1:1 Structure (e.g. Kaolonite, Halloysite)

One sheet of tetrahedral units is condensed with one sheet over octahedral units, and the resulting layer is stacked upon like layers. Between layers, oxygen ions face hydoxyl groups and the resulting Hydrogen bonds are strong.


This strong hydrogen bonding prevents ready separation of layers; thus there are no water molecules or cations in the interlayer space. The lattice is therefore said to being non-expanding and there is no tendency for soils containing these crystalline minerals to swell on wetting and shrink on drying. (Thompson and Troeh, 1978).


All the cation sites within the tetrahedral part of the layer are occupied by Si4+ ions, and generally, there is little or no isomorphous substitution of Si4+. Electrical neutrality of the crystal lattice is preserved when one-third (1/3) of the cation sites in the octahedral portion of the layer are empty i.e. only two out of three sites are occupied by Al3+.

The mineral is said to be dioctaherdal. Examples of dioctahedral 1:1 minerals include Kaolonite and halloysite.

Although these structures are electrically neutral (Theng, 1984), during the course of chrystallisation a small proportion of the Al3+ ions may be isomorphously replaced by ions of lower valency (e.g. Fe2+), and consequently the lattice overall carries a small negative charge. This charge contributes to the cation exchange capacity (CEC) of the clay, and is neutralised by cations from the soil solution.


Kaolonite crystals are pseudo-hexagonal in shape. They range in size from 1.0 – 2.0 pm across. The restricted specific surface area and limited adsorptive capacity for cations and water molecules suggest that Kaolonite does not exhibit such properties as shrinkage and swelling and cohesion to an appreciable degree.


Halloysite has a similar structure to Kaolonite but contains sheets of interlayer water, which are trapped during crystallisation. Although collapse of the structure can be induced by gentle heating in the laboratory, it is improbable that collapse occurs in the field. The high humidity present in the soil pore network is sufficient to prevent dehydration of halloysite.


The presence of water between the layers alters the distribution of stresses within the mineral lattice such that the layers curve to form a tubular structure (white, 1987).