These are the primary component of soils and are known as excellent trappers of water held between layers. Minerals within these groups are further categorized into dioctahedral and trioctahedral [ 11 ]. On the basis of number and arrangements of tetrahedral and octahedral sheets present in clay, the layer silicate are divided into three categories: type of clay mineral. Each individual layer is assembled from one tetrahedral SiO 4 and one octahedral sheet AlO 6. Kaolinite and Halloysite are examples under this category [ 20 ].
The rocks that are found to be rich in Kaolinite are identified as Kaolin or china clay [ 21 ]. The chemical weathering of aluminum silicate such as feldspar results in formation of a soft, usually white, earthy mineral dioctahedral phyllosilicate clay The dickite and nacrite are rare forms of kaolinite which are chemically similar to kaolinite but amorphous in nature.
Kaolinite is found to be electrostatically neutral having triclinic symmetry. The hydrogen bonding is found in between oxygen atoms and hydroxyl ions of the layers that are paired. Since, hydrogen bonding is weak, random movements between the layers are quite common results in lower crystallinity of kaolinite minerals than that of the triclinic kaolinite. The ideal structure of kaolinite has no charge. Hence, the structure of Kaolinite is fixed due to the hydrogen bonding therefore; there is no expansion between the layers or have low shrink-swell capacity when clay is wetted.
Due to the low surface area and little isomorphous substitution, Kaolinite has low capacity to adsorb the ions [ 15 ]. Dickite and nacrite are polytypic forms of kaolinite consisting of a double layer and have monoclinic symmetry. Dickite and nacrite differentiate themselves by different stacking sequences of the two silicate layers [ 23 ].
It is illustrated by its tubular form in contrast to the platy form of kaolinite particles. Dehydration occurs on mild heating of Halloysite and will irreversibly get transformed to kaolinite.
The dehydrated form of Halloysite has basal spacing with thickness of a kaolinite layer approximately 7. The difference of 2. Consequently, in hydrated form, the layers of halloysite are separated by monomolecular water layers that are lost during dehydration [ 23 ]. Serpentine: Serpentine is a group of hydrous magnesium-rich silicate minerals and a common rock-forming mineral having the composition Mg 3 Si 2 O 5 OH 4 [ 24 ].
Serpentine generally appears in three polymorphic forms: chrysotile, a fibrous type used as asbestos; antigorite, a variety exists in either corrugated plates or fibers; and lizardite, a very fine-grained, platy variety.
Serpentine is usually grayish, white, or green due to iron replacing magnesium but may be yellow chrysotile or green-blue antigorite. It usually occurs along the crests and axes of great folds, such as island arcs or Alpine mountain chains. Normal occurrences are in altered peridotites, dunites, or pyroxenites; serpentinite is a rock consisting largely of serpentine. Most of the layer silicate clays are commonly found in soils and based on the mica structure in which a single octahedral sheet sandwiched between two tetrahedral sheets and form an individual composite layer as shown in Figure 5.
In dioctahedral and trioctahedral layer silicates, two and three octahedral sites are occupied respectively out of the three available sites in the half- unit cell single Si 4 O 10 [ 26 ].
Different types of clay minerals. These types of clay minerals consist of one octahedral layer sandwiched between two tetrahedral layers. They are further characterized into two categories: Expanding clay minerals: Smectite group and Vermiculite.
Expanding clay minerals: This group includes mainly smectite group of clay minerals and vermiculite clay mineral. They are known for their interlayer expansion which happens during their swelling behavior when they are wet. Smectites are mainly based on either trioctahedral talc or dioctahedral Pyrophyllite structure and differ from these neutral structures due to the presence of isomorphous substitution in the octahedral or tetrahedral layer.
The Smectite group of clay minerals are further divided into Saponites trioctahedral and Montmorillonite dioctahedral. Another important member of the Smectite family is Bentonite. Bentonite clay is also known as sedimentary clay and has unique property of water retaining.
