TYPES OF CLAY

It is important to realise that clay can be exposed to many processes after it has been depos-ited. The periods are the Miocene Epoch (26 - 7 million years old), the Pliocene Epoch (7-2 millions years old), the Pleistocene Epoch (2 million to 15,000 years old) and the Holocene Epoch (up to approximately 15,000 years old). In The Netherlands no primary clay deposits are found, i.e. all clay is secondary, i.e. formed elsewhere and transported by wind, ice or water. 

Before elaborating on the different types of clay, it is important to clearly define the terms clay and clay body. A clay is a material with plastic properties, formed by natural forces and found in nature. Some examples are  

A clay mass is a mixture of clay and additives for a specific ceramic (processing) technique. The clay provides the plastic properties. Possible additives are a flux and a filler. Some examples of fluxes are potassium feldspar, bone meal, volcanic ashes and ground glass, all of which serve to affect the density and decrease the melting range. Fillers reduce the shrinkage as well as the sticky character of the clay. 

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Very clean sands are rare and normally variable amounts of c/ay will be contained in the reservoir pore system, the clays being the weathering products of rock constituents such as feldspars. The quantity of clay and its distribution within the reservoir exerts a major control on permeability and porosity. Figure 5.2 shows several types of clay distribution. 

Minerals are generally added to improve wear resistance at minimum cost. The most commonly used are ground limestone (whiting) and barytes, though various types of clay, finely divided siHcas, and other inexpensive or abundant inorganic materials may also perform this function. 

An interesting type of clay used widely in the manufacture of refractories is so-called flint clay, which is very hard and has very slight plasticity even when finely ground. Flint clays are essentially pure, extremely fine-grained kaolinite. In some cases the hardness appears to result from the presence of a small amount of free siUca acting as a cement, whereas in other cases it is the result of an intergrowth of extremely small kaolinite particles. 

Soil Clay is the primary construction material for settling basins and waste-treatment evaporation ponds. Since there is no single type of clay even within a given geographical area, shrinkage, porosity, absorption characteristics, and chemical resistance must be checked for each application. 

A great variety of aluminium-silicate bearing rocks, plastic when wet, hard when dry. Used in pottery, stoneware, tile, bricks, cements, fillers and abrasives. Kaolin is one type of clay. Some clay deposits may include appreciable amounts of quartz. Commercial grades of clays may contain up to 20% quartz. 

A type of clay composed of mixed silicates and used for refractories, ceramics, tile and stoneware. 

Clay type identification A plot of thorium versus potassium will indicate what type of clay is present. The thorium/potassium ratio can also be used. 

Sudo, T. (1954) Types of clay minerals closely a.s.sociated with metalliferous ores of the epithermal type. Sci. Repts. Tokyo Kyoiku Daigaku, Sen C, 3, 173-197. 

Two types of swelling may occur. Surface hydration is one type of swelling in which water molecules are adsorbed on crystal surfaces. Hydrogen bonding holds a layer of water molecules to the oxygen atoms exposed on the crystal surfaces. Subsequent layers of water molecules align to form a quasi-crystalline structure between unit layers which results in an increased c-spacing. All types of clays swell in this manner. 

There are a number of different clays, and some of the most common are listed in Table 55. The composition of each clay can be expressed by a formula that differs slightly from the general formula given above. Chemical composition alone, however, is not sufficient for characterizing clays their crystal structure provides the best way of characterizing any type of clay (see Textbox 21). In many clays, for example, the atoms are grouped in... 

More than 20 different types of clay can be actually distinguished. Those most appreciated for making ceramics, for example, kaolinite, are built up of combinations of the basic structural units described above. The particles of most consist of platelets (very small, flat sheets) that, when stacked together, form layered arrangements having extensive surface areas, much like the pages of a book. Other common clay particle shapes are fibrous or tubular. 

Kaolinite White platelets the type of clay most widely used for making ceramics... 

The type of clays used by the rubber industry are known to the clay industry as kaolin produced from deposits of a hydrous aluminium silicate, known as kaolinite. 

If such fillers are to be used, they should have a neutral or slightly alkaline pH, otherwise additives such as ethylene glycol and triethanolamine, which are preferentially adsorbed on the surface of the filler, should be used, preventing any undesirable interference reactions between the filler and the crosslinking peroxide. These additives must, however, always be added to the mix before the peroxide. With some mineral fillers, such as some types of clay, the polymer may be bound to the filler by means of silane treatment, and the surface of the filler becomes completely non-polar. Consequently, the interaction with the polymer matrix increases, while the adsorption of the crosslinking peroxide by the filler is severely suppressed. 

This technique uses a nitrogen atmosphere in grinding and flotation to retard oxidation of reactive sulphides, and has been successfully applied on carbonaceous ores from Nevada (USA). The effectiveness of the method depends on (a) the amount of carbonaceous gangue present in the ore, and (b) the amount and type of clay. Ores that are high in carbon or contain high clay content (or both) are not amenable for nitrogen atmosphere flotation. 

The presence of various types of clay slimes also has a significant effect on flotation properties of oxide copper minerals [2],... 

Fullers earth is a type of clay named after a fuller, whose job was to clean cloth, e.g. stripping wool of its grease. Fullers earth removes oils and grease from cloth because of its alkalinity, just like an oven cleaner solution. 

Methodologically, inevitably some questions must always be considered. Do the control sherds adequately represent the output of all of the relevant vessels from all of the relevant kiln sites There is always potentially the problem of kiln X - an unknown (or at least, uncharacterized) kiln which is producing wares similar to those under consideration, but not included either because it has not been identified archaeologically, or because it was not known to produce the type of ware under question. This problem should be at least considered since a geochemical knowledge of the type of clay deposits being exploited may restrict the geographical extent of the possible clay sources. The question of the representativeness of the samples is more difficult. Most kiln... 

The amount of swelling and contraction in smectites is quite dramatic. Typically, soils containing large amounts of this type of clay will develop cracks that are 30 cm wide at the surface and greater than 100 cm deep, and these cracks will allow surface material to fall into them during dry periods. This characteristic is so unique that these types of soils are given their own name. They are called Vertisols the name is taken from the concept that material from the surface falls to the bottom of the cracks, resulting in inversion of the soil. 

When a soil containing 2 1 clays becomes wet, the clays swell shut and water movement through it is extremely slow. In a soil profile, wetting and swelling of this type of clay will prevent downward movement of water and associated contaminants. For this reason, swelling clays are used to seal both landfills and ponds to prevent leaching or leaking. 

Identify the three maj or types of clays in soils and explain how they differ chemically. 

In practice, clay from a soil sample is prepared on a microscope slide, dried, and its X-ray diffraction measured subsequently, the same clay is placed in atmospheres saturated with, for example, glycerol, with subsequent X-ray diffraction, and again the distance between layers determined. Changes in the diffraction pattern or the lack thereof will identify the type of clay present [32],... 

The two types of clay mineral structures which are of interest in the present discussion are the expanding 2 1 structures (the smectites and vermiculites) and the 1 1 structures (the kaolins). 

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