Chapter 16
Formation Mechanisms of Tubular Structure of Halloysite J. Niu* School of Materials Science and Engineering, China University of Mining and Technology, Xuzhou, PR China * Corresponding author: e-mail:
[email protected]
16.1 INTRODUCTION As a mineral from the kaolin group, most halloysites show a unique tubular morphology, similar to carbon nanotubes, which makes it important in many applications. For this reason, a great deal of attention was paid to the rolling mechanism leading to tubular halloysite. The proper explanation of the rolling mechanism will help in understanding the microstructure and geological origin of halloysite, further providing a basis for the controllable preparation and functional modification of halloysite. Moreover, the investigation on the rolling mechanism of kaolinite is helpful in revealing certain scientific propositions, such as the evolution of the environment and the origin of life (Cleaves et al., 2012; Zhou and Keeling, 2013). So far, three rolling mechanisms of the kaolinite layer leading to tubular halloysite have been proposed: (i) the mismatch between the octahedral sheet and the tetrahedral sheet, by Pauling (1930); (ii) the attraction between interlayer hydroxyl groups in octahedrons, by Radoslovich (1963); and (iii) the surface tension of water, by Hope and Kittrick (1964). Besides tubular morphology (Honjo and Mihama, 1954), halloysite holds spherical (Birrell et al., 1955), fibre (de Souza Santos et al., 1965), crumpled lamellar (Wada and Mizota, 1982), cagelike ( Jeong, 2000) and flat morphologies (Kunze and Bradley, 1964). The prevailing view is that the morphology of halloysite may be related to the content of impure elements, such as Fe and Ti (Singer et al., 2004), which causes a change in matching between tetrahedrons and octahedrons, affecting the degree of rolling in the layer (Churchman and Lowe, 2012). For most halloysites, the Al/Si atom ratio is greater than unity, which may suggest some instances of Al for Si substitution (Merino et al., 1989). The effects of octahedral and tetrahedral substitution on rolling in Developments in Clay Science, Vol. 7. http://dx.doi.org/10.1016/B978-0-08-100293-3.00016-9 © 2016 Elsevier Ltd. All rights reserved.
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the unit layer will be described in this chapter, although the evidence for tetrahedral substitution is not unequivocal ( Joussein et al., 2005). Halloysite has versatile features, including large specific surface area (SSA), high porosity and tunable surface chemistry (Li et al., 2013), but its reserves are far less than those of another clay mineral with a similar layer structuredkaolinite (Costanzo et al., 1982). For this reason, efforts have been made over the last few decades to synthesize nanotubes from kaolinite (Singh and Mackinnon, 1996; Gardolinski and Lagaly, 2005; Matusik et al., 2009; Kuroda et al., 2011; Yuan et al., 2013). These experiments reproduce the rolling process, providing some direct evidence of the rolling mechanism of the kaolinite layer in halloysite through modern measurement techniques. Some attempts to directly observe the rolling process of the kaolinite layer in synthesis nanotubes are outlined in this chapter. Furthermore, because different natural kaolinite or halloysite samples may come from different geological periods or transformation stages, measurements of natural samples have attracted some attention, and that development will be described in this chapter as well.
16.2 FORMATION MECHANISMS OF THE TUBULAR STRUCTURE OF HALLOYSITE 16.2.1 Mismatch Between Octahedral and Tetrahedral Sheets When Pauling (1930) proposed the first kaolinite structure, he pointed out that a kaolinite layer would have a tendency to curve due to a mismatch in two faces of the constituent layer. Here, the denoted two faces are a silicon–oxygen tetrahedral sheet (or simply a tetrahedral sheet) and an aluminium–oxygen octahedral sheet (or simply an octahedral sheet). The very close approximation in dimensions of the octahedral sheet in hydrargillite (b ¼ 0.507 nm and a ¼ 0.865 nm), as well as the complete tetrahedral sheet in b-tridymite (or b-cristobalite) (a ¼ 0.503 nm and b ¼ 0.871 nm) (Pauling, 1930), show that an unsymmetrical layer composed of these two would have a slight tendency to curve that could be overcome by forces occurring between layers. According to Bates et al. (1950), when water molecules occur between the 1:1 layers, it weakens the interlayer force; therefore, the octahedral and tetrahedral sheets are free to approach their normal dimensions, which leads to a curvature of the kaolinite layer. These authors calculated the inner diameter of the resulting cylinder, which had the same order of magnitude as those of the smallest tubes that they observed. Currently, the viewpoint regarding the mismatch between sheets has been widely recognized; however, some controversy remains focused on further structural details, such as how tetrahedral rotation and curving play a role in relieving stress, how to establish contact between the microstructure and macroscopic curling morphology and the role of water, all of which will be summed up in the following sections.
