Structure of Layer Silicate Clays
Silicate clay particles are crystalline, each particle layer being comprised of two basic individual sheets. One of these sheets is dominated by a plane of silicon atoms surrounded by oxygen atoms. The oxygen atoms, in turn, act as connective links to a companion sheet dominated by an aluminum and/or magnesium plane surrounded by linkage oxygens and a few hydroxyl groups. The silica sheet is called a tetrahedral sheet because of the four-sided configuration of a given silicon atom and its associated oxygen atoms. Similarly, the aluminum/magnesium sheets are known as octahedral sheets because each aluminum/magnesium atom and associated oxygens and hydroxyls comprise an eight sided building block or octahedron.
The basic molecular and structural components of silicate clays. (a) A single tetrahedron, a four-sided building block composed of a silicon ion surrounded by four oxygen atoms; and a single eight-sided octahedron, in which an aluminum (or magnesium) ion is surrounded by six hydroxy groups or oxygen atoms. (b) In clay crystals thousands of these tetrahedral and octahedral building blocks are connected to give planes of silicon and aluminum (or magnesium) ions. These planes alternate with planes of oxygen atoms and hydroxy groups. Note that apical oxygen atoms are common to adjoining tetrahedral and octahedral sheets. The silicon plane and associated oxygen-hydroxy planes make up a tetrahedral sheet. Similarly, the aluminum-magnesium plane and associated oxygen-hydroxy planes constitute the octahedral sheet. Different combinations of tetrahedral and octahedral sheets are termed layers. In some silicate clays these layers are separated by interlayers in which water and adsorbed cations are found. Many layers are found in each crystal.
In nature, ions having nearly the same radius as a silicon atom (e.g. aluminum) can fit in the tetrahedral sheet through a process called isomorphous substitution. If the substituting ion has a lower valence than silicon, an unsatisfied negative charge within the crystal results. This is the primary source of the negative charge on the crystal. Similar isomorphous substitution can take place in the octahedral sheet with aluminum being replaced by a similar-sized lower-valent cation (e.g. magnesium) likewise giving rise to a negative charge. These negative charges can then attract cations from the soil solution. Later, these cations may be subject to exchange with plant roots thereby becoming the primary source of plant nutrients in unfertilized soils.
Types of Clay Minerals
Based on the number and arrangement of tetrahedral (silica) and octahedral (alumina-magnesia) sheets contained in the crystal units or layers, silicate clays are classified into two different groups: a) the 1:1 type minerals (one tetrahedral to one octahedral sheet) and b) 2:1 type minerals. The 1:1 type crystals (e.g. kaolinite) are larger in size than the other types and have a fixed structure with no internal surfaces and little isomorphous substitution. Consequently, these minerals have relatively low surface area and low capacity to attract (absorb) cations. They do not swell when wetted or shrink when dried.
Models of the 1:1-type clay kaolinite. The primary elements of the octahedral (upper left) and tetrahedral (lower left) sheets are depicted as they might appear separately. In the crystal structure, however, these sheets are held together by common apical oxygen atoms. Note that each layer consists of alternating octahedral and tetrahedral sheets—hence, the designation 1:1. The octahedral and tetrahedral sheets are bound together (center) by mutually shared (apical) oxygen atoms. The result is a layer with hydroxyls on one surface and oxygens on the other. To permit us to view the front silicon atoms, we have not shown some basal oxygen atoms that are normally present. The diagram at right shows the bonds between atoms. The kaolinite mineral is comprised of a series of these flat layers tightly held together with no interlayer spaces.
There are four general groups of minerals with 2:1 type crystal structures. One, the fine-grained mica group, resembles the 1:1 type in that they are somewhat larger in size than the other 2:1 types and are non-expanding, having little internal surface area. About 20% of the silicon atoms in the tetrahedral sheets have been replaced by aluminum and the very strong negative charge resulting is satisfied by potassium ions held rigidly between adjoining 2:1 layers and preventing expansion of the crystal. Cation adsorption capacity is higher than that of the 1:1 types but definitely lower than that of the other groups having 2:1 type structures.
Two clay groups with 2:1 type structures have expansive type crystals, the smectites and vermiculites. The individual 2:1 layers are held together only loosely and exchangeable cations and water molecules are attracted between the layers resulting in enormous internal adsorptive surfaces. Consequently, these clays expand when wet and shrink when dry and have very high cation adsorption capacities. In the smectite group magnesium has substituted for some of the aluminum in the octahedral sheet. Some such substitution has also occurred of aluminum for silicon in the tetrahedral sheet giving rise to the high cation adsorption capacity of this mineral.
