Clay Mineralogy Tutorial

Introduction Clay Minerology – 3D Models

Introduction
The basic building blocks of clay minerals are the tetrahedral layer and the octahedral layer. The tetrahedral layer is composed of either Si or Al in tetrahedral coordination (4 oxygens) with oxygen. The octahedral layer is composed of cations in octahedral coordination (6 oxygens) with oxygen. Whether a cation is in tetrahedral or octahedral coordination with oxygen depends on the size of the cation. The cation is stable in a particular coordination environment, as long as it is able to keep the oxygen anions from touching, thereby preventing repulsive forces from destabilizing the structure. For example, Si is small enough that only 4 oxygens are able to fit around it and the most stable arrangement of these oxygens is in tetrahedral coordination. Cations such as Mg, Fe(II) and Fe(III) are larger and thus able to accommodate 6 oxygens in their coordination environment. Aluminum’s size is in between Si and Fe/Mg, therefore, it has the ability to fit in either octahedral or tetrahedral coordination. View the first six models which show the Si-tetrahedra and Al-octahedra. These models are displayed as polyhedra, space-filling or as ball and stick representations. The space filling models are scale represenations of the size of the cation (Si or Al) and anion (Oxygen). Notice how much larger the oxygen anion is compared to the central cation. You can see that as the cation becomes larger more oxygens are able to fit in the coordination sphere without touching. Also notice that Al is coordinated by both Oxygen and hydroxyl anions. Since the proton is so small the hydroxyl anion is essentially the same size as the oxygen anion. These hydroxyls will become important later as we build the clay minerals. You can view the Silica tetrahedra and Al-octahedra in different representations by clicking the appropriate button. For most of the clay minerals we will use the polyhedral representation because it is easier to observe the arrangement of the different layers. Also, if you have access to a cheap pair of red/blue stereo glasses you can view the clay mineral models as “out of screen” red blue stereo by clicking on the clay mineral stereo button . The models range in size from about 200K to 1MB for some of the stereo models. Therefore, if you are accessing the internet via a modem the red/blue stereo models may take a long time to download.

Polyhedral
Ball and Stick
Space Filling

 

Si Tetrahedra – Polyhedral             Al Octahedra – Polyhedral
 Si tetrahedra – Ball and Stick       Al Octahedra – Ball/Stick
 Si Tetrahedra – Space Filling        Al Octahedra – SpaceFill

 

Computer Requirements

The 3D models listed on this page require the use of Quick Time Plug-ins. If your computer does not have these plug-ins, they can be downloaded by clicking here. This is the quicktime web site — follow the directions on the left-side of the screen and click either the Mac or PC button depending on your platform. Once the download is complete you may have re-boot your computer. If you are having difficulties e-mail me or talk to someone at the computer help desk by dialing 231-4357.


Use the table below to identify the atoms and molecules which make up the clay minerals:

Color Key for Clay Mineral Models
Silicon Atom Potassium Atom
Aluminum Atom Oxygen Atom
Magnesium Atom Hydroxyl Molecule

Tetrahedral and Octahedral Layers
Now let’s use these basic building blocks to create layers that compose clay minerals found in soils. If you take the Si-tetrahedra and join them at their basal oxygens (the oxygens at the base of the tetrahedra) you create a tetrahedral layer. If you take the Al-octahedra and link them by side or edge oxygens you create a octahedral layer. The octahedral layer can either be or . The di-octahedral layer is where 2 out of 3 of the octahedral sites are occupied by a trivalent cation (2 X 3 = 6). The tri-octahedral layer is where 3 out of 3 sites are occupied by a divalent cation (3 X 2 = 6). This is confusing because many students think that di- and tri- are referring to the charge on the cation. View the tetrahedral and octahedral layers. Pay particular attention to the difference between the di- and tri-octahedral layers. Also, notice the hexagonal hole that is created by the linked Si-tetrahedra in the tetrahedral layer. This will become important as we build the 2:1 clay minerals.Before we move on to the clay minerals let’s talk a little about charge development. Remember that Al can exist in tetrahedral coordination as well as octahedral coordination. If an Al tetrahedra substitutes for a Si tetrahedra in the tetrahedral layer, excess negative charge will develop. This is because Al has a +3 charge while Si has a +4 charge. Due to this charge difference the negative charge (-2) on the oxygens shared between the Al and Si tetrahedra is not satisfied. Substitution of one cation for another, either in the tetrahedral or octahedral layer, is called Isomorphic Substitution. Excess negative charge also occurs when octahedrally coordinated divalent cations such as Mg(II) or Fe(II) substitute for Al(III) in the octahedral layer. Isomorphic substitution that does not give rise to charge is Fe(III) substituting for Al in the octahedral layer. This is because both cations have a charge of +3.

Clay Minerals
The first clay mineral that we will build is Kaolinite. If we take a tetrahedral and octahedral layer and place them together we have created the 1:1 structure of kaolinite. The tetrahedral and octahedral layers are bonded together by sharing oxygen anions between Al and Si. Together, these two layers are called platelets. These platelets stack up to from a crystal of the 1:1 mineral. Since Al is in the octahedral layer, kaolinite is a di-octahedral mineral. The 1:1 platelets of kaolinite are held together strongly via hydrogen bonding between the OH of the octahedral layer and the O of the tetrahedral layer. Due to this strong attraction these platelets do not expand when hydrated and kaolinite only has external surface area. Also, kaolinite has very little isomorphic substitution of Al for Si in the tetrahedral layer. Therefore, it has a low Cation Exchange capcity (1 – 5 cmol/kg). When viewing this mineral notice the sharing of oxygens between the tetra and octa layers. Three representaions of kaolinite are displayed below (both in color and red/blue stereo). Notice how it diffcult becomes to view the individual layers when we move from the polyhedral model to the space filling model.
To view the red/blue stereo images you must have access to red/blue stereo glasses!!!

