When we studied soil texture we learned that one of the soil separates was called clay. Now we learn more about this important mineral component of the soil.
Clay minerals are the most important chemical weathering product of the soil. They are formed by the alteration of existing minerals or by synthesis from elements when minerals weather to their elemental form. It is not possible to make clays by grinding up silt or sand particles. Clay has many uses today including, pottery, ceramics, linings for landfills, computer chips, cosmetics, and pharmaceuticals.
Clay minerals are important in soils because of the negative charge they contribute for cation exchange. Understanding about soil clays is also important because of their shrink and swell potential upon wetting and drying. When soils are unsuitable for building because of a high shrink/swell potential this is because of the amount and type of clay present. Our study of clay minerals will involve looking at their structure and why this contributes to their properties.
Clay minerals are part of the larger class of silicate minerals: the phyllosilicates. Included in the phyllosilicate family are the larger true micas which include the familiar minerals muscovite and biotite. Much of what we know about clay minerals is from the macroscopic (i.e., single crystal) study of the true micas.
Silicate Clay Minerals are called aluminosilicates and they have a definite crystalline structure. Clay minerals are very small in size (by definition are less than 0.002 mm in size) and only seen with aid of electron microscope.
Building Blocks for Clay Minerals
Clay minerals are composed of two basic building blocks.
1) Silicon - Oxygen Tetrahedron (Si2O5-2)
2)Aluminum Octahedral (Gibsite Sheet) Al(OH)6-3
Tetrahedral sheets are composed of individual tetrahedrons which share
every three out of four oxygens. They are arranged in a hexagonal pattern with
the basal oxygens linked and the apical oxygens pointing up/down.
Octahedral sheets are composed of
individual octahedrons that share edges composed of oxygen and hydroxyl anion
groups with Al, Mg, Fe3+ and Fe2+ typically serving as the coordinating cation.
These octahedrons too, are arranged in a hexagonal pattern and called a gibsite
sheet.
Kaolinite - a 1:1 Clay Mineral
Kaolinite occurs when the Si-tetrahedral and gibsite sheet are brought
together with the apical oxygen ions of the tetrahedral layer also being in the
octahedral layer. As a result, the charge on these oxygen ions is balanced by
bonding to one silicon ion and two aluminum ions.
The structure illustrated is about 0.7 nm thick (from the bottom oxygen to the top oxygen) and extends 10 nm and more in the other two directions. This 3-D structure is called a micelle.
A 3-D structure of Kaolinite for viewing and moving it about is available at Museum of Minerals and Molecules. The Chime Player plug-in needs to be installed in your computer to see the models move. To view just kaolinite go to Kaolinite in Museum. Place your mouse icon on the model and move it around to see the model move.
Kaolinite is named for the ancient Chinese locality "Kauling," meaning high ridge. You can hear the pronunciation at Kaolinite Information
The kaolinite mineral is actually made up of many micelles piled one atop the other. Since the surface on one micelle contains hydrogen ions and the other surface only oxygen ions there is a tendency for hydrogen bonds to form between micelles. While individual hydrogen bonds are very low energy, the bonding energy is additive and the sum of the many hydrogen bonds between micelles results in the micelles being very strongly bonded together and nearly impossible to separate. This bonding of the layers together results in kaolinite being a nonexpanding clay mineral. Since each micelle is constructed of a layer of silicon tetrahedral units and a layer of octahedral units, kaolinite is called a 1:1 clay mineral.
Most soils of the world contain kaolinite in the clay size fraction (<2µm). In highly weathered soils, such as those of Southeastern U.S. and tropical regions of Africa, Asia, and S. America, kaolinite is usually the dominant clay mineral because of its relative resistance to chemical weathering. Since kaolinite only has negative sites at the edge of the mineral that are not satisfied, the CEC is low, 3 - 15 meq/100g. This makes the soil easy to acidify because the ability to hold on the basic cations is low.
