not a very common clay mineral in soil deposits. However, when present, it results in unusual physical properties such as a very high plastic limit, a very high plasticity index, and high frictional resistance.
Figure 5: Photomicrograph of Attapulgite
MIXED-LAYER CLAY MINERALS
Particles in some soils are composed of regularly or randomly interstratified unit sheets of two or more types of clay minerals (MacEwan and Ruiz-Amil 1975). For example, a marine or lacustrine clay deposit may include regularly alternating montmorillonite-illite particles, and a volcanic ash residual soil may contain randomly interstratified halloysite and hydrated halloysite. These mixed-layer clay minerals often represent an intermediate phase in the transformation, by weathering or through a diagenetic process, of one mineral to another mineral, such as that of mica to montmorillonite or vice versa. However, the physical properties of interstratified clay minerals are not well defined, inasmuch as a wide range of combinations is possible. Mixed-layer clay mineral particles possess properties generally representative of the constituent minerals.
ROLE OF ISOMORPHIC SUBSTITUTION
Isomorphic substitution is the replacement of a cation in the mineral structure by another cation of lower electrovalence. The difference in the valences leads to a negative charge, and the difference in size of the cations produces a distortion of the mineral structure. Both effects tend to decrease the resistance of a mineral structure to chemical and mechanical weathering. Quartz is a space-lattice silicate composed of silica tetrahedrons, (SiO4)-4 linked together by primary valence bonds to form a three-dimensional network with the formula SiO2. There is no isomorphous substitution in quartz, and each silica tetrahedron is firmly and equally braced in all directions. As a result, quartz has no planes of weakness and is very hard and highly resistant to mechanical and chemical weathering. Quartz is not only the most common mineral in sand-and silt-sized particles of soils, but quartz or amorphous silica is frequently present in colloidal (1 to 100 nm) and molecular (< 1 nm) dimensions (Mitchell 1975). In some young soils of volcanic origin, clay-sized particles may consist essentially of silica, an appreciable proportion of which is amorphous. The boundary between quartz and amorphous silica, however, is not distinct, inasmuch as physical processes such as grinding may render crystalline quartz amorphous.
In feldspar, some of the silicon are replaced by aluminum. This results in a negative charge and in distortion of the crystal structure, because Al atoms are larger than Si atoms. The negative charge is balanced by taking in cations such as K+, Na+, Ca2+ in orthoclase, albite, and anorthite feldspars, respectively. The distortion of the lattice and the inclusion of the cations cause cleavage planes that reduce the resistance of feldspars to mechanical and chemical weathering. For these reasons, feldspars are not so common as quartz in the sand-, silt-, and clay-sized fractions of soils, even though feldspars are the most common constituent of the earth's crust.
Common micas such as muscovite and biotite are often present in the silt- and sand-sized fractions of soils. In a unit sheet of mica, which is 1 nm thick, two tetrahedral layers are linked together with one octahedral layer. In muscovite, only two of every three octahedral sites are occupied by aluminum cations, whereas in biotite all sites are occupied by magnesium. In well-crystallized micas one fourth of the tetrahedral Si4+ are replaced by Al3+. The resulting negative charge in common micas is balanced by intersheet potassiums. In a face-to-face stacking of sheets to form mica plates, the hexagonal holes on opposing tetrahedral surfaces are matched to enclose the intersheet potassium. Perfect cleavage occurs along the K-plane, however, as the intersheet bond strength of (KO12)-23 is only 1/12 of the bond strength of (SiO4)-4 inside the sheets.
This is very little isomorphous substitution in kaolinite, the particles of which are formed by hydrogen bonding of unit sheets. The edge charge is important, because the particles are thick and the edge surface area is significant. The edge charge, which results from broken bonds and exposed oxygens at the edges of the particles, and give the edge a net positive charge. When the pH is high, H+ ions disassociate from the edge, which then becomes negatively charged. In mica and illite a major part of the negative charge resulting from isomorphous substitution is neutralized by K atoms which also connect the successive unit sheets to each other to form particles.
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