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THE CLAY MINERAL GROUP
 

The clay minerals are a part of a general but important group within the phyllo-silicates that contain large percentages of water trapped between the silicate sheets. Most clays are chemically and structurally analogous to other phyllo-silicates but contain varying amounts of water and allow more substitution of their cat-ions. There are many important uses and considerations of clay minerals. They are used in manufacturing, drilling, construction and paper production. They have great importance to crop production as clays are a significant component of soils.

It is the physical characteristics of clays that more so, than their chemical and/or structural characteristics, which defines this group.

Clay minerals tend to form microscopic to sub-microscopic crystals. They can absorb water or lose water from simple humidity changes. When mixed with limited amounts of water, clays become plastic and are able to be molded and formed in ways that most people are familiar with as children's clay. When water is absorbed, clays will often expand as the water fills the spaces between the stacked silicate layers. Due to the absorption of water, the specific gravity of clays is highly variable and is lowered with increased water content. The hardness of clays is difficult to determine due to the microscopic nature of the crystals. Their actual hardness is usually between 2 - 3, while many clays often register a hardness of 1 in field tests. Clays tend to form from weathering and secondary sedimentary processes with only a few examples of clays forming in primary igneous or metamorphic environments. Clays are rarely found separately and are usually mixed not only with other clays but with the microscopic crystals of carbonates, feldspars, micas and quartz

Clay minerals are divided into four major groups. Below you will find listed the primary clay mineral groups:
 

The Montmorillonite/Smectite Group

This group is composed of several minerals including pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite and montmorillonite. They differ mostly in chemical content. The general formula is (Ca, Na, H)(Al, Mg, Fe, Zn)2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that members of this group could contain. Talc's formula, for example, is Mg3Si4O10(OH)2. The gibbsite layers of the kaolinite group can be replaced in this group by a similar layer that is analogous to the oxide brucite, (Mg2(OH)4). The structure of this group is composed of silicate layers sandwiching a gibbsite (or brucite) layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches.

Uses: Are many and include a facial powder (talc), filler for paints and rubbers, an electrical, heat and acid resistant porcelain, in drilling mud, soil additives, and as a plasticizer in molding sands and other materials.
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The Kaolinite Group

This group has three members (kaolinite, dickite and nacrite) and a formula of Al2Si2O5(OH)4. The different minerals are polymorphs, meaning that they have the same chemistry but different structures (polymorph = many forms). The general structure of the kaolinite group is composed of silicate sheets (Si2O5) bonded to aluminum oxide/hydroxide layers (Al2(OH)4) called gibbsite layers. The silicate and gibbsite layers are tightly bonded together with only weak bonding existing between the s-g paired layers.
Uses: In ceramics, as a filler for paint, rubber and plastics and the largest use is in the paper industry that uses kaolinite to produce a glossy paper such as is used in most magazines.
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The Montmorillonite/Smectite Group

This group is composed of several minerals including pyrophyllite, talc, vermiculite, sauconite, saponite, nontronite and montmorillonite. They differ mostly in chemical content. The general formula is (Ca, Na, H)(Al, Mg, Fe, Zn)2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that members of this group could contain. Talc's formula, for example, is Mg3Si4O10(OH)2. The gibbsite layers of the kaolinite group can be replaced in this group by a similar layer that is analogous to the oxide brucite, (Mg2(OH)4). The structure of this group is composed of silicate layers sandwiching a gibbsite (or brucite) layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches.

Uses: Are many and include a facial powder (talc), filler for paints and rubbers, an electrical, heat and acid resistant porcelain, in drilling mud, soil additives, and as a plasticizer in molding sands and other materials
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The Illite (or The Clay-mica) Group

This group is basically a hydrated microscopic muscovite. The mineral illite is the only common mineral represented, however it is a significant rock forming mineral being a main component of shales and other argillaceous rocks. The general formula is (K, H)Al2(Si, Al)4O10(OH)2 - xH2O, where x represents the variable amount of water that this group could contain. The structure of this group is similar to the montmorillonite group with silicate layers sandwiching a gibbsite-like layer in between, in an s-g-s stacking sequence. The variable amounts of water molecules would lie between the s-g-s sandwiches as well as the potassium ions.

