The structure of clay silicates is similar to that of primary silicates, i.e., they are sheet silicates. Secondary minerals are composed of silicon tetrahedral sheets, aluminum hydroxide sheets, and / or magnesium hydroxide sheets. Under mild (generally physical) weathering conditions, secondary minerals may be inherited as colloidal fragments of primary layer silicates, such as the micas. Under more intense weathering, the primary minerals may be transformed to secondary clay minerals, as when soil hydrous micas and vermiculites are formed by the leaching of interlayer K from primary micas. Neoformation of clay minerals is a feature of intense weathering, when minerals completely different from the original primary minerals are formed. The term 'clay' is used also to identify mineral particles of the size < 0.002 mm.
Following, the most important secondary minerals are presented briefly:
Kaolinites:They are composed of one tetrahedral sheet linked to an octahedral sheet, therefore they are classified as 1 : 1 type layer silicates. The two surfaces of a 1 : 1 mineral are formed by different ions: One consists of tetrahedral oxygens and the other of OH - ions belonging to the octahedral sheet. When the 1 : 1 sheets occur in stacks, the OH- ions of one sheet lie next to and in close contact with the O2- layer of its neighbor. Because of this arrangement, the positive charge of the H+ ions in the OH - layer exerts a strong attraction for the negative oxygens of the neighboring sheets. In this way the platelets of kaolinite are tightly bound together. Kaolinite is a non-expanding mineral, hence it is unable to absorb water into the interlayer position. The non-expanding character of kaolinite explains the failure of soils high in this clay to swell or shrink much on wetting and drying. Kaolinite has a basal spacing fixed at 0.72 nm, which is small compared to the other clay minerals.
Montmorillonites (Smectite Group): These clay silicates form by crystallization from solution high in soluble silica and magnesium. Montmorillonite has a 2 : 1 layer structure. All tetrahedra in the sheets contain Si 4+ ions. Aluminium is the normal ion in the central sheet, but about one-eight of the octahedra contain Mg2+ as a substituting ion for Al3+. The negative charge caused by substitution is neutralized by various hydrated cations adsorbed to the surface of the sheets. The force of bonding between cations and the sheets is not very strong and depends on the amount of water present. In dry montmorillonites the bonding force is relatively strong. When wet conditions occur, water is drawn into the interlayer space between sheets and causes the clay to swell dramatically (expanding clay). A characteristic feature of montmorillonite is the extensive surface for the adsorption of water and ions, therefore the cation exchange capacity of montmorillonite is very high. Layers of the smectite group range in thickness from 0.98 to 1.8 nm or more.
Vermiculites: These clays have a 2 : 1 structure of primary mica minerals. Vermiculites contain either Al3+ or Mg 2+ and Fe2+ as normal octahedral ions, and tetrahedral sheets in which Al 3+ occurs as a substituted ion in place of some of the Si 4+. Vermiculite differs from the micas in that it contains hydrated cations rather than unhydrated K+ in the interlayer space. The weak bonding afforded by these ions allows vermiculite to expand on wetting. Expansion is less than in montmorillonite, however. Unlike montmorillonite and kaolonite, vermiculite does not form by crystallization from solution, but instead it is formed by alteration, or the selective replacement of ions in a structure without destroying the structure (e.g. micas are altered to vermiculites). Layer spacing ranges from 1.0 to 1.5 nm or more.
Hydrous Micas (Illites): They are 2 : 1 type minerals containing sufficient interlayer K+ to limit expansion on wetting. The K+ content of hydrous mica is less than that of micas. Charges not neutralized by K + are countered by hydrated cations. Formation of hydrous mica is favored in K-rich sediments. The process of hydrous mica formation is initiated as K+ replaces some of the interlayer cations of montmorillonites or vermiculites, and is completed when heat and pressure cause the dehydration and collapse of the clays into non-expanded forms. Hydrous micas are widespread in soils. The layer thickness of hydrous micas are about 1.0 nm.