The vermiculite group are less expansive than the smectites, since water and magnesium ions act as bridges holding the 2:1 type layers together. Vermiculites have very high cation adsorption capacities due to significant substitution of aluminum for silicon in the tetrahedral sheets as well as some substitution of magnesium for aluminum in the octahedral sheet.
Model of two crystal layers and an interlayer characteristic of montmorillonite, a smectite expanding-lattice 2:1-type clay mineral. Each layer is made up of an octahedral sheet sandwiched between two tetrahedral sheets with shared apical oxygen atoms. There is little attraction between oxygen atoms in the bottom tetrahedral sheet of one unit and those in the top tetrahedral sheet of another. This permits a variable space between layers, which is occupied by water and exchangeable cations. The internal surface area thus exposed far exceeds the surface around the outside of the crystal. Note that magnesium has replaced aluminum in some sites of the octahedral sheet. Likewise, some silicon atoms in the tetrahedral sheet may be replaced by aluminum (not shown). These substitutions give rise to a negative charge, which accounts for the high cation exchange capacity of this clay mineral. A ball-andstick model of the atoms and chemical bonds is at the right.
Another 2:1 type mineral, chlorite, is non expansive since its interlayer between two 2:1 layers is occupied by a magnesium-dominated octahedral sheet that holds the adjacent layers together. Chlorite has particle size, cation adsorption capacity, and physical properties similar to those of fine-grained micas. Layer silicate clays in which three out of three octahedral positions are occupied by metal cations are termed trioctahedral. Those with only two out three positions occupied are dioctahedral. Much of what is known about the structures of crystalline clays has been discovered using a technique called X-ray diffraction which measures the manner in which x-rays are reflected off parallel planes of atoms.
Model of a 2:1-type nonexpanding lattice mineral of the fine-grained mica group. The general constitution of the layers is similar to that in the smectites, one octahedral sheet between two tetrahedral sheets. However, potassium ions are tightly held between layers, giving the mineral a more or less rigid type of structure that prevents the movement of water and cations into the space between layers. The internal surface and cation exchange capacity of fine-grained micas are thus far below those of the smectites.
Schematic drawing illustrating the organization of tetrahedral and octahedral sheets in one 1:1-type mineral (kaolinite) and four 2:1-type minerals. The octahedral sheets in each of the 2:1-type clays can be either aluminum dominated (dioctahedral) or magnesium dominated (trioctahedral). However, in most chlorites the trioctahedral sheets are dominant while the dioctahedral sheets are generally most prominent in the other three 2:1 types. Note that kaolinite is nonexpanding, the layers being held together by hydrogen bonds. Maximum interlayer expansion is found in smectite, with somewhat less expansion in vermiculite because of the moderate binding power of numerous Mg2+ ions. Fine-grained mica and chlorite do not expand because K+ ions (fine-grained mica) or an octahedral-like sheet of hydroxides of Al, Mg, Fe, and so forth (chlorite) tightly bind the 2:1 layers together. The interlayer spacings are shown in nanometers (1 nm = 10-9 m).
The iron and aluminum oxides, common in highly weathered soils, have cation adsorption capacities even lower than that of kaolinite, are nonexpansive, and encourage good soil physical properties. Allophane, a noncrystalline silicate mineral, is nonexpansive but has a high capacity to adsorb both cations and anions. Humus is composed basically of carbon, hydrogen, and oxygen and its negative charges are associated with organic acid groups. The cation adsorption capacity of humus, like that of the hydrous oxide of iron and aluminum, is dependent on the pH, being much higher in alkaline than in acid soils.
The 2:1 type clays are most prominent in areas that have been subjected to only mild chemical weathering, while kaolinite and especially the iron and aluminum oxides dominate soils where chemical weathering has been intense. In the United States 2:1 type clays are most prominent in the northern and arid western states while the humid southeast is dominated by kaolinite and Fe, Al oxides.
8.13 A simplified diagram showing the structure of gibbsite, an aluminum oxide clay common in highly weathered soils. This clay consists of dioctahedral sheets (two are shown) that are hydrogen-bonded together. Other oxide-type clays have iron instead of aluminum in the octahedral positions, and their structures are somewhat less regular and crystalline than that shown for gibbsite. The surface plane of covalently bonded hydroxyls gives this, and similar clays, the capacity to strongly adsorb certain anions.