Phyllosilicates layer charge

FIGURE 4.6 Phyllosilicate layers having structural charges at (A) the basal planes and (B) within the layer. The layers are assumed to be at a pH where edges carry no net charge. Arrows schematize the electric field generated by the structural charges. 

Mermut, A.R. 1994. Problems associated with layer charge characterization of 2 1 phyllosilicates. p. 105—122. In A.R. Mermut (ed.) Layer charge characteristics of 2 1 silicate clay minerals. Clay Miner. Soc., Boulder, CO. 

Layer silicates, sheet-like phyllosilicates such as the familiar micas, are in primary rocks and in soils. The soil minerals are often called clay minerals. Since other components can also be in the clay fraction, layer silicates is a mom accurate term. A typical layer silicate is a combination of a layer of Al-, Mg-, or Fe(II)-0 octahedra plus one or two layers of Si-0 tetrahedra. The tetrahedral and octahedral sheets bond together by sharing oxygens at the corners of the tetrahedra and octahedra. Layer silicate minerals are differentiated by (1) the number and sequence of tetrahedral and octahedral sheets, (2) the layer charge per unit cell, (3) the type of interlayer bond... 

Pyrophyllite Phyllosilicates Low temp, metamorphic and hydrothermal R No layer charge—little chemical reactivity unstable in soils... 

The layers are characterized by a thickness of about 1 nm, and the other dimensions vary from 30 mn to several micrometres or more. Several layers are stacked in clay particles, kept together by weak van der Waals forces. The performance of polymer-clay nanocomposites strongly depends on the breaking-up of clay particles in the polymer matrix. However, the most frequently used layered silicate, MMT, is a naturally occurring 2 1 phyllosilicate, which has the same layered and crystalline structure as talc and mica but a different layer charge. A central octahedral sheet of alumina fused between two external silica tetrahedral sheets (the oxygens from the octahedral sheet also belong to the silica tetrahedral) forms the layers of the... 

It is not surprising that the magnitude of the layer charge, and secondarily the site of the charge, can influence the chemical and physical properties of the phyllosilicate minerals, although it is frequently difficult to quantify this. 

As the 2 1 layer charge decreases from that of true micas (x 1) toward talc and pyrophyllite (x 0), two major groups of phyllosilicates we encoimtered, vermiculites and smectites, in that order. It might be supposed that these two groups would have properties intermediate to the micas and talc/pyrophyllite. Such is not the case. Micas, on one hand, and talc/pyrophyllite, on the other hand, normally have well-ordered three-... 

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