Illite Cation exchange capacity

The cation exchange capacity (C.E.C.) of the illite minerals is reported to range from 10 to 40 mequiv./lOOg however, illites that afford values larger than 10—15 mequiv./lOOg usually contain some expandable layers. Ormsby and Sand (1954) showed that a good linear relation exists between C.E.C. and percent expandable layers in illites and mixed-layer illite-montmorillonites. They concluded that illite with all layers contracted would have a C.E.C. of 15 mequiv./lOOg. 

Kahr, G., and F. T. Madsen. 1995. Determination of cation exchange capacity and surface area of bentonite, illite, and kaolinite by methylene blue adsorption. Appl. Clay Sci. 

The similar structure of illite and smectite allows mixing or interstratification of 2 1 units to form mixed-layer clays. Most illites and smectites are interstrati-fied to a small degree, but they are not classified as such until detectable by X-ray diffraction. As one might expect, illite-smectite mixed-layer clays have intermediate cation exchange capacity between the end-member compositions. 

Aggregation of dissolved humic substances can also occur with particulate materials in the estuarine water column. Preston and Riley (1982) showed that the adsorption of riverine humic substances onto kaolinite, montmorillonite, and illite increased with increasing salinity and dissolved humic substance concentration. Adsorption increased in the order kaolinite < illite < montmorillonite, which they ascribed to increasing cation-exchange capacity of the clays. They found considerable quantitative differences between the extent of adsorption of riverine versus extracted sedimentary humic substances, indicating the importance of using materials of proper origin in experiments of this type. 

The cation exchange capacity of clays results from lattice imperfections or defects, isomorphous substitutions, and/or broken bonds on clay particle surfaces. Explain how the CEC s of kaolinite, the smectites, and illite, and their variation with pH, reflect these sources of their surface charge. 

Layer charge in montmorillonites is the result of substitution of lower valence cations (e.g., Mg for in the octahedral site Al for Si" in the tetrahedral site). The negative layer-charge is balanced by mono- and divalent cations between the layers (Na Ca ). Montmorillonites are stable phases within a limited range of presswe and temperature. Burial to depths greater than 4 km ( 1 kilobar) at temperatures of 75-100 C transforms the montmorillonites to illites 41) that have low cation exchange capacities and poor catalytic activity. 

Chemical Properties. An important chemical property of clays, which directly affects fines migration is the cation exchange capacity (CEC) (6-9). CEC is a measure of the capacity of a clay to exchange cations. It is usually reported in units of milliequivalents per 100 g of clay (meq/100 g). The CEC depends on the concentration of exchangeable cations in the diffuse Gouy-Chapman layer (see later). This concentration depends on the total particle charge, which may vary with pH. Unless stated otherwise, the reported values of CEC are measured at neutral pH. CEC values (meq/lOOg) of common clay minerals are as follows smectites, 80-150 vermiculites, 120-200 illites, 10-40 kaolinite, 1-10 and chlorite, <10 (10). 

Representative data for soil minerals are presented in Table 2.Ill and 2.IV, taken from the publication of Greenland and Quirk (1962). Montmorillonite has the highest surface area, ranging up to 800 m /g. This means that a 10-g sample has an area of approximately two acres or four-fifths of a hectare. This is certainly an impressive value. Kaolinites have surface areas ranging between 10 and 40 m /g, whereas illites have values intermediate between montmorillonite and kaolin. Cation exchange capacities tend to vary directly with surface areas. Of course the importance of the surface area depends upon the activity of the surface, particularly toward water. 

K = kandites, / = illites, S = smectites. Capitals denote major components. Cation exchange capacity. 

Illite [(K,H30)(AlMg,Fe) (Si,Al)p ((0H), (Hp))] is a non-expanding micaceous mineral. It occurs as aggregates of small monoclinic grey to white crystals. It is layered alumina-silicate consisting of poorly hydrated potassium Cation, which is responsible for the poor swelling behavior. Its structure consists of repetition of tetrahedron- octahedron - tetrahedron (TOT) layers. The cation exchange capacity (CEC) of illite is comparatively better than kaolinite (20-30 meq/100 g). 

Clay materials have residual electrostatic forces that attract cations and anions. This is measured as a cation exchange capacity in miUiequivalents per 100 grams. Grim (1962) stated that Kaolinites have an exchange capacity of 3-15 mUliequivalent per 100 grams, whereas illites rated higher at 10-40, and montmorillonites at 80-150. 

There are inconsistencies in the model for the calculation of activity products for the "clays. Exchangeable cations are disregarded for the low exchange capacity kaolinite, halloysite, chlorite, and moderate capacity illite. For certain expansible layer silicates and two zeolites, the logjo of the activity of selected cations is added into the sum of the activity products. 

Smectite clay catalysts are potential alternative adsorbents, although some modifications of the natural mineral are necessary. Interlayer sites in smectite dehydrate at temperatures above 200°C, collapsing to an illitic structure. Since the ion-exchange capacity of smectite centres on the interlayer site, collapse must be prevented if clay catalysts are to be used in thermal treatments of chemical organic toxins. The intercalation of thermally stable cations, which act as molecular props or pillars, is one... 

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