Environmental Engineering Reference
In-Depth Information
aluminium ions, forming sheets of two layers of oxygen (or hydroxyl) in close packing,
but only two-thirds of the possible octahedral centres are occupied by aluminium.
Clay minerals are formed by the silicon-oxygen and aluminium-oxygen structural
units being bonded together so that sheets of each result. Clay minerals thus have a platy,
crystalline structure. In the soil other ions, usually of similar size, can take the place of
silicon and aluminium by a process of
isomorphous substitution
. The different types of
clay minerals are determined by three features: the ways in which the silica and alumina
sheets are stacked into layers, the bonding between the layers, and the substitution of
other ions for Si and Al.
Figure 18.7 gives a schematic representation of the structure of five common clay
minerals.
Kaolinite
is made of a silica sheet and an alumina sheet sharing a layer of
oxygen atoms. The layers are held together by strong hydrogen bonding and the structure
is non-expanding. Illite or clay mica has repeating layers consisting of one alumina sheet
sandwiched between two silica sheets. The layers are firmly bonded together by
potassium (K) ions, which are just the right size to fit into the hexagonal holes
Figure 18.6
The basic building blocks of clay minerals: (a) a
silica tetrahedron, (b) an alumina octahedron.
