lecture 12_ clay minerals.pdf

11/27/2014 Lecture 12: Clay Minerals

http://www2.nau.edu/~doetqp-p/courses/env320/lec12/Lec12.html 1/5

Lecture 12

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. Theinorganic component of the soil makes up more than 90% of the soil solids.Inorganic components occur mainly in limited number of compounds with definitecrystalline structure called minerals. The inorganic component includes both primary andsecondary minerals.The secondary minerals normally are found in the clay fraction of the soil which is thefraction of the soil solids which is less the 2 micron or 0.002 mm. Clay minerals areminerals 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. Thetotal colloidal area of soil colloids may range from 10m2/g to more than 800 m2/gdepending the external and internal surfaces of the colloid.Soil colloids also carry negative or positive charges on their external and internalsurfaces. The presence of charge influences their ability to attract or repulse charge ionsto or from surfaces.Soils colloids play a very important role in the chemical reaction which take play in soiland influence the movement and retention of contaminants, metals, and nutrients in thesoil.

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 ordecomposition and recrystallization.Clay mineral types are normally determined by the types of minerals and acidity of theleaching water.Based on their origins clays may classified as Inherited, Modified, Transformed orNeoformed (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 isthe substitution of one element for another in ionic crystals with out change of thestructure. It takes place during crystallization and is not subject to change afterwards. Ittakes 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 andnot pH-dependent.In allophanes, some silicate clays e.g. kaolinite, and the metal oxides the main source ofcharge are termed pH -dependent charges because these charges depend on the pH ofthe soil. pH depend charges are variable and may either be positive or negativedepending on the pH of the soil. In the metal oxides acid soils tend to develop positivecharges 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 tetrahedronand 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 certainpositions 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 calledtrioctahedral.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 theirelectronegativity and their capacity to absorb cations.In 2:1 clay mineral an octhehedral sheet is bonded to two tetrahedral sheets. Theoctahedral sheet is generally sandwiched between the two tetrahedral sheets. The 2:1clays can be classified into expanding (smectites) and non-expanding clays (Illite andmicas) on the basis of the sheet where isomorphous susbstitution is taking predominantlytaking place (Refer to Figure 5-3).In the 2:1:1 lattice clays, a positively charge brucite sheet sandwiched between layersrestricts swelling, decreases effective surface area, and decreases the effective CEC ofmineral. 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 similarto mica. It occurs commonly in sedimentary rocks.

Table 12.1 Silicate Clay Mineral Groups:

Group

LayerType

LayerCharge (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 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 soissuch 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 arealso 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 isreadily 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 plantnutrient, 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 componentof 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-silicatesand 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, suchas long chain alkyl amine cations can be been used as liners in landfills to reducetransport of contaminants. Organoclays also could be used in wastewater treatment andspill control situations.

Vocabulary

TetrahedraOctahedraIsomorphic substitutionChloriteIlliteMontmorilloniteSoil colloidsVemiculliteKaoliniteSesquioxidesAllophane

Web sites

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

Journal of Natural Resources and Life Sciences Education. This is an excellent electronicjournal 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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