Halloysite structure

Zvyagin [1967] is convinced that the halloysite structure is not merely disordered kaolinite. When the degree of disorder is great in either kaolinite or halloysite, there is little to distinguish between the two minerals, but the stacking sequence of the layers in halloysite cannot be derived from that of kaolinite. Souza Santos et al. [1965] have also commented that dried fiber bundles of halloysite from Brazil show some degree of regularity in their structures when examined by X-ray diffraction and selected-area electron diffraction. 

Halloysite Similar in composition to kaolin but of different structure... 

Halloysite-10A represents a structure with few if any interlayer cations, allowing one to investigate the relatively simple case of water interacting with a clay surface. Similarly, ice-like models have been proposed for water adsorbed on smectite and vermie-ulite surfaces (2, 12, 12). These represent cases of charged clay layers with adsorbed exchangeable cations. 

Kaolin Minerals. The 1 1 structures include a group of aluminosilicate minerals which are termed collectively the kaolin minerals specifically these are kaolinite, dickite, nacrite, and halloysite. The basic 1 1 layer for all of these minerals has the composition AlgSigOj-fOHJj, there is a small amount of substitution of iron for aluminum, ana fluoride for hydroxyl ion. All, except halloysite, are normally anhydrous and do not expand (as do the smectites) upon exposure to water and most organic molecules. As a result, they generally have a rather small surface area, on the order of 10 nr... 

Tubular fibrous morphology has also been described for a hydrated kaolin (Honjo et al., 1954), a mineral known to have a structure different from that of kaolinite or halloysite. 

Because the clays have a basic layered structure of aluminosilicate sheets the expectation was a platy habit. The creation of tubular forms was studied by Bates (1959). He postulated that the curved forms, observed by electron microscopic investigation of halloysite and chrysotile, originated through the relief of strain between sheets of unequal dimensions. 

Fig. 2.14 The structure of halloysite. Hydrogen bonded interlayer water is shown in this 10 A or 0.01 nm. form.

Kaolins. The kaolin minerals include kaolinite. dickite. and nacrite which all have composition AUOi 2 SiO 2 FLO halloysite (7 At. AGO, 2 SiO, 2 H.O and halfoysile (10 A). Al 0, 2 SiO 4 H 0 The struclural formulas for kaolinite and halloysite tIO At. which are shown in Figure I, are AbSiaOiotOH)). and AljSi.iO, i(OH) -4 H,(). respectively. The so-called fire day mineral is a h-axis disordered kaolinite halloysite (7 A) and halloysite (10 A) are disordered along both the a- and h-Mcs Indeed, most variations in the kaolin group originate as structural polymorphs, related to variations in layer slacks. 

Halloysite has a tubular morphology (Bates et al.,1949 1950) with the c axis radial. Most halloysites are highly disordered along both the a and b axes and around the circumference. More recent studies have indicated that some tubular halloysites have an appreciable degree of structural regularity (Honjo et al., 1954 Takahashi, 1958 De Souza Santos et al., 1965). 

Allophane, in any abundance, is most commonly formed from volcanic material although it can presumably form from any alumino-silicate minerals and indeed is probably present as a transitory stage in the alteration of any material to a clay mineral if any significant structural re-arrangement is required. In its most pure form it occurs as veins and incrustations. Most analyses (Table LXXII) are of samples from this type, of deposit. In volcanic soils (Japan, Australia, N.W.U.S., etc.) where allophane is abundant it is usually intimately mixed with halloysite and collection of... 

De Sousa Santos, P., De Sousa Santos, H. and Brindley, G.W., 1966. Mineralogical studies of kaolinite-halloysite clays, 4. A platy mineral with structural swelling and shrinking characteristics. Am. Mineralogist, 51 1640-1648. 

Hydrated halloysite has the same structure as kaolinite with water (single layer of H20 molecules) sandwiched between the mineral layers (10-A spacing). This mineral is commonly encountered in tropical soils... 

Particularly attractive method for preparation of synthetic zeolite is recrystallization of natural aluminosilicates, such as kaolinite (halloysite), previously formed for elimination of plastic flow of highly thixotropic, pulverized zeolite. Some additional components of initial mixtures, such as texture modifiers (hard coal, lignite, cellulose, silica, aluminum oxide) are also introduced. They enrich the structure of zeolite adsorbent in transport pores and prevent an excessive compression of the clay material during the formation process. This results in an increase in product efficiency during the crystallization of zeolite phase. 

The adsorbents have been prepared fi-om the halloysite (H) - mineral fi-om kaolinite group with an admixture of carbonaceous materials refinery waste deposits (RSI), sediment communal sewage (CSew) and cellulose (Ce), and the fiaction of these mixtures were within 30 - 70 wt.%. The mbcture of raw material was thermally (carbonaceous materials carbonization, 973 K) and hydrothermally (crystallization of the amorphous metahalloysite in alkaline solution to zeolitic structure of NaA type, 373 K) pretreated in order to cilitate their specific structure [1,2]. 

Because several spatial stacking arrangements are possible there are several kaolin minerals, each with the same chemical composition, namely Al2Si205(0H)4, but with different properties. Nacrite, dickite, kaolinite, halloysite, and livesite are well recognized species. No positive evidence has so far been published linking other trivalent cations with a single layer lattice structure, but it has been suggested that iron(iii) can replace aluminium in part in the kaolin lattice. 

The structure of halloysite is equivalent to that of kaolinite, but has a layer of water molecules between each pair of silica and alumina layers (see Chap. 9 Deer et al. 1992). Writing halloysite dissolution in the same form as in Eq. (7.35) for kaolinite, its solubility product is = 10 - (Hem et al. 1973 Stecfel and van Cappellen 1990), versus = 10" for kaolinite. In other words, halloysite is about 80 times more soluble than kaolinite. If plotted in Fig. 7.9, also assuming Si02(aq) = 17 ppm, the solubility curve for halloysite is parallel to but 1.9 log units above the curve for kaolinite at any pH. 

Because of their alternating tetrahedral (T) and octahedral (O) layers, the two-layer phyllosil-icates are said to have T 0 structures. Listed in Table 9.1 are other T 0 minerals, including the kaolinite polymorphs nacrite, dickite, and halloysite, and the trioctahedral serpentine minerals lizardite, antigorite, and chrysotile, in which brucite layers alternate with layers of silica tetrahedra. 

Individual kaolinite layers are 7 A thick, so that the stacked layers produce a repeat distance (c-axis spacing) of 7 A, a fact used to identify kaolinite in soils by x-ray diffraction. The mineral halloysite, which has the same 1 1 layer structure as kaolinite, diflFers from kaolinite in the fact that it has a single sheet of water molecules between the layers. This increases the c-spacing to 10.1 A. Mild heating of halloysite will dehydrate and collapse it irreversibly to the kaolinite 7-A spacing. 

Layer Inclusion-compounds In this type of complex formation, the layer structure of the host is made use of. Certain clays, such as montmorillonite and halloysite, have a sandwich or layer structure that will readily include organic compounds of a polar nature. Alcohols, ethers, nitriles and amines are among the guests included by these clays. The layer width may vary, depending upon the compound included. Ionic attraction and van der Waals forces are involved in this type of inclusion. 

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