However, very small amounts of isomorphous substitution occur: most commonly Fe 3+ for Al 3+ (up to 1:30) and rarely Al 3+ for Si 4+ and Fe 2+ for Al 3+. The kaolinite structural formula is electrically neutral. Halloysite is a hydrated 1:1 structure and occurs in nature as spheres and tubes the latter are typically 30 nm in diameter and 0.5–10 µm in length (Brigatti et al. Dickite and nacrite are less common polymorphs in the kaolin group. The ideal formula of the pseudohexagonal unit cell is Si 2Al 2O 5(OH) 4. Kaolinite is the most common kaolin-group mineral, and it occurs in nature as particles composed of stacks of several layers. There are three important consequences of this arrangement: (1) each kaolinite layer is a strong dipole (2) the kaolinite layers are strongly held together by H-bonding and by dipolar interaction and (3) the siloxane surface is hydrophobic, while the aluminol surface is hydrophilic. This surface is called the aluminol surface. The basal surface on the opposite side of this sheet is composed of OH groups bonded to Al (FIG. One basal surface of the layer is composed of oxygen atoms bonded to Si and is called the siloxane surface. The tetrahedal and octahedral sheets share a layer of oxygen atoms (FIG. ![]() The notation 1:1 means that one layer of kaolinite is composed of a tetrahedral silica sheet and an octahedral alumina sheet. This paper gives an account of how kaolin-group minerals can be modified and nanocomposited with other materials to develop new materials. One might expect that kaolinite–polymer nanocomposites will be developed that can compete with smectite–polymer nanocomposites in specific applications. However, in recent years our knowledge of the intercalation of kaolinite has increased significantly. Intercalation, the reversible insertion of a molecule or ion into layered compounds, is difficult in the case of kaolinite because the H-bonding between the layers has to be broken up. Kaolinite in contrast has a negligibly small cation-exchange capacity. Much work has been done on the smectite-group minerals, or “swelling clays,” because cationic molecules can be easily intercalated via the ion-exchange reaction. Common impurities, such as quartz, micas, illite, montmorillonite, goethite, hematite, anatase, and rutile, can adversely affect the use of kaolins and commonly must be removed.īeyond the traditional uses of kaolins, much effort has been devoted to the synthesis of clay–polymer nanocomposites and the development of their applications (Ruiz-Hitzky and Van Meerbeek 2006). It is advantageous that kaolinite readily disperses in water at high solid contents-up to 70%. For most industrial applications, mined kaolinite has to be refined and processed to achieve the desired properties, such as viscosity, whiteness, purity, and crystal size and shape (Kogel 2014 this issue ). INTRODUCTIONĬrystalline order/disorder and layer stacking are the properties most important in determining the use of kaolin-group minerals in industrial materials. Hydrochloric acid cleans clay minerals by removing free iron oxide from the surface acetic acid is less effective.KEYWORDS: Kaolinite, intercalation, grafting, exfoliation, delamination, nanocomposites, nanohybrids. Acetic acid is preferred to hydrochloric acid for this purpose. ![]() These experiments show that treatment in dilute acids has no harmful effect in the preparation of clays for X-ray diffraction. Sodium hydroxide attacked the halloysite structure, as shown by chemical analysis and X-ray diffraction. Acid treatment did not destroy the structure of the clays, but the halloysite structure was partially destroyed. The samples most strongly attacked by HCl and NaOH were examined by X-ray diffraction. Hydrochloric acid removed iron oxide coatings from soil clay minerals, but acetic acid did not remove them completely. Halloysite was more strongly attacked by hydrochloric acid than was any of the other experimental minerals. ![]() ![]() Sodium hydroxide attacked the kaolin group minerals more strongly than it did montmorillonite, metabentonite, or illite. All the solutions removed some SiO 2, Al 2O 3, and Fe 2O 3 from the samples, but the quantities were small. The supernatant solutions were removed from the clay minerals and analyzed for SiO 2, Al 2O 3, CaO, MgO, Na 2O, and K 2O. One-g samples of a montmorillonite, a metabentonite, an illite, two kaolinites, and three halloysites were treated with 50 ml of hydrochloric acid (6⋅45 N, 1:1), acetic acid (4⋅5 N, 1:3), sodium hydroxide (2⋅8 N), sodium chloride solution (pH 6⋅10 Na = 35‰ Cl = 21⋅5‰), and natural sea water (pH 7⋅85 Na = 35⋅5‰ Cl = 21⋅ 5‰) for a 10-day period in stoppered plastic vials.
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