The most prominent members of this group are Montmorillonite. Beidellite, nontronite, and saponite. The flake-like crystals of smectite e. Each layer is composed an octahedral sheet sandwiched between two tetrahedral silica sheets. Slight attraction is found between oxygen atoms present in the bottom tetrahedral sheet of one unit and in the top tetrahedral sheet of another unit.
This allows a variable space between layers, which is occupied by exchangeable cations and water. Therefore, the exchangeable cations and water can easily enter the interlayer space resulting in the expansion of layers that may vary from 9.
In Montmorillonite, magnesium ions are replaced aluminum ions in some sites of octahedral sheet and likewise, some silicon ions in the tetrahedral sheet may be replaced by aluminum ions. This type of replacement is known as isomorphic substitution which give rise to a negative charge on the surface of clay minerals.
Therefore, the layer charge density of these minerals is found to be in between 0. The general structural formula of smectite group of clay minerals is Na, Ca 0. The structure, chemical composition, exchangeable ions are responsible for their several unique properties such as high cation exchange capacity, high surface area and high adsorption capacity.
The quantity of cations required to balance the charge deficiency induced by these substitutions is referred to as the cation exchange capacity CEC.
The CEC for Montmorillonite ranges from 80 to milliequivalent per grams. Montmorillonite clays have very poor thermal stability. These minerals show some prominent characteristics like high cation exchange capacity, swelling and shrinkage capacity. When smectite dominated soils e. Chemical composition of the unit cell has been represented as [ Si 8. It is a versatile mineral due to its platelet structure.
The platelet consisting of a tetrahedral silicon oxide layer in which some silicon replaced by trivalent cations sandwiched between two octahedral aluminum oxide layers in which aluminum replaced by divalent cations.
The hydroxide group is present on the edge of each platelet results in thixotropic nature [ 28 ]. The different types of bentonite are found based on their respective dominant element, such as potassium K , sodium Na , calcium Ca , and aluminum Al.
For industrial purposes, three main classes of bentonite exist: sodium, calcium and potassium bentonite. Sodium bentonite: Sodium bentonite expands in wet condition where it absorb the water as much as several times its dry mass. Because of its excellent colloidal properties, [ 29 ].
It is often used in drilling mud for oil and gas wells and also in boreholes for geotechnical and environmental investigations [ 30 ]. Due to its swelling capacity, sodium bentonite is used as a sealant, since it offers a self-sealing, low permeability barrier.
Enhancement in some rheological or sealing performance are observed after various surface modifications of sodium bentonite for example, the addition of polymers [ 31 ]. Calcium bentonite: Calcium bentonite is considered as a useful adsorbent for ions, fats and oils [ 32 ]. Hence, some properties of sodium-beneficiated calcium bentonite or sodium-activated bentonite such as viscosity and fluid loss of suspensions may not be fully comparable to those of natural sodium bentonite [ 30 , 33 ].
Potassium bentonite: This is also known as potash bentonite or K-bentonite. Potassium bentonite is formed from alteration of volcanic ash and considered as potassium-rich illitic clay. Vermiculite: Vermiculite also belongs to group of clay minerals where one octahedral sheet occurs between two tetrahedral sheets. Most of the Vermiculites are Al dominated showing dioctahedral structure. Vermiculite is known as hydrous phyllosilicate mineral which undergoes substantial expansion when heated results in exfoliation and commercial furnaces can routinely generate this effect.
Vermiculite formed by the weathering or hydrothermal modification of biotite or phlogopite [ 34 ]. Vermiculite was first described in in Millbury, Massachusetts. Therefore, both the tetrahedral and octahedral units are joined together tightly rather than driving apart from each other resulting the less expansion of interlayer spacing on wetting [ 36 ].
Vermiculite clays are weathered micas where the potassium ions are replaced by magnesium and iron ions between the molecular sheets [ 37 , 9 ]. Non-expanding clay minerals: This group includes mainly Mica illite clay mineral which is a secondary form of mineral precipitate. This group is an example of a phyllosilicate, or layered alumino-silicate. Muscovite and biotite are also found in the clay fractions which are also called fine grained Mica.
