After reading this article you will learn about:- 1. Concept of Silicate Clays 2. Structure of Silicate Clays 3. Classification 4. Sources of Negative Charges.
Concept of Silicate Clays:
(i) Size and Chemical Composition:
The chemical analysis of clay indicates the presence of silica, alumina, iron and combined water. These make up from 90-98 per cent of the colloidal clay. The soil colloidal matter contains plant nutrients like Ca, Mg and K etc. Clay is a mixture of hydrated aluminoferro silicates of varying composition mixed in some cases with an excess of sesquioxides or silica.
The term “clay” has three meanings in soil usage:
(1) it is a particle fraction composed of any particles less than 2 microns (< 2Âµ) in effective diameter,
(2) it is a name for minerals of specific composition; and
(3) it is a soil textural class. Many materials of the clay size fraction, such as gypsum, carbonates or quartz are small enough to be classified as “clay” on the basis of size but are not “clay minerals”. Sometimes some clay minerals have a size of 4 or 5 p (double the upper size limit of the “clay size fraction”).
Silicate clay minerals have been examined by electron microscope and found that the particles are laminated made up of layers of plates or flakes or even rods. Each clay particle is made up of a large number of plates like structural units.
The different units or flakes of clay minerals are held together with varying degrees of force depending upon the nature of the clay mineral. The edges of some clay particles are clean cut and others are frayed or fluffy. In all cases, clay minerals are developed more in the horizontal axis than that of vertical axis.
(iii) Surface Area:
The surface area of a clay particle is usually defined as the area of the particle that is accessible to ions or molecules when the clay is in an aqueous solution. All clay particles (finer fraction of soil) must expose a large amount of external surface.
In some clay there are extensive internal surfaces as well. This internal exists between the plate like crystal units that make up each particle (Fig. 9.1). So the large surface area of clay colloids is not only due to its fineness but also its plate-like structure.
Surface areas of clay particles can be measured by using cetylpyridinium bromide (dissolving in water) for fully dispersed clay suspensions. Surface area for clays like sodium montmorillonite 700-800 m2/g; vermiculites and some mixed layer clays 300-500 m2/g; micaceous clays 100-300 m2/g; and kaolinitic clays 5-100 m2/g.
Amorphous clays (oxygen and other atoms less regularly oriented) have surface areas between 100 and 500 m2/g.
(iv) Electronegative Charge:
Clay micelles (micro cells) carry negative charges and so a number of oppositely charged ions (cations) are attracted to each colloidal clay crystal. The colloidal clay particles have inner ionic layer (surfaces of highly negative charge) and the outer ionic layer (cations swarming layer).
(v) Adsorbed Cations:
Clay micelles adsorb a number of cations-humid, arid and semiarid regions colloids-cations are H+, Al3+, Ca2+, Mg/+, Na+ and K+. The amount of these cations held by clay varies with its kind. Cations adsorbed (if dominant) on the clay colloids very oftenly determines the physical and chemical properties of the soil and thereby influence the plant growth.
Alkali soil dominated by sodium on the surface of the clay micelle causes poor physical condition of the soil.
They are mixtures of silica and aluminium that have not formed well-oriented crystals. Technically they are not minerals because they lack crystallinity. These clays occur where large amounts of weathered products existed but have not had the conditions or time for good crystal formation. Amorphous clays are common in soils forming from volcanic ash.
Their properties are inconsistent, such as having high positive charges (high anion exchange capacities) or even high cation exchange capacity. Because almost all of their charge is from hydroxyl (OH”) ions, which can gain a positive ion or lose the H+ attached, these clays have a variable charge that depends on how much H+ is in solution (the soil acidity).
Sesquioxide Clays (Metal Oxides and Hydrous Oxides):
Under conditions of extensive leaching by rainfall and long-time intensive weathering of minerals in humid warm climates, most of the silica and much of the aluminium are dissolved and slowly leached away. The remanant materials, which have lower solubilities, are sesquioxides.
