By Alfred R. Conklin, Jr.

Clay is a common part of our everyday life. In school we may have worked with modeling clay and professional artist use it in their work. In drilling operations a specific type of clay, bentonite, is used as a lubricant and sealant. In the chemical industry clay is used as a catalyst to increase the rate of chemical reactions. Many consumer products such as paper use clays as fillers. Some researchers have suggested that clays, particularly soil clays, acted as catalyst for the beginning of life on earth. On a day-to-day basis soil clays are important to plant growth and the decomposition of pollutants in the environment.

In soil there are a seemingly infinite variety of clays, all of which are important because they are the smallest, most active inorganic components. This component is all the inorganic particles less than 0.002 mm in diameter. At this size these particles are colloidal and thus tend not settle out of suspension. They have high sorptive capacity, hold large amounts of water and have cation exchange capacity (CEC). The most common cations on the exchange sites are calcium, magnesium and potassium.

The most important group is the silicate clays, which are made up of layers of silica and alumina. It is the arrangement of and isomorphous substitution in these layers, which determines the type of clay and its physical and chemical characteristics. For instance some clays expand and are extremely sticky and plastic when wet, others are slippery and plastic while still others do not expand and are not sticky when wet. Some clays have CEC which is pH dependent and some have CEC which is permanent regardless of pH.

There are also amorphous clays and clays made up of the oxides of aluminum, iron and titanium.

Clays can be considered as coming from two different sources. Some are remains of decomposed rock while others are synthesized in soil. The latter are called secondary minerals and are the most common. One might think of clays as being stable unchanging soil components. However, all clays, even those derived from rock, are constantly changing sometimes in a matter of days or weeks. The four most common silicate clays in soil regardless of their source are kaolinite, fine grained mica, smectite, and amorphous. Of these the first three have definite shape, repeating structure and composition while the last does not.

Kaolinite, fine grain mica and smectite are general terms for groups of silicate clays. They are called silicate clays because they are made up of alternating layers of silica and aluminia. The silica layer is composed of tetrahedral silicon atoms bonded to four oxygens, which are in turn bonded to either other silicon or aluminum atoms. The silicon layer has oxygens on the surface, which allows for attraction water, other clays, and organic and inorganic ions and molecules in the surrounding soil water. On the other hand the alumina layer is octahedral and its surface has hydroxyl groups (-OH), which allow for hydrogen bonding. The alumina layer thus has both partially negative oxygens and partially positive hydrogens on the surface, which are available to attract components in soil water.
Kaolinite is the dominant clay in areas with high weathering or where soil has been developing for a long time. It is said to be a one to one (1:1) clay because it is made up of one layer of silica and one layer of aluminia. The crystals of kaolinite are relatively large sometimes larger than 0.002 mm. The large size is made possible because kaolinite is very stable. Stability comes in part from the fact that the silica layer of one kaolinite crystal can hydrogen bond to a hydroxyl on the alumina layer of another crystal.
 

Because of their relatively large size and simple structure kaolinite is the least active of the silicate clays. Large size means less surface area and thus less sorptive capacity. All cation exchange capacity (CEC) comes from unsatisfied bonds at the surface of the crystals. The availability of these sites is dependent on the pH of the solution surrounding the crystal. At low pH (high H+ concentration) protons fill these sites and the cation exchange capacity is less. At high pH the reverse is true. This is termed pH dependent charge or CEC.

Find grained mica crystals are smaller than kaolinite and have a two to one composition. Their crystal structure is made up of one sheet or layer of alumina sandwiched between layers of silica. Their smaller size means they have larger surface area, which leads to more sorptive capacity and more CEC. However, in this case charges on the crystal come not only from edge effects but also from isomorphous substitution. There is substitution of aluminum (Al) for silicon (Si) in the tetrahedral sheet. In a simplistic way we can think of aluminum as having a charge of +3 and silicon as having a charge of +4. Thus, when an aluminum atom replaces a silicon atom not all the negative charges in the structure are satisfied. This results in the crystal having both permanent or pH independent negative charge and pH dependent charge and its total CEC comes from both sources.
In fine grain mica isomorphous substitution deforms the crystal structure. It turns out that this deformation forms a “pocket” on the surface, which is just big enough to accommodate either a potassium ion (K+) or an ammonium ion (NH4+). Two crystals can come together and share one of these ions between them. Because the substitution is on the surface the interaction with cations is strong. The consequence is that the crystals are held together strongly. The clay does not expand nor is the internal surface between two crystals available for exchange of cations.

Smectite is also a 2:1 clay mineral. Because of its small size it has high sorptive capacity. It also has cation exchange capacity as a result of both edge effects and isomorphous substitution. In this case the isomorphous substitution, of magnesium for aluminum, occurs in the alumina layer. Magnesium has a +2 charge and so there is unsatisfied negative charge in the interior of the crystal. The shape of the crystal is not changed and all the surfaces are available for adsorption and cation exchange.

Because the charge is further from the surface crystals are not strongly held together. This leads to extensive swelling and shrinking upon wetting and drying. Soils high in smectite clays develop cracks 30 cm wide and 100 cm deep when dry. Pollutants spilled on these types of soil when dry can immediately move 100 cm deep. When wetted the soil swells and the cracks close. Structures built on soils high in smectites are often destroyed by the shrinking and swelling.

In addition to the well crystallized clays there are poorly organized silicates and sesquioxides in soil. The most commonly studied amorphous silicate clay called allophane is common in soils developing from volcanic ash. These clays have pH dependent cation exchange capacity and high phosphate sorbing capacity. Sesquioxides are made up of oxides of aluminum such as gibbsite [Al(OH)3] iron such as goethite (FeOOH) and hematite (Fe2O3) and oxides of titanium. They are most commonly found under intense weathering conditions such as in the tropics.

It is common to find a mixtures of these clays in most soils. As one moves from the equator to the poles the type of clay changes from a predominance of kaolinite and oxides of iron and aluminum to a predominance of smectite and fine grain mica. In between there are likely to be a mix of various clay types with no one dominant type.

The sub-soil or B horizons are usually higher in clay than the overlying A horizons. Thus buried pipelines and tanks are surrounded by soil higher in clay. This means that the spread of leaks is retarded by the high sorptive capacity and small pores in clays. However clean up is often a problem because clays are hard to handle. When they are dry they are like powders and can be easily blown. When wet they can be slippery, sticky and plastic.

Expanding clays such as bentionite, which is a specific type of smectite, have been used as sealants. They can seal pond bottoms, the bottoms of landfills and around well linings. As long as they are kept wet the seal is good. However if allowed to dry the seal is lost and may not reform effectively on re wetting.

The high sorptive and cation exchange capacity of soil clays make them important in plant growth. Clay particles are surrounded with a swarm of cations, which are available to plants. The higher the clay content the more available nutrients there will be. Clays also hold large amounts of water. Thus, they provide water for plants during periods of drought. Exchangeable protons on clays give them buffering capacity, which tends to counter act rapid changes in soil pH. Although sometimes hard to handle clays are beneficial if one is attempting to do either bio- or phyto-remediation.

Two good sources of information about clays in the soils of a particular area are the Soil Conservation Service, which has extensive knowledge about the soils in a county. A second source is the Soil Survey for the local area. These surveys are available from the local Soil Conservation Service and the agricultural extension agent. They contain information about the soil types, their clay content and the engineering and hydrologic characteristics of these soils and the clay they contain.
 

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