Uses of Kaolinite
China clay is one of the purest of the clays and is composed chiefly of kaolinite. It forms usually when granite is changed by hydrothermal metamorphism. Usage of the terms china clay and kaolin is not well defined; sometimes they are used synonymously for a group of similar clays, and sometimes kaolin refers to those obtained in the United States and china clay to those that are imported.
China clays have long been used in the ceramic industry, especially in fine porcelains, because they can be easily molded, have a fine texture, and are white when fired. France's clays are made into the famous Sèvres and Limoges potteries. These clays are also used as a filler in making paper. Kaolinite is the basic raw material for ceramics, and large quantities are also used in the manufacture of coated paper. In the United States, deposits are found primarily in Georgia, North Carolina, and Pennsylvania; china clay is also mined in England (Cornwall) and France.
For a look at Minnesota Kaolin mines go to Minnesota Kaolin Mines
2:1 clay minerals
The basic structure of 2:1 clay minerals is two silicon
tetrahedral layers and one aluminum octahedral layer. This layer is weakly held
to another 2:1 layer to make the 2:1 family of clay minerals. An interlayer or
the space between the sheets becomes an important difference between 2:1 and
1:1 clay minerals. Smectite and Vermiculite are two kinds of 2:1 clay minerals.
Smectite Clay Mineral
The term "smectite" is used to describe a family of expansible 2:1 phyllosilicate clay minerals having permanent layer charge because of the isomorphous substitution in either the octahedral sheet (typically from the substitution of low charge species such as Mg2+, Fe2+, or Mn2+ for Al3+) or the tetrahedral sheet (where Al3+ or occasionally Fe3+ substitutes for Si4+). It is common for smectites to have both tetrahedral charge and octahedral charge.
A three dimensional structure of Smectite clay minerals is available for viewing at Museum of Minerals and Molecules. or go directly to Smectite Clay
These isomorphous substitutions lead to net negative charges on the clay structure which must be satisfied by the presence of charge-balancing cations somewhere else in the structure. The interlayer is hydrated, which allows cations to move freely in and out of the structure. Because the interlayer is open and hydrated, cations may be present within the interlayer to balance negative charges on the sheets themselves. These cations between the layers are part of the cation exchange capacity of the soil. Smectites will have a CEC of around 80 to 150 meq/100 g
Swelling Properties of smectite The interlayer in smectites is not only hydrated, but it is also expansible; that is, the separation between individual smectite sheets varies depending on: 1) the type of interlayer cations present (monovalent cations like Na+ cause more expansion than do divalent cations like Ca2+), 2) the concentration of ions in the surrounding solution, and 3) the amount of water present in the soil.
Because the interlayer is expansible, smectites are often referred to as "swelling clays". Soils having high concentrations of smectites can undergo as much as a 30% volume change due to wetting and drying or these soils have a high shrink/swell potential. These dramatic changes in soil volume are responsible for the properties of soils in the Vertisol order, which form deep cracks upon drying. Most soils in Minnesota will have smectite as the dominant clay mineral.
Mining of smectite (bentonite) Early Americans found bentonite vital to their lives. Pioneers found moistened bentonite to be an ideal lubricant for squeaky wagon wheels. The mixture was also used as a sealant for log cabin roofing. The Indians found bentonite useful as a soap. Small amounts of Wyoming bentonite were first commercially mined and developed in the Rock River area during the 1880s. Newer, more substantial deposits were discovered in other parts of Wyoming during the 1920s and the first processing plant in Wyoming was built during this period. since that time many other processing plants have been built for the purpose of processing Wyoming sodium bentonite.(source: Black Hills Bentonite, LLC)
Wyoming bentonite (see Black Hills Bentonite) is composed essentially of smectite clay. Wyoming bentonite is capable of absorbing 7 to 10 times its own weight in water, and swelling up to 18 times its dry volume.