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16.2.1.1 Tetrahedral Rotation and Tetrahedral Curving After Newnham and Brindley (1956) found that the oversized silica layer was compressed by rotations of tetrahedrons when evaluating the structure of dickite, the structural disorder of kaolin minerals began to attract attention. Singh (1996) suggested that tubular halloysite formed by means of a tetrahedral curving mechanism rather than a tetrahedral rotation mechanism to correct the mismatch between the octahedral and tetrahedral sheets. Tetrahedral rotation reduces the lateral dimension of the tetrahedral sheet by shrinking the distances to an equal amount for basal oxygen, Si and apical oxygen in all the directions of the a–b plane; on the other hand, in the tetrahedral curving mechanism, the constriction occurs only along the rolled axis, and the apical oxygen could generate a greater constriction than the Si and basal oxygen, due to being relatively more inside the curvature (Fig. 16.1). Both of the dimensions of tetrahedral and octahedral sheets correspond to the equilibrium between three different kinds of forces: cation–cation repulsion, anion–anion repulsion and cation–anion bonds. Cation–cation repulsion is the most influential force in causing individual departure from ideal structures because oxygen atoms only partially shield the cations from each other (Radoslovich, 1963; Bailey, 1980). Based on this finding, Singh (1996) thought that cation–cation repulsion played a major role in the determination of cell dimensions, stability and stacking of layer silicates. The Si–Si repulsion caused by tetrahedral curving and rotation in order to correct a given amount of mismatch was determined using Coulomb’s law for the quantitative comparison of the two mechanisms: F¼
A
q q0 r2
(16.1)
B a c
b
b a
FIG. 16.1 Tetrahedral (A) rotation and (B) curving. Reproduced with permission from Singh (1996). Copyright (1996) from The Clay Minerals Society, publisher of Clays and Clay Minerals.
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where F is the repulsion force, q and q0 are the charges in electrostatic units and r is the distance between charges. The relative increase in repulsion can then be calculated from the relation r 2 h Frel ¼ (16.2) r where Frel is the relative repulsion, rh is the Si–Si distance in a planar hexagonal tetrahedral sheet and r is the Si–Si distance in a curved or ditrigonal tetrahedral sheet. The value of r for the hexagonal planar and ditrigonal sheets can be calculated from the b dimension using the relation: r ¼ b/3, where b is the cell dimension. In order to calculate r for a curved tetrahedral sheet, the b dimension of a planar tetrahedral sheet (btet) can be determined from btet ¼ k(R + t), where R is the inner radius of the curvature of the 1:1 layer and t is the thickness of the 1:1 layer; and the b dimension of an octahedral sheet (boct) can be determined from boct ¼ kR, where k is the angle of curvature in radians. Then R can be given by R¼
boct t btet boct
(16.3)
The radius for the Si atom plane (RSi) is RSi ¼ R + t h
(16.4)
where h is the distance between the Si atoms and the outer periphery of the tube with a value of 0.615 Å (Bates et al., 1950). Applying this procedure determined the Si–Si distances for hexagonal, ditrigonal and curved tetrahedral sheets, as well as Coulomb repulsion before and after deformation. The results showed that the curving mechanism resulted in only 1% increase in the Coulomb repulsion between adjacent Si atoms, whereas tetrahedral rotation encountered 12 times greater repulsion in comparison to the curving mechanism to correct the same amount of mismatch. Without interlayer water, the hydrogen bond between adjacent layers is strong, requiring the structure to correct the alignment between the basal oxygen and the outer hydroxyl (OH) groups, which leads to a shorter hydrogen bond (Bailey, 1988). This hydrogen bond provides an additional driving force for the tetrahedral rotation to correct the mismatch on both sides of the tetrahedral sheet, and therefore kaolinite cannot show a curved morphology. With interlayer water, Bailey (1990) pointed out that the water could minimize the interaction between adjacent layer surfaces, and tetrahedral rotation was blocked by the dynamic disorder of hydrogen bonds from H2O molecules, so basal oxygen could not all rotate in the same direction and ‘hole water’ or exchangeable cations might be inserted into ring openings. In other words, interlayer water plays a dual role in the rolling of halloysite: (i) to block
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tetrahedral rotation and (ii) to relax interlayer hydrogen bonding. As a result, the hydrogen bond between adjacent layers will be greatly weakened, the basal oxygen side will contract at a lower amount than does the apical oxygen side and a rolling of the kaolinite layer will occur. It is noted that the mismatch also could be corrected by a combination of tetrahedral curving and rotation, and the ratio of two mechanisms could vary with the tube radius. Tetrahedral curving and rotation mechanisms provide a generally reasonable explanation for the rolling/curving/curling/bending of kaolinite layers; however, the relationship between tetrahedral curving and macrorolling orientations of the layers is still unclear. This requires further study of the spatial distribution of tetrahedral curving in kaolinite layers, which will be discussed further in the next section.
16.2.1.2 How Water Molecules Enter the Interlayer Space As stated earlier in this chapter, it is generally accepted that water molecules play an important role in halloysite rolling, but there is still some controversy over how water molecules enter the interlayer space. Bailey (1990) attributed the introduction of water and hydrated cations into the interlayer space to the substitution of Si by Al, producing a net negative charge and providing a driving force. However, using 27A1 nuclear magnetic resonance (NMR) spectroscopy, Newman et al. (1994) found the four coordinated Al (AlIV) contents of two kaolinite and six halloysite samples were all
