In this article we will discuss about:- 1. Composition of Clay Minerals 2. Isomorphous Substitution 3. Properties of Clay Minerals.
Clay minerals in soils belong to phyllosilicates group of minerals, which in turn belong to silicate minerals. Silicate minerals make up about 90% of the rock-forming minerals of the earth’s crust. The word Phyllon in the Greek means leaf and thus the silicate minerals having leaf or sheet-like structure with thickness considerably smaller than the other two dimensions, are known as phyllosilicate minerals. The other mineral classes under phyllosilicate group along with clay minerals are serpentine, pyrophyllite, talc, mica, and chlorite.
Clay minerals occur in small particle sizes, and their unit cells ordinarily have a residual negative charge that is balanced by the adsorption of cations from solution. Clay minerals are oxides of aluminum and silicon, which are called aluminosilicates. Clay minerals are hydrated aluminum silicates and hydrous oxides of aluminum, magnesium, and iron.
Clay minerals are made up of the following two basic structural units or building blocks:
i. Silica tetrahedralunit.
ii. Aluminum or magnesium octahedral unit.
Individual silica tetrahedral units are combined with their neighboring units on the sides to form a thin silica sheet. Similarly, several aluminum octahedral units in the neighborhood on the sides are combined to form a thin aluminum octahedral sheet.
The different ways in which the silica and alumina sheets are combined in layers of these units and the manner in which two successive two- or three-sheet layers are held together result in the formation of different types of clay minerals. The type and amount of isomorphous substitution within the crystal structure brings about the differences among minerals within the clay mineral groups.
The silica unit consists of a silicon atom surrounded by four oxygen atoms, equidistant from the silicon, at the corners of the tetrahedron with the silicon at the center of the tetrahedron. Thus, the silica tetrahedral unit has four (tetra = four) triangular faces with three faces on the sides and one at the bottom. The O-O distance is 2.55 Å (Å = Angstrom = 10 –10 m), with the silicon of radius 0.5 Å, fitting into the central space of radius 0.55 Å between the oxygen atoms.
Several individual silica tetrahedral units are combined in such a way that the three oxygen atoms, forming the base of the tetrahedral unit, are shared each with one neighboring silica tetrahedral unit. Each oxygen atom at the base being common to two adjacent tetrahedral units, shares their negative charges.
Silicon atoms have one oxygen atom at the apex of the tetrahedron, which is exclusive to the atom and is not shared by the neighboring atoms. The contribution of negative charge by the oxygen atoms to each silicon atom is – (1 × 2 + 1/2 × 4 × 2) = – 5. Thus, each silica unit has a negative charge of – 5 + 4 = – 1.
Some of the silicon atoms (Si4+) in the silica sheet may be replaced by aluminum atoms (Al3+) by a process known as isomorphous substitution. As the aluminum atom has lesser positive charge (+3) compared to the silicon atom (+4), the negative charge of the silica sheet further increases to – 2 for the substitution of every silicon atom by aluminum.
The silica sheet can be visualized as a row of silicon atoms at the middle, with a row of oxygen atoms at the top and bottom. The number of oxygen atoms at the top and bottom layers is in the ratio of 1:3. The silica sheet has a thickness of 4.93 Å.
An aluminum octahedral unit has six OH ions (known as hydroxyl ions) at each of the six corners of an octahedron, with the aluminum atom at the center of the octahedron. The octahedral unit has eight triangular faces, six on the sides and one each at the top and bottom of the octahedron, as shown in Fig. 3.2(a).
When several aluminum octahedral units are combined, an octahedral sheet is formed. In an octahedral sheet, each hydroxyl ion is shared by the neighboring octahedral units both at the top and at the bottom. For each octahedral unit, the charge contribution from six hydroxyl ions is –3, which is balanced by the positive charge (+3) of aluminum atom (Al3+), resulting in an electrically neutral octahedral sheet. The alumina sheet can be visualized as the two rows of oxygen or hydroxyl ions with a row of aluminum atoms in the middle.
In an alumina sheet, only 2/3 of the aluminum positions are usually occupied by the aluminum atom, the third position being left vacant. Such units are called dioctahedral units. The deficiency of aluminum atom in the octahedral units gives a charge of –1 to the alumina sheet for each unit, which is 5.05 Å thick.
When the alumina sheet consists of only hydroxyl ions and aluminum atoms, this sheet is called gibbsite sheet, represented by the letter G. However, the aluminum (AP+) atoms in some of the octahedrons of the octahedral sheet may be replaced by magnesium (Mg2+) atoms due to isomorphous substitution.
This gives a net charge of -1 per each octahedral unit of the octahedral sheet. When aluminum atoms are replaced by magnesium by isomorphous substitution, all the aluminum positions are filled (called trioctahedral) by magnesium and such a sheet is called brucite.
The distance between OH ions in octahedral arrangement in the alumina sheet is 2.94 Å. When oxygen atoms occupy the positions of OH ions, the distance between oxygen atoms is 2.6 Å. The alumina sheet is formed by bonding of individual structural units of the aluminum octahedron in longitudinal and lateral directions, as shown in Fig. 3.2(b). Alumina sheet is schematically represented as shown in Fig. 3.2(c) and the brucite sheet as in Fig. 3.2(d). When silica and alumina sheets are combined in different ways, different types of clay minerals are formed.