Illite is considered as an modified product of muscovite and feldspar formed from weathering and hydrothermal environments; known as component of sericite. It is commonly found in soil and argillaceous sedimentary rocks as well as in some low grade metamorphic rocks [ 38 ]. The adsorption capacity, swelling, shrinkage capacity is less than Montmorillonite and Vermiculite but more than Kaolinite interstratified layers are present.
Chlorite is mainly belongs to silicate group which are basically iron magnesium silicates with some aluminum atoms. The typical chlorite clay crystal composed of layers, such as in vermiculites clay mineral alternate with a magnesium dominated tri-octahedral sheet also known as brucite giving rise to ratio.
All the octahedral positions in chlorite are occupied by magnesium ions as in the brucite layer [ 41 ]. The negative charge of chlorites is less than smectite or vermiculites but about the same as that of fine grained mica. There is no water adsorption between the layers responsible for the non — expanding nature of this crystal. Chlorites having a muscovite-like silicate layer and an aluminum hydroxide sheet are called donbassite and show the ideal formula of Al 4.
In many cases, the aluminum ions present in octahedral layer are partially replaced by magnesium ions as in magnesium-rich aluminum dioctahedral chlorites called sudoite. Another type of dioctahedral chlorite is Cookeite in which lithium substitutes for aluminum in the octahedral sheets [ 42 , 15 ]. The structure of palygorskite consists of extended silicon-oxygen sheets results in the retention of the mineral in the layer silicate family whereas the tetrahedral SiO 4 groups forming silicon-oxygen sheets are oriented in such a manner so that extended lathlike features could be developed which create the fibrous morphology.
The chain silicate mineral found in sediments from playa lakes, saline deposits in desert soils and in calcareous material. Attapulgite is one type of palygorskite found in Attapulgus, Ga. For chemical formula and physical properties of attapulgite [ 44 , 45 ]. It is opaque and white, gray, or cream in color. It may seem like the bones of the cuttlefish Sepia. The name of Sepiolite is derived from cuttlefish Sepia. The structures of sepiolite and palygorskite are almost similar consisting of narrow strips or ribbons of layers that are attached to each other at the corners.
One ribbon is attached to the another by inversion of the direction of the apical oxygen atoms of SiO 4 tetrahedrons; in other words, an extended rectangular box comprising of continuous layers is enclosed to the nearest boxes at their extended corner edges.
Therefore, due to the absence of silicate layers, channels or tunnels occur on the extended sides of the boxes results in the fibrous morphology of the minerals. Since the octahedral sheet is irregular, some of the magnesium ions present in octahedral layer are exposed at the edges and hold on bound water molecules H 2 O.
The width of the ribbons is found to be greater in sepiolite than in palygorskite which is a major difference between these chain silicates. The width of ribbons defines the number of octahedral cation positions per formula unit [ 5 , 46 ]. Sesquioxide clays are produced from heavy rainfall and leached most of the silica and alumina from alumino — silica clay by leaving less soluble iron oxide Fe 2 O 3 , iron hydroxide Fe OH 3 and aluminum hydroxide Al OH 3.
Sesquioxides of iron and aluminum are found in soil. A sesquioxide is an oxide comprising three atoms of oxygen and two another element. For example, aluminum oxide Al 2 O 3 is a sesquioxide. They are not adhesive in nature and do not swell in presence of water. They have ability to hold large amount of phosphate as they have tendency to hold phosphorous tightly make them unavailable for absorption by plants.
They have low CEC. They are found in both crystalline and amorphous form. Crystalline Sesquioxide are either metal oxide or hydroxide whereas amorphous Sesquioxide are Allophane and Imogolite.
The shape of imogolite is cylindrical consisting of a modified gibbsite sheet where the hydroxyl of one side of a gibbsite octahedral sheet lose protons which form bond with silicon atoms located at vacant octahedral cation sites of gibbsite.