Sesquioxides (metal oxides) are mixtures of aluminium hydroxide, Al(OH)3, and iron oxide, Fe2O3 or iron hydroxide, Fe(OH)3. Sesquioxides refer to the clays of iron and aluminium because their formula can be written Al2O3.xH2O and Fe3O3; xH2O, one and one-half times more oxygen than of Al or Fe. These clays may be amorphous or crystalline and do not swell. They are not sticky and do not behave as like that of the silicate clays.
Structure of Silicate Clays:
Most of the silicate clays are made up of planes of oxygen atoms with silicon and aluminium atoms holding the oxygen together by ionic bonding, which is the attraction of positively and negatively charged atoms. Three or four planes of oxygen atoms with intervening silicon and aluminium ions make up a layer.
One clay particle is composed of many layers stacked like a deck of cards. Most silicate clays are aluminosilicates (aluminium and silicon components of the clay structure). These two basic molecular components are shown in Fig. 9.2.
One silicon atom surrounded by four oxygen atoms forming silica tetrahedron (because of its four sided configuration) and the unit has tetrahedral coordination. The planes of oxygen held together by Si4+ are tetrahedrally oriented and are referred to as a silica tetrahedral sheet. The aluminium octahedron is an eight sided building block consisting of central aluminium atom surrounded by six hydroxyls or oxygen’s (Fig. 9.2).
Large numbers of aluminiumoctahedra, bound to each other by shared oxygen atoms in an octahedral layer, are arranged in a plane forming aluminium octahedral sheet. One silica sheet per one aluminium sheet is a 1: 1 lattice A 2: 1 lattice has 2 silica sheets per 1 aluminium sheet.
Different combinations of these two general structural units (tetrahedral and octahedral sheets) form the structures of the various layer silicates like mica, vermiculite, montmorillonite, chlorite, kaolinite and other interstratified and intergradient layer silicates.
Classification of Silicate Clays:
On the basis of the number and arrangement of silica and alumina sheets, silicate clays may be classified into four different groups:
(i) 1: 1 type minerals;
(ii) 2: 1 type minerals (expand between crystal units);
(iii) 2: 1 type non-expanding minerals; and
(iv) 2: 2 type minerals.
(i) 1: 1 Type Minerals:
The most important mineral in this type, commonly found in soils, is kaolinite whose structure is shown in the Fig. 9.3. The chemical composition of kaolinite is Si4Al4 O10 (OH)8. The two sheets of each crystal unit of kaolinite are held together by oxygen atoms which are mutually shared by the silicon and aluminium atoms in their respective sheets.
These units are held together very rigidly by hydrogen bonding (-H-) to the oxygen plane of the adjacent layer. So the lattice is fixed and kaolinite mineral does not allow water to penetrate between the layers and has almost no plasticity, cohesion, shrinkage and swelling properties. Besides kaolinite, there are other minerals in this type namely halloysite, anauxite and dickite.
(ii) 2: 1 Type Minerals (Expanding Lattice Type):
There are some important minerals in this type which includes montmorillonite (smectitic group), vermiculite and other smectitic group of minerals like beidellite, nontronite and saponite etc. Montmorillonite is best known example of this type of minerals and its structure is shown (Fig. 9.4).
The flake like crystals of this mineral are composed of 2: 1 type crystal units (Fig. 9.4). These crystal units are loosely held together by very weak oxygen to oxygen linkages. Water molecules as well as cations are attracted between crystal units, causing expansion of the crystal lattice.
The spacing (C-Axis) of the layers ranges from 12 to 18 Ã… (1.2-1.8 nanometers) and is variable with the exchangeable cation species and the degree of inter layer solvation.
Montmorillonites are the swelling and sticky clays. Internal surface, cohesion and plasticity of this mineral are also very high. The size of this mineral is very small (0.01-1.Op m or microns). On drying it shows shrinkage and as a result wide cracks usually form as soils dominated this type of mineral. The dry aggregates or clods are very hard, making such soils difficult to till.
Vermiculite clays are common in most soils. The structure of vermiculite (Fig. 9.5) is similar to that of hydrous mica structure but also has the layers held more weakly together by hydrated magnesium (six water molecules in octahedral coordination with Mg2+) rather than tightly held by potassium ions (K+). Thus, vermiculite has swelling but not as much as montmorillonite. It has a high cation exchange capacity.