During the cretaceous age, volcanoes in the
Yellowstone area of Wyoming were subject to long periods of eruptions. Ash
falling from these eruptions dropped into seas which covered much of Wyoming,
forming a sediment as much as 50 feet deep. These sediments were slowly altered
into the clay we know today as bentonite. In these deposits are contained 70%
of the world's known supply of western or sodium type bentonite.
Bentonite is mined by surface "open pit"
methods. Various types of heavy equipment including bull dozers and
rubber-tired scrapers are used to remove the shale rock overlying the
bentonite. Topsoil, as well as the underlying material, is carefully removed
and stockpiled. These "overburden" materials as they are called will
be placed back and reseeded once the bentonite has been removed.
The bentonite which is exposed during this process can
be as little as 1 1/2 feet or as much as 10 feet thick. This is the material
which is mined and processed. Many bentonite manufactures prefer to "field
dry" the exposed bentonite prior to hauling it to the processing plants.
This is accomplished by plowing and discing while taking advantage of the low
humidity and sunny days to dry the bentonite prior to its removal. The moisture
level prior to "field drying" can exceed 30%. This process will
normally extract 15 to 20% of the moisture from the clay prior to hauling.?
Uses of Bentonite (Smectite)
Drilling mud, or drilling gel, has bentonite as a major component. Drilling mud is crucial in the extraction of drill cuttings during the drilling process. Bentonite, when mixed with water, forms a fluid (or slurry) that is pumped through the drill stem, and out through the drill bit. The bentonite extracts the drill cuttings from around the bit, which are then floated to the surface.
Taconite, a low grade iron ore, has been developed as an economic source for iron. During processing, the taconite is ground into a very fine powder. The ground taconite is then mixed with small amounts of bentonite which serves as a binder to the taconite. This mixture is processed into balls or pellets. The process is finished when these pellets are sintered in rotary kilns that give the pellets a hard surface.
The metal casting industry needs bentonite as an economical bonding material in the molding processes associated with the metal casting industry. Bentonite, when mixed with foundry molding sands, forms a pliable bond with the sand granules. Impressions are formed into the face of the bentonite/sand mixtures. Molten metal is pored into the impressions at temperatures exceeding 2,800 F. The unique bonding characteristics of bentonite insures the durability of the mold during these high temperatures. Once the process is complete, the bentonite/sand mold can then be broken away from the casting face and reused.
In recent years, bentonite has become a major component in the
manufacturing of cat litter.
Because of the unique water absorption, swelling, and
odor controlling characteristics of bentonite, it is ideal for use in
"clumping" types of cat litters. Clumping cat litter has become
widely accepted as an economical alternative to conventional non-clumping type
cat litters. Because bentonite forms clumps when wet, the clumps can easily be
removed and disposed of. The remainder of the unused material stays intact and
can continue to be used. clumping cat box litters will last longer with less
frequency of changing.
For many years bentonite has been used as a binder in the feed pelletizing industry. Small amounts of bentonite can be added to feed products to insure tougher, more durable pellets. Bentonite has also proved helpful in sealing freshwater ponds, irrigation ditches, reservoirs, sewage and industrial water lagoons, and in grouting permeable ground. In addition, it has been used in detergents, fungicides, sprays, cleansers, polishes, ceramic, paper, used as a base for cosmetics and medicines, and applications where its unique bonding, suspending or gellant properties are required.
Vermiculite Clay Mineral.
Vermiculite is a high-charge 2:1 phyllosilicate clay mineral. It is generally regarded as a weathering product of micas. The charge in vermiculites may be both tetrahedral and octahedral in nature, but most vermiculites have mainly tetrahedral charge due to the substitution of Al3+ for Si4+. Typically, between 0.6 to 0.9 out of 4 Si are replaced by Al. This negative charge must be balanced by the presence of positively-charged sites somewhere else in the mineral structure. See 3-D Vermiculite Clay Mineral or Museum of Molecules and Minerals. Generally vermiculite is like smectite except for more tetrahedral charge, which results in vermiculite having a CEC of 150 meq/100g. Therefore, to achieve charge-balance the presence of exchangeable cations in the interlayer is necessary.