Isomorphous Substitution in Clay Minerals:
In an ideal silica sheet, silicon atoms occupy all tetrahedral spaces. In clay minerals, however, some of the tetrahedral spaces are occupied by aluminum instead of silicon atom. This substitution of one atom by another may occur during the initial formation or subsequent alteration of clay minerals.
The substitution of an atom or ion in the tetrahedral positions of silica sheet or octahedral positions in the gibbsite or brucite sheet in clay minerals by other atoms or ions of approximately the same size without any change in the crystal structure is known as isomorphous substitution.
Similarly, in an ideal brucite sheet, all the octahedral spaces are filled by magnesium. Some of the magnesium atoms of the brucite sheet in clay minerals may be replaced by iron (Fe2+) by isomorphous substitution. Some of the aluminum atoms (Al3+) of the gibbsite sheet in clay minerals may also be replaced by magnesium (Mg2+) by isomorphous substitution.
A silicon (Si4+) atom has four positive charges and aluminum has three positive charges. When silicon is replaced by aluminum (Al3+) in the silica sheet, there is a deficiency of a positive charge, resulting in a net negative charge for each substitution. Similarly, the deficiency of a positive charge occurs causing a net negative charge for each substitution of aluminum (Al3+) by magnesium (Mg2+) in the brucite sheet or by iron (Fe2+) in a gibbsite sheet .Thus, the effect of isomorphous substitution in clay minerals is to give a negative charge to the particles. The negative charge acquired by the clay particles due to isomorphous substitution brings about important properties of plasticity and compressibility to the clays in the presence of water.
In clay mica (illite), aluminum substitutes for 20% of silicon in the tetrahedral sheet. In vermiculite, the substitutions are in all sheets. In montmorillonite, vermiculite, and mica, aluminum substitutes for silicon in the silica sheet. Kaolinites showed occasionally the small isomorphous substitution of Al3+ for Si2+ in the tetrahedral layer and Mg2+ and Fe3+ for Al3+ in the octahedral layer, leading to a permanent negative charge.
Isomorphic substitution is one of the sources of permanent negative charge in soils. It is the primary source of a soil’s ability to hold and exchange cations. This ability to hold and exchange cations is referred to as CEC.
The following properties of clay minerals are important to understand their influence on the behavior of soils:
1. Specific surface.
2. Cation exchange capacity.
Specific surface is the surface area of the given clay mineral per unit weight. The higher the specific surface, the more is the magnitude of the surface forces of attraction and repulsion of electrical nature per unit weight of soil and their influence over the gravity forces. Clays with higher specific surface, in general, have higher liquid limit, free swell index, activity, and CEC and lesser shear strength.
Clay minerals acquire negative charge on the particle surface due to isomorphous substitution. The negatively charged particle surface attracts positively charged ions called cations (pronounced as cat-eye-ons) available in the surrounding pore water to balance the negative charge for equilibrium. An ion with a positive charge is called a cation that can be either basic or acidic.
The cations attracted to the clay particle surface are not rigidly fixed or permanently held by the clay particle and can always be replaced by other cations available in the pore water. Thus, the cations attracted to the clay particle surface are called exchangeable cations.
The maximum mass of all cations that a soil is capable of holding, at a given pH value for exchanging with the cations in the soil solution, is known as cation exchange capacity (CEC). CEC is expressed in milliequivalent (meq) of hydrogen per 100 g of dry soil. One meq is equivalent to 1 mg of hydrogen or its equivalent mass of any other cations that will replace 1 mg of hydrogen.
The term CEC originated in agriculture as a measure of fertility and nutrient retention capacity. However, it also became an important parameter in the study of clay minerals to understand their engineering behavior. For agricultural soils, the CEC is ideally between 10 and 30 meq/100 g.
Organic matter present in soils also has negative charge and attracts exchangeable cations. Thus, the CEC in soils increases with the increase in clay content and organic matter.
The larger the CEC, the more cations the soil can hold. A clay soil will have a larger CEC than a sandy soil. Kaolinite has a very little capacity to hold cations.
The most common exchangeable cations in soils in the alkaline environment are Na, Ca, K, and Mg. The part of the exchange capacity contributed by the base cations of Na, Ca, K, and Mg is known as the base exchange capacity. In the acidic environment, hydrogen and aluminum are the common exchangeable cations. Thus, the CEC is equal to the sum of the base exchange capacity and the exchange capacity contributed by the acidic ions such as hydrogen and aluminum. Each clay mineral has different quantum of negative charges and hence different CEC.
The word exchange signifies the fact that the cations attracted to the negatively charged particle surface may be replaced or exchanged by other cations present in pore water.
The ability of a cation to replace other cations depends on (a) The valence of the cation, (b) The size of the cation, and (c) Geometrical conditions.
The higher the valence of the cation, the higher is its replacing power and the harder it is to replace such cations. For example, magnesium which is divalent (Mg2+) has the power to replace Na+ and K+ which are monovalent. Na+ and K+ cannot replace Mg2+ under normal conditions.
When the valence of cation is the same as the valence of the cation to be replaced, the larger the cation, the more replacing power it has. Certain geometrical conditions sometimes influence the replacing power of cations. For example, potassium, which is a monovalent cation, exactly fits into the hexagonal holes of the silica sheet and hence is hard to be replaced.
Cations can be arranged in a series based on the ability of a cation to replace other cations as follows:
Li+ < Na+ < H < K+ < NH4+ < Mg2+ < Ca2+ < Al3+
Following are the two standard methods for determining the CEC:
i. Extraction with ammonium acetate.
ii. Silver-thiourea method (one-step centrifugal extraction).