Gibbsite sheet

The silicate sheet in kaolinite, for example, has an 0-0 repeat distance in the sheet of 0.893 nm, whereas the octahedral or gibbsite sheet repeat is smaller, about 0.862 nm. Mismatch of the 1 1 sheets induces curvature with the smaller dimension sheet on the interior. The octahedral gibbsite layer in clays is postulated to be situated on the inside of the curve. This relationship contrasts with the hypothesis for chrysotile, in which the tetrahedral silicate sheet is smaller and is postulated to be the interior unit in the scrolled serpentine mineral. 

The basic structural units in layer silicates are silica sheets and brucite or gibbsite sheets. The former consist of SiO tetrahedra connected at three corners in the same plane forming a hexagonal network. The tips of the tetrahedra all point in the same direction. This unit is called the tetrahedral sheet. The bijucite or gibbsite sheet consists of two planes of hydroxyl ions between which lies a plane of magnesium or aluminum ions which is octahedrally coordinated by the hydroxyls. This unit is known as the octahedral sheet. These sheets are combined so that the oxygens at the tips of the tetrahedra project into a plane of hydroxyls in the octahedral sheet and replace two-thirds of the hydroxyls. This combination of sheets forms a layer. 

Most of the chlorite-like material formed in soils is dioctahedral rather than trioctahedral. In the process of weathering, illite and muscovite are stripped of their potassium and water enters between the layers. In these minerals and in montmoril-lonites and vermiculites, hydroxides are precipitated in the interlayer positions to form a chlorite-like mineral (Rich and Obenshain, 1955 Klages and White, 1957 Brydon et al., 1961 Jackson, 1963 Quigley and Martin, 1963 Rich, 1968). Al(OH)3 and Fe(OH)3 are likely to be precipitated in an acid to mildly basic environments and Mg(OH)2 in a basic environment. The gibbsite sheets in the soil chlorites are seldom complete and the material resembles a mixed-layer chlorite-vermiculite. The gibbsite may occur between some layers and not between others or may occur as islands separated by water molecules. 

Figure 3.7. Schematic of the kaolin structure showing one silicate and one gibbsite sheet in each layer, which has been expanded along the c-axis to show bonding. The basic unit is repeated along the two horizontal axes to form layers. Adjacent layers are held together by hydrogen bonding (from Taylor and Ashcroft, 1972, with permission).

Figure 3.9. Schematic of the illite structure showing one gibbsite sheet between two silicate sheets. Potassium holds the layers together in a 12-fold coordination, preventing expansion. Isomorphous substitution can occur in the layers marked with an asterisk (from Taylor and Ashcroft, 1972, with permission).

Figure 3.10. Schematic of the chlorite structure showing two gibbsite sheets between two silicate sheets (from Jackson, 1964, with permission).

Figure 3B. Schematic of a portion of gibbsite sheet showing the relative locations of all AI3+ by removing the top layer of oxygen. Minerals may contain any combination of Al3+ and Mg2+, while Fe3+ or Fe2+ may also substitute isomorphously (from Taylor and Ashcroft, 1972, with permission).

The EXAFS results reported for the untreated samples (see Section 8.3.4) led to the conclusion that Zn may form highly ordered inner-sphere sorption complexes with gibbsite surfaces or substitute into an octahedral Al-hydroxide layer of some sort. The use of sequential extraction enabled more concrete conclusions to be made. For the nonextracted soil samples (bulk and coarse), second-shell Al coordination numbers did not exceed four, in fine with the dioctahedral structure of gibbsite sheets (only two out of three metal positions are occupied). Elsewhere, a gradual increase was observed in Al coordination up to six with each extraction step, indicating that Zn is part of a fully occupied, trioctahedral Al-Zn2+ layer and not part of gibbsite or another dioctahedral Al compound.67 While dioctahedral Al-hydroxide layers are... 

Figure 10. Comparison of As(V)-metai distances obtained for different binding modes of As(V) with (a) corner-sharing Al octahedra as found in gibbsite or in the gibbsite sheet of kaolinite (b) corner-sharing Fe octahedra as found in goethite (c) a Si tetrahedron (d) an aluminum octahedron and (e) an aluminum tetrahedron.

When considering the natural mineral form, clays mainly consist of aluminosilicates alternated silicate and gibbsite sheets constitute the basic components of clays. In the silicate layer, three oxygen atoms of the tetrahedral Si04 units are shared by different units to form the layered structure reported in Fig. 7.1a. Free oxygen atoms, shown just above each Si atom in the figure, do not belong to one sheet, but may serve to link two adjacent sheets with each other. 

Gibbsite sheets (Fig. 7.1b) contain octahedral [Al(OH)6] units, where each hydroxide ion is co-ordinated to two Al " cations. Thus, only two-thirds of the hydroxide sites are occupied in bridging two A1 ions. Analogously, brucite sheets, typical of some natural cationic and anionic clays, contain Mg(OH)2 units where each hydroxide ion is bonded to three cations. 

Fig. 7.2 Schematic representation of a 2 1 silica and gibbsite sheet typical of smectite clays...

Dioctahedral gibbsite sheet, dioctahedral mica sheet. 

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