Chlorites: This group embraces a range of minerals that have certain outstanding characteristics in common. All have a basic 2 : 1 layer structure, and they are non-expanding. Chlorites differ from other 2 : 1 layer minerals in one unique respect, i.e., they contain a stable, positively charged octahedral sheet rather than adsorbed cations in the interlayer space. The octahedral sheet consists of two layers of OH- ions that enclose either Mg2+, Fe 2+, or Al3+ as the central octahedral cations and leads to a positive charge of the sheet. By virtue of its positive charge, the interlayer sheet neutralize the negative charge of the 2 : 1 sheets. Because chlorite contains two octahedral sheets, it is called a 2 : 1 : 1 layer mineral. Sometimes, octahedral materials in chlorite neither totally fill the interlayer space between sheets nor completely neutralize the negative charge of the sheets. This unsatisfied charge is neutralized by various cations adsorbed to the particle surfaces from the solution phase. The thickness of the chlorite layer is 1.4 nm.
Allophanes: These are poorly categorized substances, which are sometimes regarded as clay minerals and at other times considered among hydroxides. It is for sure that they are poorly structured, i.e., amorphous in character and they consist of silica and hydrous oxides. They are abundant in soils derived from volcanic ash deposits. The structural formula of Allophane can be written as Si3Al 4O12*nH 2O. Another similar silicate is Imogolite which is Si 2Al4O 10*5H2O.
It should be stressed that clay minerals scarcely occur in pure form in soils, but they are a mixture of the clays presented in theoretical form above.
The properties of clay minerals are summarized in Table 1 and discussed in the following.
Table 1. Summary of clay mineral properties.
All clay minerals show different expansions, whereas kaolinite, hydrous mica, and chlorite are non-expanding minerals and the others are expanding minerals. In kaolinite the bonding is strong because of tight H-OH bonding between the layers. The interlayer bonding of hydrous mica is mostly by K+ ions which is relatively strong. Montmorillonite and vermiculite show very weak to weak bonding due to various cations between the sheets, therefore they show a great expansion, especially in wet conditions. In chlorite the bonding is moderate to strong because of the positively charged octahedral layer.
The smaller the size of a fragment, the greater the ratio of its surface to volume, which defines the specific surface area. The specific surface area is low for kaolinite and hydrous mica and high for montmorillonite, vermiculite, and allophanes.This is because the surface area outside of the silicates (external surface) is increased by the surface area between the sheets, called interlayer area (or internal surface). In comparison coarse sand has a specific surface area of about 0.01 m2 /g, fine sand 0.1 m2/g, silt 0.1 - 1 m 2/g, and humic acids 800 - 1000 m2 /g (White, 1987).
The cation exchange capacity (CEC) is quite variable within and between mineral groups. For example, the CEC of the kaolinites, hydrous micas, chlorides, and allophanes is relatively low, whereas the CEC is high for montmorillonites and vermiculites. For comparison humic acids show the highest CEC in soils with 180 - 300 cmol/kg. There are two mechanisms, which are driving the CEC, both associated with the negative charges of silicate clays. The first involves unsatisfied valences at the broken edges of the silica and alumina sheets. Also, the flat external surfaces of the minerals have some exposed oxygen and hydroxyl groups, which act as negatively charged sites. Especially at high pH, the hydrogen of these hydroxyl dissociates slightly and the colloidal surface is left with a negative charge carried by the oxygen. In moderately to strongly acid soils the hydrogen is apparently tightly held and not subject to ready replacement by other cations. The magnitude of this pH-dependent charge varies with the type of the colloid. It accounts for most of the charge of the 1 : 1 type minerals and up to one fourth of that of some 2 : 1 type minerals. The cation exchange capacity of soils with a high amount of 2 : 1 type clays originates mainly from isomorphous substitution. These negatively charged sites are not affected by pH and constitute the permanent charge. Minerals such as kaolinite with their absence of isomorphous replacement have their exchange sites confined to the broken edges of crystals, therefore the CEC is low. On the other hand montmorillonite and vermiculite have a relatively large amount of isomorphous replacement resulting a large number of exchange sites and a high CEC.
