Composition of Clay Constituents

Classification of clays by some rational basis into groups facilitates the convenient studies of its properties and application. Clays belonging to the same group have similarities in physical and chemical properties, and may be related to corresponding industrial and environmental applications. Chemical composition is one of the major criteria for classification of clays, along with internal structures, origin, natural occurrence etc. 

Most of the solid components of the Earth s crust, i.e. rocks, sediments, clays etc. are largely made up of various mineral species. Amineral species is deiined as A naturally occurring, inorganic, homogeneous solid, having a definite (but not necessarily fixed) chemical composition and a fixed ordered internal structure, i.e. crystalline. This ordered internal structure is reflected in the external morphology when the mineral has a well crystalline form. When the crystalline form is not perceptible externally called cryptocrystalline, its crystalline nature can be detected by scientific analytical techniques like X-Ray Difiraction. Amorphous natural solids like coal, volcanic glasses etc. do not qualify as minerals. The abimdance of any mineral in the Earth is decided by the availability of the constituent elements of the mineral in the earth s crust and also the stability of that mineral in the surface or near-surface environment. 

Silicon and oxygen being the most abundant elements of Earth crust, silicate, oxide and hydroxide class of minerals are most commonly available 

Mukherjee, The Science of Clays Applications in Industry, Engineering and Environment, 23 

Several groups with structural and chemical similarities are bunched together as mineral subclass. In silicate minerals the basic silica tetrahedral may form rings, sheets etc. forming different mineral subclasses like sorosilicates, sheet silicates etc. And the silicate subclass forms a mineral class. A particular mineral class have the same anion or anionic groups. 

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Classification and Composition of Clay Constituents 29 Table 2.7 Description of sheet silicate (some of which occurs in clay)... 

The book is divided into three parts. At the begirming of Part I, precise and unambiguous definitions of clays and their constituents are given along with clear and succinct explanations. The subsequent discussions of Part I cover the vast and diverse spheres of the science of clays that include their origin and evolution in nature their composition and internal structure their physical and chemical properties and soil mechanics. The analytical techniques for determination of clay constituents are also described in this part. 

Clays are composed of extremely fine particles of clay minerals which are layer-type aluminum siUcates containing stmctural hydroxyl groups. In some clays, iron or magnesium substitutes for aluminum in the lattice, and alkahes and alkaline earths may be essential constituents in others. Clays may also contain varying amounts of nonclay minerals such as quart2 [14808-60-7] calcite [13397-26-7] feldspar [68476-25-5] and pyrite [1309-36-0]. Clay particles generally give well-defined x-ray diffraction patterns from which the mineral composition can readily be deterrnined. 

The chemical and physical aspects of crud can dilfer for each separate operation and will vary in inorganic composition, organic content, color, and density. The composition of many cruds appears to have in common such constituents as Si, Al, Fe, P, SO4, particles of gypsum, clay, and other fine particles together with the solvent. Often there is a direct relation between the feed liquor and the crud compositions, indicating possible aqueous carryover as well as inefficient clarification before solvent extraction [33]. Various researchers have reported on the formation of crud and its characterization in their circuits [42-45]. 

In the composition of the mass, certain proportions of potter s and plastic clay, clay marl, and quartz or sand, will always bo found—these constituents varying in their nature according to tbo different localities in which they occur. Thus Paris earthonwaro consists of—... 

The fusibility of a substance is not solely Influenced by tho elements which enter into its composition, but also by tho manner in which these elements ate arranged and oombined together. The chief constituent of clay—alumina—is a base which, In combination with silica, forms one of the most refractory substances, and this property is possessed by the clays in proportion as they ore unmixed. with other bases, as alkalies, oxide of iron, lime, arid magnesia, in the order here given. In tho purer clays, which for ajl ordinary purposes may be considered fire-proof, the refractory quality is augmented in proportion to the quantity of silica they contain. 

Under the microscope, important observations include composition (the mineral and organic contents), texture (size and sorting of sediments), and especially the fabric - the geometric relationships - of the constituents. At the early Neolithic site of Catalhoyuk in Turkey, for example, micromorphology revealed that some of the house floors had been plastered with a thin coat of clay at least 50 times. These thin layers of plaster incorporated many small finds of plant remains and other evidence. 

However, fractional separation has been the basis for most asphalt composition analysis (Fig. 15.5). The separation methods that have been used divide asphalt into operationally defined fractions. Three types of asphalt separation procedures are now in use (a) chemical precipitation in which n-pentane separation of asphaltenes is followed by chemical precipitation of other fractions with sulfuric acid of increasing concentration (ASTM D-2006) (b) adsorption chromatography with a clay-gel procedure in which, after removal of the asphaltenes, the remaining constituents are separated by selective adsorption/desorption on an adsorbent (ASTM D-2007 and ASTM D-4124) and (c) size exclusion chromatography in which gel permeation chromatographic (GPC) separation of asphalt constituents occurs based on their associated sizes in dilute solutions (ASTM D-3593). 

However, it must be emphasized that there has to be some attempt to recognize the limitations of the method before any projection relating to the mineral composition of coal is possible. For example, the high temperature required for the ashing may result in the loss of the volatile constituents of the minerals or the mineral constituents will undergo a chemical change. In the former case, certain of the mineral elements will escape detection while in the latter case the constituents of clays or shale (to cite an example) will lose water of hydration or the carbonate minerals will lose carbon dioxide and the oxides so produced may even undergo further reaction with sulfur oxides or with silica to produce completely different mineral species ... 

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