Tetrahedral sheet

Clay minerals that are composed of two tetrahedral layers and one octahedral layer are referred to as 2 1 clay minerals or TOT minerals. The apical oxygens of the two tetrahedral sheets project into the octahedral sheet. The 2 1 stmcture has a basal spacing (nominal thickness) of 1.0 nm (10 E). Pyrophjlhte [12269-78-2] Al2Si40 Q(0H)2, is the dioctahedral mineral, ie, AF" in the octahedral sites, and talc [14807-96-6], Mg3Si402Q(0H)2, is the trioctahedral, ie, in the octahedral sites. Both these minerals are essentially free of substitution in the octahedral site and therefore do not have a net... 

Palygorskite and sepioHte are different from other clay minerals in the manner in which the 2 1 layers are joined. Rather than being joined in a continuous manner, the tetrahedral sheets are joined to an adjacent inverted tetrahedral layer, making the octahedral layers noncontinuous and leaving an open channel in the mineral stmcture (37,38,148). The dimension of palygorskite is teI.S nm (18 E) the dimension of sepioHte is 9e2.7 nm (27 E) (37). 

Many varieties of clay are aluminosilicates with a layered structure which consists of silica (SiOa" ) tetrahedral sheets bonded to alumina (AlOg ) octahedral ones. These sheets can be arranged in a variety of ways in smectite clays, a 2 1 ratio of the tetrahedral to the octahedral is observed. MMT and hectorite are the most common of smectite clays. 

The brownmillerite structure type can be described as a sequence. .. OTOT... where O stands for the octahedral sheet and T the tetrahedral sheet. A number of other phases closely related to this structure have been characterized, including Ca2LaFe308, with a stacking sequence. .. OOTOOT... and Ca4Ti2Fe20i 1 and... 

Figure 3.4. Two types of isomorphous substitution. The middle structures are two-dimensional representations of clay without isomorphous substitution. On the left is an isomorphous substitution of Mg for A1 in the aluminum octahedral sheet. On the right is isomorphous A1 substitution for Si in the silicon tetrahedral sheet. Clays are three-dimensional and -OH on the surface may be protonated or deprotonated depending on the pH of the surrounding soil solution. There will be additional water molecules and ions between many clay structures. Note that clay structures are three-dimensional and these representations are not intended to accurately represent the three-dimensional nature nor the actual bond lengths also, the brackets are not intended to represent crystal unit cells.

Figure 3.11. Upper diagram micelle of sodium octanoate in water lower diagram micelle of 2 1 clay (the center gray layer is an aluminum octahedral sheet the upper and lower are silicon tetrahedral sheets).

Two cases of isomorphic substitution can be distinguished In the tetrahedral sheet, or in the octahedral sheet (Sposito, 1984). 

If isomorphic substitution of Si(IV) by AI(III) occurs in the tetrahedral sheet, the resulting negative charge can distribute itself over the three oxygen atoms of the tetrahedron (in which the Si has been substituted) the charge is localized and relatively strong inner-sphere surface complexes (Fig. 3.10a) can be formed. 

Kaolinite particles. Typically about 50 unit layers of hexagonal plates are stacked irregularly and interconnected through H-bonding between the OH-groups of the octahedral sheet and the oxygens of the tetrahedral sheet (Fig. 3.9) (Sposito, 1989)... 

The layer silicates comprise tetrahedral sheets of silica and octahedral sheets of aluminium and magnesium hydroxide, with varying amounts of the Si, Al and Mg replaced by cations of lower valence giving the lattice a net negative charge. Two basic combinations occur 1 tetrahedral sheet with 1 octahedral (e.g. kaoUnite, halloysite), and 2 tetrahedral with 1 octahedral (e.g. smectite, vermiculite, illite). 

In each 2 1 mixed layer, the upper tetrahedral sheet is translated by a/3 with respect to the lower one, thus creating the octahedral oxygen coordination around the cations of the intermediate sheet. Translation may occur along any positive or negative direction defined by structural axes Xj, X2, and X3 of a pseudohexagonal lattice, as shown in figure 5.43. 

Figure 5.42 Structural scheme of micas. (A) Tetrahedral sheet with tetrahedral apexes directed upward. (B) Structure of mixed layers along axis Y, a, b, and c are edges of elementary cell unit.

Figure 5.43 Sliding between tetrahedral sheets of mixed 2 1 talc layer, (a) Set of positions 1 occupied, (b) Set of positions II occupied. From Bailey (1984a). Reprinted with permission of The Mineralogical Society of America.

Figure 5.44 shows the conformation of the hexagonal rings on the tetrahedral sheet for limiting values of the rotational angle = 0° and 12°, respectively). Due to structural constraints, the value of depends directly on the mean cation-to-oxygen distances in [TO4] tetrahedra (4) and in octahedra (4,), according to... 

The 2-3 subscript for the B site in the formula expresses the fact that there are two families of mica structures, the dioctahedral and trioctahedral micas, based on the composition and occupancy of the intralayer octahedral sites. The trioctahedral micas have three divalent ions—for example, Mg or a brucitelike [Mg(OH)2] intralayer, and the dioctahedral group—two tri-valent ions—for example, Al or a gibbsitelike [AlfOHfa] intralayer, between the tetrahedral sheets. In the dioctahedral micas, therefore, one-third of the octahedral sites are vacant or unoccupied (Fig. 2.12C). 

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