(iii) 2: 1 Non-Expanding Type Minerals:
In this group, hydrous mica or illite is the most important in soils. Like montmorillonite, illite has a 2: 1 type lattice. The structure of illite is presented diagrammatically in Fig. 9.6. However, about 15 per cent of silicon in silica sheets is substituted by aluminium.
The excess of negative charge is satisfied largely by potassium in the inter-lattice layers, thus making the lattice structure of the non-expanding type. Hence illite is relatively non-expansive. Thus hydrous mica has slight to moderate swelling. The properties of water adsorption, cation exchange and other physical properties lie in between those of kaolinite and montmorillonite type of minerals.
(iv) 2: 2 Type Minerals:
Chlorite (2: 2 or 2: 1: 1 layer silicates) occurs extensively in soils. Chlorites are basically silicates of magnesium with some iron and aluminium present and it is composed of alternate talc (similar to a montmorillonite crystal unit) and brucite [Mg(OH)2] layers (Fig. 9.7). Chlorite mineral is similar to the unit lattice of vermiculite, except the hydrated Mg in vermiculite is a firmly bonded magnesium hydroxide octahedral sheet.
Thus, a layer of chlorite has 2 silica tetrahedra, an aluminiumoctahedra and a magnesium octahedra sheet (2: 2 or 2: 1: 1). Chlorite does not swell on wetting and has low cation exchange capacities. It is almost non-expanding type of mineral because of its very little water adsorption.
The comparative properties of most important types of silicate clay minerals found in soils are given in table 9.1.
Sources of Negative Charges on Silicate Clays:
There are generally two types of charges i.e. one pH dependent and the other pH independent originate from exposed crystal surfaces (as a result of ionizable hydrogen ions) and isomorphous substitution respectively.
(i) Exposed Crystal Edges:
The negative charge on the silicate clays develops due to unsatisfied valences at the broken edges of silica and aluminium sheets. Besides, the flat external surfaces of silicate clay minerals also serve as the sources of negative charge.
Ionizable hydrogen ions are hydrogen’s from hydroxyl ions on clay surfaces. The Al-OH or -Si-OH portion of the clay ionizes the H and leaves a temporary negative charge on the oxygen (-Alâ€“O– or -Si â€“O–). The mechanism is shown in (Fig. 9.8).
Reactions are as follows:
â€”SiOH + H–Dâ€”SiO– + H2O
â€”AlOH + OH Dâ€”AlO– + H2O
The extent of ionized hydrogen depends on solution pH; more ionization occurs in more alkaline solutions. The magnitude of this pH dependent charge varies with the type of colloid. This type of charge is dominant for organic colloids.
(ii) Isomorphous Substitution:
Isomorphous substitution is the substitution of one ion for another of similar size but lower positive valence. Substitutions that are common are the Si4+ replaced by Al3+, and even more extensive replacement of Al3+ by one or more of these: Fe3+, Fe2+, Mg2+ or Zn2+. The mechanism of isomorphous substitution is given in Fig. 9.9.
Without substitution, the positive and negative energy are in balance. The three positive charges of Al are fully satisfied by an equivalent of three negative charges from the surrounding oxygen’s or hydroxyls. There is no net negative or positive charge.
However, when a magnesium (Mg2+) ion of about same diameter as an aluminium ion (Al3+) replaces one of the aluminums by isomorphous substitution, an imbalance occurs. Consequently, the aluminium octahedral sheet assumes one negative charge for each Mg2+ for Al3+ substitution. In a similar way the negative charge develops due to isomorphous substitution in the silica tetrahedral sheet (Fig. 9.10).
The charges arise from such isomorphous substitution are not dependent on pH and therefore, these charges are commonly referred to as permanent or pH independent charges.
(iii) Anion Exchange:
Some clay minerals exhibit positive as well as negative charges. This will cause anion exchange between hydroxyl ions (OH) and anions like phosphate, sulphate, chloride etc.