Page 135 - Geotechnical Engineering Soil and Foundation Principles and Practice

Soil Minerals
130 Geotechnical Engineering

6.3.6 Building a Layer Silicate
Mica shares silica tetrahedral in only two directions instead of three directions as
in quartz, and therefore readily cleaves into flakes. Clay minerals also have shared
bonding in only two directions, as silica tetrahedral link together at three corners
instead of all four. This kind of sharing characterizes layer silicates. The sharing
arrangement is shown in the top part of Fig. 6.4.

In addition, octahedrally coordinated aluminum ions can share corners to form a
continuous layer, which is shown in the bottom part of Fig. 6.4.

6.3.7 Combining Layers
The final step is to make a sandwich from the two kinds of layers by sharing apical
ions. One type is open-faced with one silica layer and one alumina layer. This is
illustrated at the top in Fig. 6.5, where oxygen ions in the layers are shown
touching. This is the mineral kaolinite, which is abundant in soils in tropical and
semitropical climates.

Kaolinite is nonexpansive because a hydrogen ion, the dynamic proton, slips
between two opposing oxygens and creates a hydrogen bond—the same kind of
bond that occurs organized in ice and less well organized in liquid water.

A thicker sandwich is formed when two silica layers enclose one alumina layer, as
shown at the bottom in Fig. 6.5. This is a three-layer silicate and will be the focus
of attention as it includes expansive clays.

6.3.8 As Easy as AB

A silica tetrahedron viewed from the side somewhat resembles a letter ‘‘A’’ and an
aluminum octahedron is kind of like a ‘‘B,’’ so a graphic shorthand for the two
classes of layer silicates are ‘‘AB’’ and ‘‘ABA.’’ Kaolinite can be represented by
AB AB AB ... where the represents hydrogen bonding.

Kaolinite is named for a mountain in China, and is the main constituent in
China clay used for the manufacture of porcelain. Less pure deposits are used for
stoneware or bricks. Kaolinite dominates the clay fraction of highly weathered
soils in tropical and semitropical climates, but also is common in shales,
particularly in tropical paleosols occurring underneath coal strata.

˚
Kaolinite is identified by X-ray diffraction from its basal spacing d 001 ¼ 7.14 A,
which is the thickness of an AB layer. It is approximately the thickness
˚
of three oxygen atoms, 3 2.6 ¼ 7.8 A, with allowance for the stacking
arrangement.

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