Thus, three oxygen atoms and one hydroxyl present around silicon atom make up an isolated SiO 4 tetrahedron as in orthosilicates which make a planar array on the edge of a gibbsite sheet. Because of shorter bond length between silicon-oxygen bonds than aluminum-oxygen bonds sheet change into curve shape results in a tube like structure with inner and outer diameters of about 6. Allophane are considered as a group of naturally occurring hydrous aluminosilicate minerals. They are not totally amorphous but are short-range partially ordered.
Allophane are described by the dominance of Si-O-Al bonds where most of the aluminum atoms are tetrahedrally coordinated. Unlike imogolite, the morphology of allophane varies from fine, rounded ring-shaped particles to irregular aggregates which indicates that the ring-shaped particles may be hollow spherules or polyhedrons. Despite their indefinable structure, their chemical compositions surprisingly fall down in a relatively narrow range as the SiO 2 :Al 2 O 3 ratios are mostly found to be in between 1.
In general, the SiO 2 :Al 2 O 3 ratio of allophane is higher than that of imogolite [ 46 ]. Clay minerals are considered as gift for human beings as they are exploring the clay minerals continuously through research as these are of very low cost, environment friendly, easily available and non — toxic. The clay minerals are so widespread that in the world, there is hardly any country where there are no deposits of one or other kind of clay minerals. In nature, clay minerals are found with certain physical and chemical characteristics due to which these clay minerals play an important role in different fields from research to industries [ 47 ] The clay minerals are a class of rock-forming minerals having porous like sheet structure with different distances between the sheets.
The cation exchange capacity, a number of possible charged ions by the negatively charged surface of clay materials, depends on the number of sheets and the cations located in these structures [ 48 ].
Therefore, physical and chemical properties of clay minerals depend significantly on their sheet structure, cation- and anion-exchange capacity and adsorption ability which mainly determines their importance in different applications [ 49 ].
The exchange of these adsorbed ion takes place with other ions. The quantitative relationship between different reacting ions makes the exchange reaction completely different from simple sorption. Cation-exchange capacity CEC is the measurement of number of cations retained on the surface of soil particles [ 50 ].
Negatively charged ions present on the surface of soil particle bind with positively charged ions but allow them to exchange in the surrounding soil water with other positively charged particles results in alteration of chemistry of soil [ 51 ]. The various aspects of soil chemistry are affected by CEC.
CEC Wikipedia CEC depends on particle size, crystallinity perfection and adsorbed ion therefore for a given mineral, values exist in range rather than single specific capacity. The exchange capacities also depend on pH due to the presence of hydroxyl group on the surface of certain clay minerals like allophane and kaolinite. The original negative charge layer is either replaced or exchanged by the adsorbed cations. This ability of colloidal particles such as clay minerals to maintain and exchange positively charged ions is important because it governs the mobility of positively charged chemical species both in soils and in general geochemical cycling of cations as shown in Figure 6 [ 52 ].
CEC is a reversible process and normally correlated with clay minerals due to the presence of interlayer exchangeable cations such as smectites. The cation-exchange capacities of the clay minerals is given in the Table 1. Clay minerals showing the cation exchange capacity. There are various cations which do not have same replacing power and not equally replaced under a given set of conditions. For example, calcium will easily replace sodium than sodium will replace calcium.
Due to similar size potassium and ammonium ions, they easily fit in the hexagonal cavities of the silicate layer. Vermiculite and vermiculitic minerals preferably and irreversibly adsorb these cations and fix them between the layers.
Heavy metal ions such as copper, zinc, and lead are strongly attracted to the negatively charged sites on the surfaces of the layer minerals, allophane and imogolite, which are caused by the dissociation of surface hydroxyls of these minerals. The indirect method to determine CEC mainly involves the exchange of naturally occurring cationic species in clay minerals with organic cations such as alkylammonium. Depending on the method used for determining CEC, if the exchanged cations are present in excess, they are removed in subsequent step and the cations retained on the surface of clay are determined.
Methylene blue used for determination of CEC in a rapid qualitative procedure but when compared with other methods, results were not appropriate [ 53 , 54 ]. Due to high affinity of clay minerals for these ions, CEC can be determined directly. There are some other techniques like potentiometric titration, surface tension measurement which are used for determination of CEC. The potentiometric titration used for different types of clay minerals like Colay 90Wyoming bentonite, Na-Montmorillonite and Illite bearing shale whereas surface tension measurement used for Montmorillonite, Kaolinite and Illite [ 58 ].