Kaolinite Polyhedral              Kaolinite Ball and Stick            Kaolinite Space Filling

Kaolinite Polyhedral 3d         Kaolinite Ball and Stick 3d        Kaolinite Space Filling 3d

 

 

Now if we take that same kaolinite 1:1 layer and add a second tetrahedral layer below the octahedral layer we create a 2:1 mineral. It is a 2:1 mineral because a single octahedral layer is sandwhiched by two tetrahedral layers. The di-octahedral mineral with essentially no ispomorphic substitution is called  pyrophyllite, pyrophyllite3d .The tri-octahedral from of this mineral (Mg in the octa layer) is called Talc. The 2:1 platelets of talc and pyrophyllite are held together by Van Der Waals bonds. These bonds are weaker than the hydrogen bonds that hold the 1:1 layers of kaolinite together. However, similar to kaolinite these layers are essentially non-expanding when hydrated. Therefore, these minerals have only external surface area similar to kaolinite and essentially no cation exchange capacity (CEC).

How are we doing so far? Basically, we have built the framework for all of the clay minerals. If you understand what we have covered so far the rest will be a breeze. Now if we take the same basic structure of the 2:1 mineral Pyrophyllite and isomophically substitute Al for Si in the tetrahedral layer we have muscovitemuscovite3d (Mica). In this mineral there is a good deal of isomorphic substitution. Consequently, the mineral has a large amount of excess negative charge. This excess negative charge is balanced by interlayer potassium. Because of potassium’s ionic radii it is able to fit nicely in the hexagonal hole created by the Si/Al tetrahedral layer. Consequently, the interlayers collapse and hold this potassium tightly. The 2:1 layers are held together due to this electrostatic attraction between the Si tetra layer and potassium and also because potassium fits so nicely in the hexagonal hole. Notice the K in the interlayers. Since the layers collapse upon the interlayer K, micas are non-expanding and therefore have only external surface area. Also, since K satisfies the excess negative charge created by isomorphic substitution, pure micas have no avaliable CEC.


Let’s again take the basic Pyrophyllite structure and this time let’s isomorphically substitute Mg (divalent cation) for Al (trivalent cation) in the octahedral layer. We have just created the mineral montmorillonite, montmorillonite3d . The isomorphic substitution is less than in Muscovite so the overall charge will be less. Also, the isomorphic substitution is primarily in the octahedral layer (compared to the tetrahedral layer for muscovite). Remember Coloumb’s Law which deals with the force of attraction between two charges; the farther the distance between the 2 charges the smaller the force. Because of the overall lower negative charge and because the charge is located in the octahedral layer, this clay mineral can expand when hydrated. Montmorillonite is part of a group of clay minerals called Smectites. These minerals expand when they are wet and contract when they are dry. This shrinking and swelling causes problems for houses and roads. The mineral vermiculite has properties in between muscovite and montmorillonite. It has less isomorphic substituiton than muscovite but more than montmorillonite. Also, the isomorphic substitution is primarily located in the tetra layer. Hence, this mineral is semi-expanding. Both montmorillonite and vermiculite have internal and external surface area and significant CEC (Mont: 80 – 150 cmol/kg; Verm: 100 – 200 cmol/kg). When viewing montmorillonite notice the Mg (yellow) substituting for Al (light-blue) in the octahedral layer.

Have you had enough? Only one more mineral to go. Let’s take the 2:1 mineral muscovite again. Remember that this mineral has isomorphic substitution of Al for Si in the tetrahedral layer. Now let’s add a hydroxyl sheet made of Al and Mg and place it in the interlayer between the 2:1 layers. We just constructed the 2:1:1 or or 2:2 mineral chloritechlorite3d . This mineral is termed 2:1:1 (or 2:2) because it has another octahedral sheet in the interlayer. This sheet is held together in the interlayer because we have Al (trivalent) isomorphically substituting for Mg (divalent). This is opposite to the isomorphic substitution we have been previously discussing. This isomorphic substitution gives rise to a net positive charge on the interlayer octahedral sheet. Therefore, the platelets are held together electrostatically and this mineral is non-expanding!!! When viewing this mineral notice the octahedral layer between the 2:1 layers. Also, notice the Al (light blue) substitution for Si (dark blue) in the tetrahedral layer.

That’s it!! You are now experts in clay mineralogy. Many of the clay minerals we just discussed are important in soils found in SW Virginia. These minerals give rise to the CEC that is essential for holding nutrient elements in an available form for agriculture.

This web page was constructed by M.J. Eick and R.B. Burgholzer from Department of Crop and Soil Environmental Sciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061-0404. If you have comments or suggestions, email us at eick@vt.edu or rburghol@vt.edu

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Welcome to my Environmental Soil Chemistry blog.  This blog will be used primarily to house tutorials and links to bu used for students enrolled in my Environmental Soil Chemistry Course.  While I may make posts periodically most course information will be located on the Scholar course site.

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