Minerals kaolinite

Of special interest to intercalation studies are complex non-stoichiometric systems, such as the so-called misfit layer chalcogenides that were first synthesized in the 1960s [45]. Typically, the misfit compounds present an asymmetry along the c-axis, evidencing an inclination of the unit cell in this direction, due to lattice mismatch in, say, the -axis therefore these solids prefer to fold and/or adopt a hollow-fiber structure, crystallizing in either platelet form or as hollow whiskers. One of the first studied examples of such a misfit compound has been the kaolinite mineral. 

Kaolin clays are naturally occuring sedimentary deposits composed largely of kaolinite mineral. Typical impurities in these deposits are iron oxides, titanifer-ous minerals, silica, feldspar, mica, sulfides and organic matter. The majority of kaolin clay produced in the world is used in the paper industry as coating and filler materials. This mineral also makes an excellent filler, carrier, opacifier and diluent in a variety of industrial products such as paints, plastics, cement, rubber, pharmaceuticals, etc. 

The 1 1 clay-mineral type consists of one tetrahedral sheet and one octahedral sheet. These two sheets are approximately 7 A thick. This two-sheet type is divided into kaolinite (dioctahedral) and serpentine (trioctahedral) groups. The kaolinite minerals are all pure hydrous aluminum silicates. The different members are characterized by the manner of stacking of the basic 7 A layers (Brindley, 1961b). 

Hinckley believes the differences between the two types of clays is largely a function of the environment of deposition and post-depositional leaching. The poorly crystallized hard clays were believed to have been deposited in a marine environment where face to face flocculation occurred. The well-crystallized soft clays were presumably flocculated edge to face in a fresh-water environment. The more porous soft clays were more thoroughly leached and re-crystallized than the less porous hard clays. Like other kaolinite studies this one does not provide any specific information about the actual chemical composition of the kaolinite minerals. 

Fire clays, ball clays, flint clays are kaolinite-rich clays, usually of the 6-axis disordered variety, which contain a relatively high impurity content. Illite, montmoril-lonite, diaspore, boehmite, quartz, and organic material are the minerals usually associated with these deposits. Few, if any, of the kaolinite minerals in these clays have been concentrated enough to afford meaningful chemical data. 

One of the major differences in the reported chemical composition of the kaolinite minerals is in the H20+and H20 values. In part, these variations may be real but many must be due to the presence of halloysite and other impurities, variation in grain size and surface area, and in the methods of dehydration. H20 increases linearly with increase in surface area and with decrease in grain size. 

The cation exchange capacity of the kaolinite minerals is relatively low but due to... 

The Si02 /Al2 03 ratios for the various kaolinite minerals suggest that most of the variations are a function of impurities. Ninety percent of the analyses show a ratio less than 2 1 suggesting that an excess of A1203 is much more common than an excess of... 

Keller et al. (1971) reported on the occurrence of halloysite formed by the action of hot spring waters on rhyolitic volcanic rock in Michoacan, Mexico and suggested that high concentrations of Si and Al in solution, low pH (about 3.5) and sulfate as the solvent anion allows the formation of halloysite rather than other kaolinite minerals. 

A number of copper-bearing halloysites have been reported associated with copper deposits (Chukhrov et al 1970). They believe Cu, equivalent to as much as 7—8% CuO, can occur in the octahedral sheet. When more than this is present, it occurs as chrysocolla, in part in concordant orientation with the halloysite. It would appear that the poorly crystallized kaolinite minerals will allow a much broader range of isomor-phous substitution than the better crystallized members. 

Dehydrated halloysites have C.E.C. in the range of 6—10 mequiv./lOO g (Van der Marel, 1958 Garrett and Walker, 1959). Garrett and Walker have shown that the exchangable cations are located on the external surfaces of the crystals and not in the interlayer position of halloysite. Until it is possible to obtain accurate chemical analyses of the kaolinite minerals, it will be difficult to determine their exchange capacity and the source of the charge. 

The Si02/Al203 ratios vary from 84 100 (Ross and Kerr, 1934, report a low value of 74 100) to 197 100. Most values are considerably lower than the theoretical value 200 100 of the kaolinite minerals. It appears that when silica and alumina precipitate, alumina is usually in excess of that required for a stable 1 1 structure. 

The experimental studies of interactions of organic molecules adsorbed or intercalated in the interlayer space of clay minerals are very extensive. These experimental investigations were reviewed in several monographs [15-17]. In this part, we will review only experimental studies concerning systems of kaolinitic minerals (specifically dickite and kaolinite) with formamide (FA), N-methylformamide (MFA) and dimethylsulfoxide (DMSO) since theoretical studies of interactions of small organic molecules with clay minerals are devoted to intercalates and adsorbates of kaolinitic minerals with these organic molecules. This will allow to compare theoretical results with available experimental data. 

The kaolinite mineral species studied are kaolinite, kaolinite d (disordered kaolinite), dickite and nacrite. These polytypes have been described by Bailey (1963) on the basis of sense and degree of displacement of 1 1 layers and the position of vacant octahedral positions in the layer sequence. For the hydrated kaolinitic minerals, we have used the terminology of Keller and Johns (1976) which is based on endellite as the completely hydrated species and halloysite as the partly or completely dehydrated species. The polytypes of chlorite have been described by Bailey and Brown (1962) and Hayes (1970). In Fig. 8.3b it is shown that montmorillonite, the mixed-layer clays and illite are located between pyrophyllite without interfoliar charge and the dioctahedral... 

In this sequence the authors ignored halloysite/endellite, important to dominant kaolinite minerals resulting from the alteration of feldspars in recent sediments. We propose the following diagenetic sequence ... 

The clay minerals are distinguished by their size (<2-3 pm) and extremely active surface chemistry. A few clay minerals, such as micas, do occur in silt and larger sizes. The most common clay mineral groups are those of kaolinite, smectite, and illite. Kaolinitic minerals are characterized by a rigidly held, hydrogen-bonded, nonexpansive, double-layer... 

It had recently been shown that thermal disruption of the kaolinite mineral layer structure produced a highly reactive disordered aluminosilicate with a silica to alumina molar ratio of two. Higher calcination temperatures produced what is known as mullitized kaolin, which also contained some reactive free silica (86). This served to supply the additional silica needed for the synthesis of zeolite Y, which has a silica to alumina molar ratio in the 4.5 to 5.0 range. 

Fig. 21.3 Two views of the optimized structure and maps of electrostatic potential of thymine adsorbed on hydrated tetrahedral and octahedral surface of kaolinite mineral (a — K(3t)NaW-TH, b - K(3o)NaW-TH) obtained at the B3LYP/6-31G(d) level of theory...

Kalsilite. K20.Al203.2Si02 together with a small amount of Na20. This mineral is sometimes formed when alkali vapour attacks fireclay refractories. Kandife. Group name for the kaolinite minerals, i.e. kaolinite, nacrite, dickite and halloysite. (Clay Minerals Bull., 2, 294, 1955.)... 

Jain, V., Singh, S. S., Pradip, and Waghmare, U. V. 2011. A first-principles study of the adsorption of starch on hematite and kaolinite mineral surfaces. Unpublished results. 

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