Octahedral-substituted layered silicates

Layered silicates dispersed as a reinforcing phase in polymer matrix are one of the most important forms of hybrid organic-inorganic nanocomposites. MMT, hectorite, and saponite are the most commonly used layered silicates. Layered silicates have two types of structure tetrahedral-substituted and octahedral substituted figure 6. In the case of tetrahedrally substituted layered silicates the negative charge is located on the surface of silicate layers, and hence, the polymer matrices can react interact more readily with these than with octahedrally-substituted material. 

Minnesotaite resembles talc with Fe substituting for some of the Mg the octahedral sheet (Fe, Mg)3Si40 o(OH)2. Fibers of this material are abundant in the rock formations mined for iron in Minnesota (Gruner, 1944)— hence the name. In its fibrous form, about three-fourths of the octahedral sites are occupied by Fe, rather than Mg. Although also a layer silicate, minnesotaite has a complex structure that is not yet fully understood. 

The mineral phase. Mineral colloids are composed of layered silicates and amorphous metal hydroxides. The two basic building layers of the silicates are (i) a tetrahedral silicon dioxide layer modified by occasional substitution by Al and (ii) an octahedral A1 oxyhydroxide layer with occasional substitution by Mg2+, or... 

These diagrams indicate that when the total layer charge is less than 0.7, A1 will be the dominant cation when the seat of the charge is largely in the octahedral sheet as the predominant charge shifts to the tetrahedral sheet the larger Fe ion substitutes for A1 in the octahedral sheet. Radoslovich (1962) found that montmorillonite was the only layer silicate in which tetrahedral A1 caused the layer to increase in size in the b direction. He explains this by suggesting ... 

An important feature of the smectites, vermiculites and other 2 1 layer silicates is that isomorphous substitutions can occur in both the tetrahedral and octahedral sheets. Thus, substitution of Si by A1 occurs in the tetrahedral sheet, together with replacement of A1 by Mg, Fe, Li or other small atoms in the octahedral sheet. The substitutions lead to a deficit of positive charge, which is compensated by the presence of exchangeable, interlayer cations. 

The 2 1 layer silicate minerals are sometimes defined on the basis of the number of octahedral positions occupied by cations. When two-thirds of the octahedral positions are occupied, such as in pyrophyllite (Al2Si40io(OH)2), the mineral is call dioctahedral, when all three positions are occupied, such as in talc (MgjSL OioCOH ), the mineral is called trioctahedral. This difference in composition of layer silicates can be fairly easily determined by x-ray diffraction, because each substitution slightly changes the dimensions of the unit cell. 

AI phosphate. When hectorite (a 2 1 layer silicate in which Mg2+ is the dominant octahedral cation rather than Al3+) is substituted for kaolinite, much less phosphate is retained. This points out the importance of Al to phosphate retention. In acid soils, phosphate retention is often closely related to the amounts of extractable Pe3+ and Al. 

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