If dry clay minerals are allowed to adsorbed water in a controlled environment, water is added into their interlayer space in more or less discrete forms of layer causing swelling or expansion of the interlayer space. The swelling of interlayer space of clay minerals is due to the hydration energy forces associated with the particles interaction [ 59 ].
The swelling clays are prone to large volume change which are related to change in water content. The swelling capacity of clay minerals depends on the following factors: The layer charge density of clay minerals.
The swelling capacity phenomenon depends on granularity as well as superficial activities of clay itself. The main mineral components of clay minerals are dispersed layer silicate [ 60 ].
The swelling properties of clay minerals play an important role in design of structures of light buildings. There are several mineralogical research which discuss about the swelling and non — swelling clay minerals. The mobilization of swelling capacity may result stability concerns and foundation threats, tunnels and slopes which required certain assumption and factors like swelling potential which need to be mobilized by applying the swelling pressure.
The factors affecting the swelling potential are known as internal factors cations present in clay minerals and properties of clay minerals and factors having significant effect on swelling potential are known as external potential properties of ions and available pore water [ 61 ]. Swelling pressure mainly depends on the average specific surface area, more surface area, more Swelling pressure mainly depends on the average specific surface area, more surface area, more force acting on the surface results in high volume change and high swelling pressure [ 62 ].
During the swelling process, deformation of crystal structure in clay minerals takes place results in change in mineral strength and hydraulic conductivity. The structure of clay minerals showing swelling capacity is represented by sheets of SiO 4 -tetrahedrons and Al OH 3 -octahedrons which are bonded by oxygen molecules and combined to each other through interlayer having free and exchangeable cations and water molecules with variable thickness and ratio [ 63 ].
The clay minerals showing more expansion belongs to group of clay minerals with tetrahedrons and octahedrons layers where cations and water molecules are stored in the interlayer spaces [ 64 ]. The difference between swelling and non-swelling clay minerals is the size of interlayer space. Greater the interlayer space, more change will be in swelling behavior. The interlayer spacing is measured using X-Ray Diffraction analysis.
The swelling clay show more interlayer spacing while non-swelling clay show less interlayer space Table 2. The mechanism of swelling of clay minerals is shown in Figure 7. The water molecules attached to the surface of clay minerals which is negatively charged results in hydration during the swelling process for most expandable clay minerals. The hydration is the first step in swelling process. The hydration process results in osmotic swelling where water molecules flow toward interlayer of clay minerals having high ion concentration as there is difference in concentration of ions between the unit layers and in the pore water [ 64 ].
The surface charge of clay minerals affects various chemical properties of clay minerals by varying the quantity of electrical and surface charge density. The surface charge properties play an important role in the formation of organo complexes of clay minerals, migration of ions, swelling and shrinkage. On the basis of difference in surface properties, clay minerals are categorized into two categories: Clay minerals having permanent negative charge: The permanent negative charge in clay minerals occurs due to the isomorphous substitution in the tetrahedral and octahedral layers.
This type of charge is also known as structural charge and is pH independent. The negative charges of clay minerals are mainly balanced by the interlayer ions present in the interlayer space of clay minerals.
The basal plane of clay minerals are permanently charged through which overall negative charge originated which is measured over pH from 2 to 12 for many clay platelets as shown in Figure 8. The edges faces are pH dependent results in anisotropy of clay particles [ 65 ].
Clay minerals having Variable charge: The charge arises due to the protonation of Si — OH group present on the edges of clay surface. This type of charge is pH dependent. It may either be positive or negative due to the protonation or deprotonation of functional group of clay minerals depending on the pH [ 65 , 66 ]. Madison: American Soc Agron, Parikh, S. Soil: The foundation of agriculture. Nature Education Knowledge 3 10 , 2 Rasiah V.