Flocculation and dispersion are further important characteristics of clay minerals. Flocculation is the process, where the individual particles of clay are coagulated to form floccular aggregates. The degree and permanence of flocculation depend upon the nature of the ions present. For example, calcium and hydrogen tend to increase flocculation. Dispersion is defined as a process in which the individual particles are kept separate from one another. This is accomplished by potassium and sodium. Thus, depending upon the cations present in a soil, it may be either in a flocculated (aggregated) or in a dispersed (massive) state. Sodium saturated clays have a thick electric double layer surrounding the ion, that means the clays remain is suspension. Calcium suppresses the double layer and cause flocculation, while tri- and tetravalent ions are more efficient in causing flocculation. Clay translocation is closely related to flocculation and dispersion, respectively. The movement of clay requires that the clay be dispersed so that it can remain in suspension and be transported by water moving through pores and cracks in soil.
The usual source of kaolinites and montmorillonites is the precipitation at weathering sites. Hydrous micas are formed due to alteration of vermiculite or montmorillonite, so is the formation of vermiculite (alteration of mica or hydrous mica). Chlorites are formed by alteration of vermiculite or montmorillonites or in metamorphic rocks.
The stages in weathering are listed in Table 2. It is apparent that the composition of weathering solutions is strongly dependent on minerals that are undergoing weathering. First the original minerals dissolve and secondary minerals can form from it. Leaching of elements such as calcium, magnesium, sodium, potassium, and soluble silica supports further transformation processes. The gradual loss of soluble silica results in the formation and disappearance of clays in an ordered sequence, starting with those highest in silica content and ending with those containing no silica, i.e., the hydrous oxides. Over long periods the clays that form first eventually become unstable, decompose and they are replaced by other secondary clay minerals which are more stable. First 2 : 1 clay minerals are formed. Iron oxides may also appear early and they seem to persist almost infinitely in the weathering environment, which attests to their great stability under most conditions. As weathering proceeds kaolinites appear, or even kaolinites are decomposed, the silica released from it is leached, and the aluminum transforms to a hydrous oxide, usually gibbsite. These minerals tend to persist as the final products of long and intense silicate mineral weathering. The stages of weathering are time related functions, whereas the rate of weathering depends primarily on the climatic factors (temperature, precipitation). Silicate mineral weathering and clay synthesis are limited under either dry or cold conditions, but they proceed rapidly under hot, wet conditions, as in tropical regions. The time span required for a full weathering cycle shown in Table 2 is several tens of thousands of years .
Table 2. Stages in the weathering of minerals in the < 2 mm fraction of soils (modified table, after White, 1987).
In the drier and cooler regions of North America 2: 1 type clays tend to dominate soils because of limited weathering. For example, sedimentary or metamorphic rocks containing mica have been an important source of minerals in glacial deposits and therefore those soils are rich in hydrous mica, montmorillonite and vermiculite. If parent rock is sedimentary shale, which is clay rich material, weathering produces 2 : 1, 1 : 1 type clay minerals, or hydrous oxides. Soils with clays high in kaolinite and hydrous oxides tend to be restricted to older landscapes that are both warm and wet. For instance, such soils can be found in the wet and warm climate of the Southern United States. Soils containing hydrous oxides such as Al- and Fe-Oxides as the dominant clays are limited to tropical regions, e.g. soils on the Hawaiian Islands. Parent material high in bases, or a climate which discourage the leaching of bases, encourage montmorillonite formation. For this reason, hydrous mica and montmorillonite are more likely to occur in soils in semiarid and arid climate.
Clay coatings (argillans) are often different in color from and with a higher reflectance than the S-matrix of the ped. They are easily recognizable in sandy and loamy soils, but difficult to distinguish from slickenside surfaces in clay soils. In soil horizons the accumulation of silicate clay is denoted by a 't' , that are clay coatings on ped faces and / or in pores. The clay coats may be either formed by illuviation or concentrated by migration within the horizon. If slickensides are present, which are formed by shear failure as clay minerals swell upon wetting (vertic charactersitics) the donation is a 'ss'.