Uses: A common constituent in shales and is used as a filler and in some drilling muds.
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The Chlorite Group

This group is not always considered a part of the clays and is sometimes left alone as a separate group within the phyllo-silicates. It is a relatively large and common group although its members are not well known. These are some of the recognized members:

Amesite (Mg, Fe)4Al4Si2O10(OH)8
Baileychlore (Zn, Fe+2, Al, Mg)6(Al, Si)4O10(O, OH)8
Chamosite (Fe, Mg)3Fe3AlSi3O10(OH)8
Clinochlore (kaemmererite) (Fe, Mg)3Fe3AlSi3O10(OH)8
Cookeite LiAl5Si3O10(OH)8
Corundophilite (Mg, Fe, Al)6(Al, Si)4O10(OH)8
Daphnite (Fe, Mg)3(Fe, Al)3(Al, Si)4O10(OH)8
Delessite (Mg, Fe+2, Fe+3, Al)6(Al, Si)4O10(O, OH)8
Gonyerite (Mn, Mg)5(Fe+3)2Si3O10(OH)8
Nimite (Ni, Mg, Fe, Al)6AlSi3O10(OH)8
Odinite (Al, Fe+2, Fe+3, Mg)5(Al, Si)4O10(O, OH)8
Orthochamosite (Fe+2, Mg, Fe+3)5Al2Si3O10(O, OH)8
Penninite (Mg, Fe, Al)6(Al, Si)4O10(OH)8
Pannantite (Mn, Al)6(Al, Si)4O10(OH)8
Rhipidolite (prochlore) (Mg, Fe, Al)6(Al, Si)4O10(OH)8
Sudoite (Mg, Fe, Al)4 - 5(Al, Si)4O10(OH)8
Thuringite (Fe+2, Fe+3, Mg)6(Al, Si)4O10(O, OH)8

The term chlorite is used to denote any member of this group when differentiation between the different members is not possible. The general formula is X4-6Y4O10(OH, O)8. The X represents either aluminum, iron, lithium, magnesium, manganese, nickel, zinc or occasionally chromium. The Y represents either aluminum, silicon, boron or iron. However, it is most often aluminum and silicon.

The gibbsite layers of the other clay groups are replaced in the chlorites by a similar layer that is analogous to the oxide brucite. The structure of this group is composed of silicate layers sandwiching a brucite or brucite-like layer in between, in an s-b-s stacking sequence similar to the above groups. However, in the chlorites, there is an extra weakly bonded brucite layer in between the s-b-s sandwiches. This gives the structure an s-b-s b s-b-s b sequence. The variable amounts of water molecules would lie between the s-b-s sandwiches and the brucite layers.

Uses: No industrial uses.
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Some of the minerals listed above (specifically chlorite, pyrophyllite and talc) are noted as belonging to one of the clay groups and are often excluded by some mineralogists. Usually the reason is that their crystal size and character do not consistently conform to those unique parameters that define a clay. Such minerals are listed here more for their structural similarities, however, all three minerals are quite often found associated with and do behave like clays, occasionally.

Clay Minerals

1. Introduction:

Soils are made up of a complex mixture of solids, liquids and gases. The solid fraction of all 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 a limited number of compounds with definite crystalline structure called minerals. The inorganic component includes both primary and secondary minerals. Most secondary minerals normally are found in the clay fraction of the soil which is a small portion of the soil solids, often less than 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 a 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(s) which take place in the soil and influences 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 re-crystallization. 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.

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 dependent 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 group 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 electro-negativity 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 substitution is taking predominantly taking place. 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 formula of half cell is Al Mg2(OH)6)K (Mg3(Si4-x Alx)O10(OH)2. Substitution occurs in the tetrahedral layer and this layer change is variable but similar to mica. It occurs commonly in sedimentary rocks.

Table 12.1 Silicate Clay Mineral Groups:

Group
Layer Type Layer Charge (x)
Type of Chemical Formula

Kaolinite
1:1
<0.01
[Si4]Al4O10(OH)8.nH2O (n= 0 or 4)
Illite
2:1
1.4-2.0
Mx[Si6.8Al1.2]Al3Fe.025Mg0.75O20(OH)4
Vermiculite
2:1
1.2-1.8
Mx[Si7Al]AlFe.05Mg0.5O20(OH)4
Smectite
2:1
0.5-1.2
Mx[Si8]Al3.2Fe0.2Mg0.6O20(OH)4
Chlorite
2:1:1
Variable
(Al(OH)2.55)4[Si6.8Al01.2}Al3.4Mg0.6)20(OH)4

Adapted from Sposito 1989. The Chemistry of Soils. Oxford University Press.

7. Sesquioxide Clays (Metal Oxides and Hydrous Oxides)

Also found in finer components. 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 weathered soils such as oxisoils in tropical areas and ultisols found predominantly in the southeastern U.S. The most commn iron oxides are Goetihte (FeO(OH) and Hematiite (Fe2O3). These 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 a natural oxidant to certain metals such as As3+ and Cr3+. Birnessite (MnO2) is the most common 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 often associated with clay minerals of the kaolinite group. Imogolite has the empirical formula SiAl4O10.5H2O.

9. Carbonate and Sulfate Minerals

The carbonate 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 Dolamite (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. Organo-clays, 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. Organo-clays also could be used in wastewater treatment and spill control situations.

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

The Mineral MONTMORILLONITE