Nitrate accumulation under cropping in the Ferrosols of far north Queensland wet tropics. Australian Journal of Soil Research 39, Sparks, D. Physical chemistry of soil potassium. In Potassium in Agriculture , ed. Munson, R. Environmental Science and Technology 29, Thompson, A. Introduction to the sorption of chemical constituents in soils. Nature Education Knowledge 4 4 , 7 Soil: The Foundation of Agriculture. Sustainable Agriculture. What Are Soils? Food Safety and Food Security. Introduction to the Sorption of Chemical Constituents in Soils.
Pests and Pollinators. Soil erosion controls on biogeochemical cycling of carbon and nitrogen. The Influence of Soils on Human Health. Use and Impact of Bt Maize. Aquaculture: Challenges and Promise. Soil Carbon Storage. Soil Minerals and Plant Nutrition. Soil Water Dynamics. The Conservation of Cultivated Plants. The Soil Biota. Transgenic Animals in Agriculture. Schulze, Ph. Citation: Singh, B. Nature Education Knowledge 6 1 How do chemical reactions involving soil minerals play a crucial role in controlling the availability of essential plant nutrients?
Aa Aa Aa. Primary Minerals and Soil Fertility. Secondary Minerals and Soil Fertility. In contrast to the primary minerals, secondary minerals in soils are usually formed by low-temperature reactions during the weathering of primary minerals in the aqueous environment at the Earth's surface.
Secondary minerals primarily control nutrients through adsorption-desorption, dissolution-precipitation, and oxidation-reduction reactions. Secondary Nutrients. These cations are retained at negatively charged sites of phyllosilicates via electrostatic attraction outer-sphere complexation Figure 1.
Secondary carbonates are considered to be important scavengers of some nutrients through incorporation in the mineral structure e. Among the micronutrients, Fe, Mn, Cu, Zn, and Ni are taken up by plants in their cationic forms, and B, Mo, and Cl are taken up by plants in their anionic forms.
Fe and Mn are often present in large quantities in most soils, and adsorption reactions play little role in controlling their plant availability in soils. Oxidation and precipitation reactions predominantly control the soil solution concentration of Fe and Mn. Goethite, hematite, and ferrihydrite are the most commonly occurring secondary Fe oxides in soils. Due to the microcrystalline size of Fe oxides, these minerals possess high specific surface areas and provide numerous adsorption sites for both cationic and anionic elements in all varieties of soils.
The two most stable Fe oxides, goethite and hematite, are known to have substantial structural substitution of trace elements, including Mn, Ni, Zn, and Cu. Manganese minerals are not as abundant and common as Fe oxides. Often, they exist in soils as mineral coatings, as nodules, or as finely dispersed particles in the soil matrix. Both Fe and Mn oxides are common mineral constituents in many soils and are important substrates for the retention of many macronutrients and micronutrients.
Plant availability of both Fe and Mn is greatly reduced in calcareous soils due to the extremely low solubility of Fe and Mn oxides and of Mn carbonates. In such situations, plants induce biochemical responses, such as release of reducing and chelating compounds and acidification of rhizosphere, which can increase the availability of Fe, Mn, and other micronutrients.
Soil minerals serve as both sources and sinks of essential plant nutrients. As primary minerals that originally formed at high temperatures and pressures in igneous and metamorphic rocks are weathered in soils, they release plant nutrients into the soil solution.
New minerals form in the aqueous phase of soil environments. These secondary minerals serve as sources of nutrients themselves, or they precipitate or adsorb essential elements, keeping them from being taken up readily by plants.
In many cases, secondary minerals serve as important reservoirs where nutrients are held strongly enough to prevent leaching, yet weakly enough to allow plants to draw on them to meet their nutritional needs.
In some soils and in certain topsoils, the soil organic matter contains and releases plant nutrient elements.
References and Recommended Reading Brady, N. Goldich, S. A study in rock-weathering. Journal of Geology 46, Lapidus, D. Environmental Soil Chemistry , 2nd ed. Wilson, M. Weathering of the primary rock-forming minerals processes, products and rates.
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