Clay minerals such as kaolinite, smectite, chlorite, micas are main components of raw materials of clay and formed in presence of water. A large number of clays used to form the different structure which completely depends on their mining source. They are known as hydrous phyllosilicate having silica, alumina and water with variable amount of inorganic ions like Mg2+, Na+, Ca2+ which are found either in interlayer space or on the planetary surface. Clay minerals are described by presence of two-dimensional sheets, tetrahedral (SiO4) and octahedral (Al2O3). There are different clay minerals which are categorized based on presence of tetrahedral and octahedral layer in their structure like kaolinite (1:1 of tetrahedral and octahedral layers), smectite group of clay minerals (2:1 of tetrahedral and octahedral layers) and chlorite (2:1:1 of tetrahedral, octahedral and octahedral layers). The particle size of clay minerals is Clay mineralscation exchange capacityswelling capacityadsorptiontetrahedral
Neeraj KumariSBAS, K. R. Mangalam University, Gurugram, IndiaChandra Mohan*SBAS, K. R. Mangalam University, Gurugram, India
*Address all correspondence to: chandra.mohan
Clay and Clay MineralsEdited by Gustavo Morari Do Nascimento
Clay and Clay Minerals
Prof. Gustavo Morari Do Nascimento
Georgius Agricola (1494–1555), the founder of geology, was seemingly the first who gave the definition of clay in 1546. It has been modified several times due to which the clay definition raises the questions related of constituents of clay and implicitly which was very important <1>. The latest effort to solve all these issues was done by the Joint Nomenclature Committees (JNCs) of the Association Internationale pour l’Etude des Argiles (AIPEA) and the Clay Minerals Society (CMS). According to these societies, clay, a naturally occurring material, composed mainly of fine-grained minerals, become plastic in presence of water and become hard when dried or fired. By this definition of clay, engineered clays and clay-like materials can be distinguished as clay (fine grained minerals) exhibiting plasticity in presence of water and become hard on drying and firing <2, 3>.
1.2 Clay minerals
Clay minerals are the characteristics minerals on the earth found near planetary surface (the surface where the outer crust of the object comes in contact with atmosphere) environment with variable amount of ions like iron, magnesium, alkali metals, alkaline earth metals and other cations. They are considered as important constituents of soil and form by diagenetic and hydrothermal alteration of rocks in presence of water <9>. They are commonly found in fine grained sedimentary rocks such as shale, mudstone and siltstone. Clay minerals act as “chemical sponges” as they have capacity to hold water and dissolved plant nutrients eroded from other minerals due to the presence of some unbalanced electrical charge on their surface <8>. As water is essential for clay minerals formation, therefore, most of the clay minerals are known as hydrous alumino silicate or hydrous aluminum phyllosilicate.
The formation of clay minerals is due to the chemical weathering of rock <9, 10>. The chemical and structural composition of clay minerals is found to be similar to the primary minerals which originate from the crust of earth mainly from igneous or metamorphic rocks. Transformations may occur in ambient conditions. Although some of the most resistant primary minerals such as quartz, micas and feldspar may remain in soils whereas other less resistant primary minerals (pyroxenes, amphiboles) are susceptible to breakdown by weathering, thus forming secondary minerals. The resultant secondary minerals are the formed due to either modification of the primary mineral structure (incongruent reaction) or neoformation through precipitation or recrystallization of dissolved constituents of primary minerals into a more stable structure (congruent reaction). These secondary minerals are most probably defined as phyllosilicates because, as the name suggest (Greek: phyllon, leaf), they exhibit a platy or flaky structure with irregular edges; while one of their most important basic structural units is an extended SiO4 tetrahedra sheet <11>.
As the clay minerals are most important component of the soil, they are usually ultra-fined particles having less than 2μm sized particles. Clay minerals are found to be the most interesting class of minerals that have attracted substantial worldwide attention and investment in research and development. In 1930, the nature of clay can be defined with advanced development in X-Ray diffraction technology used to investigate the molecular nature of clay particles.
Most of the chemical and physical properties of the soil including swelling – shrinking capacity, cation exchange capacity etc. are due to presence of the clay minerals in soil. Clay minerals are look like micas due to their chemical composition <12>.
2.1 Tetrahedral sheet
The main dominating atom in the tetrahedral sheet is found in form of Si4+ cation. The basic building block of tetrahedral sheet is a unit of Si atom surrounded by four oxygen atom known as silica tetrahedra. The tetrahedral sheet is formed by sharing of three oxygen of each tetrahedra with three nearest tetrahedra as shown in Figure 2. These oxygen atoms are known as basal oxygen which connect pairs of all tetrahedra together (more or less) in one plane whereas the fourth oxygen atom remain free and form the bond with other polyhedral elements known as apical oxygen. Apical oxygens are all in a separate plane and provide a link between both tetrahedral and the octahedral sheet <15>. As only one apical O is present per tetrahedron therefore, each tetrahedron shares a corner with an octahedron in the octahedral sheet.
Cation exchange capacity and specific surface area of different clay minerals.
3.1 Layer silicates
A silicate comprising of planar octahedral layer bound to tetrahedral layer above and below with a distinctive repeating distance between t-o-t layers. 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:1:1 type of clay mineral
2:1 type of clay mineral
2:1:1 type of clay mineral
3.1.2 2:1 Type of clay mineral
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. Therefore, they are referred as 2:1 layer silicates in which Talc
3.2 Chain silicates
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 (Fe2O3), 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 (Al2O3) is a sesquioxide. Many sesquioxides contain the metal atom having +3 oxidation state and the oxide ion such as Al2O3, La2O3 except the alkali metal sesquioxides which contain both peroxide, (O22−) and superoxide, (O2−) ions, e.g., Rb2O3 is formulated <(Rb+)4(O22−)(O2−)2> <45>. 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.
Imogoliteis an aluminosilicate having the composition of SiO2·Al2O3·2.5H2O. In 1962, this mineral was discovered in a soil obtained from glassy volcanic ash known as “imogo.” Electron-optical observations of imogolite suggest a unique morphological feature with smooth and curved threadlike tubes differing in diameter from 10 to 30 nanometers which can further extend upto several micrometers in length. 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 SiO4 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 6.4Å and 21.4Å, respectively, and with all hydroxyls exposed at the surface.
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 1:1 layer minerals, allophane and imogolite, which are caused by the dissociation of surface hydroxyls of these minerals.
The method used to determine the CEC mainly include the complete exchange of cations by a cationic species like ammonium, Na, K, Co(III) hexamine complex, Ba and Cu(II)ethylene diamine complex. 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>. Some other complexes like an Ag thiourea complex, Co(III) hexamine complex, and Cu(II) ethylenediamine complex <55, 56, 57>. 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>.
Basal spacing of different types of clay minerals on the basis of swelling potential.