Chemical Properties of Soil
Types of Crystalline Silicate Clays
Based on the number and arrangement of tetrahedral and octahedral sheets contained in the crystal units or layers, crystalline silicate clays may be classed into three-layer types based upon the number and arrangement of tetrahedral and octahedral sheets in their basic structure. On the basis of number and arrangement of tetrahedral and octahedral sheets, silicate clays are classified into 3-different groups: 1:1 type, 2:1 type, and 2:1:1 type of minerals
1:1 Clay Minerals
The 1:1 layer minerals contain one tetrahedral and one octahedral sheet in their basic structural unit. Kaolinite is by far the most common 1:1 silicate clay in soils (others include halloysite, nacrite, and dickite). Compared to some 2:1 silicate clays, 1:1 clays like kaolinite exhibit less plasticity, stickiness, cohesion, shrinkage, and swelling and can also hold less water.
2:1 Clay Minerals
The joining of two tetrahedral sheets (one from each side) to one octahedral sheet produces a three-sheet mineral type, which is called 2:1 and is represented by the mica, vermiculite, montmorillonite, smectite, and illite. Smectites and vermiculites are expanding type of clay, while mica and illite is non-expanding.
2:1:1 Clay Minerals
The 2:1:1 clay mineral consists 2 silica tetrahedral sheets and 1 aluminum octahedral sheet with a magnesium hydroxide sheet in the interlayer space between 2:1 sheets. Chlorite is sometimes said to have a 2:1:1 structure. The properties of chlorite are very similar to those of the fine-grained micas. They have a limited CEC and moderate hydration, expansion, and plasticity. The exception is that it is a good source of magnesium (Mg2⁺) during weathering.
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