Lecture 12

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Chapter 5:145-157

Clay Minerals


1.   Introduction:

Soils are made up of a complex mixture of solids, liquids and gases.
The solid fraction of soils are made up of organic and inorganic components. The inorganic component of the soil makes up more than 90% of the soil solids.
Inorganic components occur mainly in limited number of compounds with definite crystalline structure called minerals. The inorganic component includes both primary and secondary minerals.
The secondary minerals normally are found in the clay fraction of the soil which is the fraction of the soil solids which is less the 2 micron or 0.002 mm. Clay minerals are minerals which mainly occur in the clay sized fraction of the soil.


2.    Importance of Clay Minerals:

The clay minerals and soil organic matter are colloids.
The most important property of colloids is their small size and large surface area. The total colloidal area of soil colloids may range from 10m2/g to more than 800 m2/g depending the external and internal surfaces of the colloid.
Soil colloids also carry negative or positive charges on their external and internal surfaces. The presence of charge influences their ability to attract or repulse charge ions to or from surfaces.
Soils colloids play a very important role in the chemical reaction which take play in soil and influence the movement and retention of contaminants, metals, and nutrients in the soil.


3.       Origin of Clay Minerals.

Clay minerals are formed weathering a variety of minerals.
The two main processes may involve slight physical and chemical alteration or decomposition and recrystallization.
Clay mineral types are normally determined by the types of minerals and acidity of the leaching water.
Based on their origins clays may classified as Inherited, Modified, Transformed or Neoformed (see pages 147 and 148 of text for definitions).


4.       Charge Development on Clays.

Two main sources of charge in clay minerals are isomorphous substitution and pH-dependent charges.
Charge development of on silicate clays is mainly due to isomorphous substitution. This is the substitution of one element for another in ionic crystals with out change of the structure. It takes place during crystallization and is not subject to change afterwards. It takes places only between ions differing by less than about 10% to 15% in crystal radii.. In tetrahedral coordination, Al3+ for Si4+ and in octahedral coordination Mg2+, Fe2+, Fe3+ for Al3+. Charges developed as a result of isomorphous substitution are permanent and not pH-dependent.
In allophanes, some silicate clays e.g. kaolinite, and the metal oxides the main source of charge are termed pH -dependent charges because these charges depend on the pH of the soil. pH depend charges are variable and may either be positive or negative depending on the pH of the soil. In the metal oxides acid soils tend to develop positive charges because of the protonation of the oh ggoud on the oxide surfaces.


5.    Type of Clay Minerals.

There are four major types of Clay minerals ( see Table 5-1).
These include the layer silicates, the metal oxides and hydroxides and oxy-oxides, amorphous and allophanes, and crystalline chain silicates.


6.    Silicate Clays.

The silicate clays are layers of tetrahedral and octahedral sheets.
The basic building blocks of tetetrahedral and octahedral sheets are the silica tetrahedron and the aluminum octahedra.
The Si+4 cation occurs in fourfold and tetrahedral coordination with oxygen whilst the Al3+ is generally found in sixfold or octahedral coordination.
Layer silicate minerals are sometimes defined on the basis of the number of certain positions occupied by cations. When two-thirds of the octahedral positions are occupied , the mineral is called dioctahedral; when all 3 positions are occupied it is called trioctahedral.
When one octahedral sheet is bonded to one tetrahedral sheet a 1:1 clay mineral results. Presence of surface and broken - edge OH groups gives the kaolinite clay particles their electronegativity and their capacity to absorb cations.
In 2:1 clay mineral an octhehedral sheet is bonded to two tetrahedral sheets. The octahedral sheet is generally sandwiched between the two tetrahedral sheets. The 2:1 clays can be classified into expanding (smectites) and non-expanding clays (Illite and micas) on the basis of the sheet where isomorphous susbstitution is taking predominantly taking place (Refer to Figure 5-3).
In the 2:1:1 lattice clays, a positively charge brucite sheet sandwiched between layers restricts swelling, decreases effective surface area, and decreases the effective CEC of mineral. The Idealized formular of half cell is Al Mg2(OH)6)K (Mg3(Si4-x Alx)O10(OH)2. Substitution occurs is in the tetrahedral layer and the layer change is variable but similar to mica. It occurs commonly in sedimentary rocks.


Table 12.1 Silicate Clay Mineral Groups:



Layer Type Layer Charge (x)  

Type of Chemical Formula




[Si4]Al4O10(OH)8.nH2O (n= 0 or 4)

















Adapted from Sposito1989. The Chemistry of Soils. Oxford University Press.


7.    Sesquioxide Clays (Metal Oxides and Hydrous Oxides)

Also found in finer component. These tend to form in soils as Si is depleted by leaching.
Gibbsite is the most common Al oxide mineral and is often found in highly weatherd sois such as oxisoils in tropical areas and ultisols found predominatly in the southeastern U.S.
The most commn iron oxides are Goetihte (FeO(OH) and Hematiite (Fe2O3) Thess are also found in highly weathered soils and gives many red soils their color.
The metal oxides gibbsite and goethite tend to persist in the environment because Si is readily leached than Al, or Fe and significant amount of soluble organic matter is present.
Manganese oxides are also quite common in soils. Apart from being an essential plant nutrient, they are an nutrural oxidant to certain metals such as As3+ and Cr3+. Birnessite(MnO2) is the most comon Mn oxide found in soils.
Most of the charges developed on the metal oxides are pH-dependent.


8.    Allophanes and Imogolite

These are structurally disordered aluminosilicates.
They are normally derived from volcanic ash materials and constitute a major component of volacnic soils.
Allophane is is often associated with clay minerals of the kaolinite group.
Imogolite has the empirical formula SiAl4O10.5H2O.


9.    Carbonate and Sulfate Minerals

The carboate and sulfate minerals are highly soluble compared to the alumino-silicates and are more prevalent in arid and semi arid regions.
The major carbonate minerals are calcite (CaCO3) and Dolomite (CaMg(CO3)2.
The major sulfate mineral is gypsum.


10.    Use of Clay Minerals.

Clay minerals have many industrial uses in the chemical and oil industries.
Organoclays, which have the metals in the clay replaced by large surfactant cations, such as long chain alkyl amine cations can be been used as liners in landfills to reduce transport of contaminants. Organoclays also could be used in wastewater treatment and spill control situations.



Isomorphic substitution
Soil colloids


Web sites

Worldwatch Institute Homepage. This organization provides environmental data and news to individuals and organizations interested in the environment. URL: www.worldwatch.org

Journal of Natural Resources and Life Sciences Education. This is an excellent electronic journal about natural resource education. Abstracts are free, articles are not. URL: www.agronomy.org/journals

The NSCSS Pedology Page has lots of useful information and links to other sites. URL: www.nscss.org/ped.html


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