Like smectite, Vermiculite is also hydrated and somewhat expansible though less so than smectite because of its relatively high charge. Soils rich in vermiculite do not display the high shrink-swell characteristics of highly smectitic soils. In some soils the interlayer of vermiculite is filled with potassium ions which act as a bridge holding the layers rigidly together. This mineral is called Illite or Hydrous Mica.
Exfoliation of Vermiculite
Vermiculite possesses the special property of expanding to between six and twenty times its original volume when heated to approximately 1,000 degrees Celsius. This process, called exfoliation, liberates bound water from between the mica-like layers of the mineral and literally expands the layers apart at right angles to the cleavage plane.
The resulting exfoliated Vermiculite has a characteristic concertina (worm like) shape (the name vermiculite is derived from the Latin 'vermiculare' - to breed worms). This characteristic of exfoliation, the basis for commercial use of the mineral, is the result of the mechanical separation of the layers by the rapid conversion of contained water to steam. The increase in bulk volume of commercial grades is 8 to 12 times, but individual flakes may exfoliate as many as 30 times.
The bulk density of crude vermiculite or vermiculite concentrate is in the range of 40-70 lb/ft³ and exfoliated or expanded vermiculite is in the range of 4-10 lb/ft³.
Vermiculite is used to loosen and aerate soil mixes.
Mixed with soil, it improves water retention and fertilizer release, making it
ideal for starting seeds. Also used as a medium for winter storage of bulbs and
flower tubers.
Vermiculite has been used in various industries for over 80 years. Exfoliated vermiculite has a high insulation value, is lightweight and will absorb and hold a wide range of liquids. These granules are non-combustible, and are insoluble in water and all organic solvents. Expanded vermiculite is easily poured, is light, clean, highly absorbent and provides baffle against impact shock when used for packaging. Exfoliated vermiculite is used in the friction brake linings market, high temperature insulation, loft insulation, fire proofing, sound proofing, various construction products, and animal feeds,
The cation exchange properties of vermiculite, and its layered structure and surface characteristics, are utilized in products such as intumescent coatings and gaskets, and the treatment of toxic wastes.
Vermiculite mines are located in Australia, Brazil, China, Kenya, South Africa, Zimbabwe, and the states of South Carolina and Montana. Vermiculite mines are surface operations where ore is separated from other minerals, and then screened or classified into several basic particle sizes.
Sources of Negative Charge .
1) Soil Humus
Soil organic matter will under go disassociation of the H ion from the COOH group or carboxyl group (a carboxyl group consists of a carbon atom joined to an oxygen atom by a double bond and to a hydroxyl group, OH, by a single bond). The H+ ion will now exist in the soil solution and the COO - (organic matter ) will have a negative charge site which can attract cations.
R—COOH <—> R—COO-
The creation of the negative charge on the carboxyl group is pH dependent, or as the pH increases more OH- are available, more H+ leaves the COOH group , and therefore, more CEC is created. Conversely acid soils have a lower CEC from OM because the H stays in the COOH group.
2) Silicate Clay Minerals
1) Broken bonds at edge of clay minerals. All clays have a finite size, and at the edge of the mineral there are some oxygen atoms that do not have their negative charges satisfied by cations in mineral structure, thus at the edge of the mineral negative charges exist.
2) Isomorphous Substitution - internal charge not satisfied because
of ionic substitution of Al 3+ for Si 4+ or Mg2+ for Al 3+
In vermiculite the replacement of Si4+ with Al+3 in 25% of tetrahedrals creates
1 negative site for each substitution.
In smectite Mg2+ replaces Al3+ in Octahedral layers about 1/4 of the time and
creates negative sites
Summary - Kaolinite clays only have the edges contributing to CEC, while 2:1 clays have the edges and isomorphous substitution.
© Terence H. Cooper & Regents of the University of Minnesota, 2008. The University of Minnesota is an equal opportunity educator and employer.