Imogolite structure

Figure 7 Imogolite structure viewed down tube axis. (From Ref. 39.)...

The microporosity of a new tubular silicatelayered silicate nanocomposite formed by the intercalation of imogolite in Na -montmorillonite has been characterized by nitrogen and m-xylene adsorption. The nitrogen adsorption data yielded liquid micropore volume of -0.20 cm g as determined by both the t-plot and the Dubinin-Radusikevich methods. The t-plot provided evidence for a bimodal pore structure which we attributed to intratube and intertube adsorption environments. The m-xylene adsorption data indicated a much smaller liquid pore volume (-0.11 cm g ), most likely due to incomplete filling of intratubular pores by the planar adsorbate. The FTIR spectrum of pyridine adsorbed on the TSLS complex established the presence of both Bronsted and Lewis acid sites. The TSLS complex was shown to be active for the acid-catalyzed dealkylation of cumene at 350 C, but the complex was less reactive than a conventional alumina pillared montmorillonite. 

Imogolite is a naturally occurring aluminosilicate mineral with a unique tunnel-like or tubular structure. The... 

We have been investigating the use of imogolite as a pillaring agent for smectite clays with layer lattice structures ". The regular intercalation of the tubes within the layered host results in the formation of a tubular silicate-layered silicate (TSLS) complex. These new nanocomposite materials may be viewed as pillared clays in which the pillars themselves are microporous. Significantly, the TSLS structure is thermally stable up to 450 C when montmorillonite is selected as the layered host . 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

Fig. 4. Schematic view of part of an imogolite cylinder formed from twelve gibbsite structural type units, each corresponding to (Al203-Si02 2H202)2) and indicated by dotted lines (Reprinted from [29a], copyright (1986) from John Wiley and Sons)...

Ildefonse Ph, Kirkpatrick RJ, Montez B, Calas G, Flank A-M, Lagarde P (1994) 27Al MAS NMR and aluminum X-ray absorption near edge structure study of imogolite and allophanes. Clays Clay Minerals 42 276-287... 

A more pronounced structural disorder often exists in freshly precipitated silica or metal hydroxides in soils, since these compounds typically are amorphous. Structurally disordered aluminosilicates, known collectively as allophane and imogolite, are common in the clay fractions of soils formed on volcanic ash parent material. ... 

Figure 2. Cross sectional view of the structure of imogolite. The tube walls consist of five tiers of atoms. The composition of the tiers as read from the inner to outer tier is (HO) Si O3 AI2 (OH>3.

Figure 3. Structure of a TSLS complex formed by intercalation of an imogolite monolayer in the galleries of Na" -montmorillonite.

Figure 1-2. Cross-sectional view of the structure of imogolite. The tube walls consist of five...

Fig. 47 The left part shows the structure of two imogolite nanotubes [i.e., a) the (12,0) zigzag one and b) the (8,8) armchair one]. The left part shows the variation in the total energy as a function of tube radius for (filled circles) zigzag and (open circles) armchair nanotubes. Reproduced with permission of American Chemical Society from ref. 143. 

---