Agriculture Reference
In-Depth Information
together with a layer of potassium ions. Most illites appear to form through transforma-
tion of micas present in the soil parent materials (Brown, 1990).
The smectites (Figure I.8) are a group of 2:1 clay minerals in which water and other
polar solvents are able to penetrate between the layers. The relative weakness of the inter-
layer bonds means that their distances apart are not fixed but vary with the humidity
and the sizes of the hydrated and exchangeable cations present between the layers.
Interlayer spacing normally varies from 1.2 to 1.5 nm although it may be much larger at
high humidities when sodium is the dominant interlayer cation present (Brown, 1990).
The surface areas of smectites are large since internal interlayer surfaces also
contribute to their total surface areas. In the 33 smectite-rich soils described by de Kimpe
et al.
(1979) (Table I.5), the internal surfaces contributed an average of 82 % of the total
surface area. Soils containing appreciable smectites (vertisols) shrink extensively and
reversibly on drying to produce distinctive patterns of polygonal surface cracking.
In the chlorite group, the 2:1 unit layers are bound together (at a distance of 1.4 nm)
by layers of hydrated magnesium, aluminium or iron.
Not all clay minerals have the structure and behaviour typical of the major groups
listed above. Mixed layer clays occur widely in soils and the inclusion of clays of
