The presence of even small amounts of certain clay minerals can have significant effect on the properties of the soil. The identification of clay minerals requires special techniques and equipment.
The techniques include microscopic examination, X-ray diffraction, differential thermal analysis, optical property determination and electron micrography. Even qualitative identification of the various clay minerals is adequate for many engineering purposes. Detailed treatment of clay minerals is considered out of scope of the present text.
An indirect method of obtaining information on the type and effect of clay mineral in a soil is to relate plasticity to the quantity of clay-size particles. It is known that for a given amount of clay mineral the plasticity resulting in a soil will vary for the different types of clays.

‘Activity (A)’ is defined as the ratio of plasticity index to the percentage of clay-sizes:

where c is the percentage of clay sizes, i.e., of particles of size less than 0.002 mm.
Activity can be determined from the results of the standard laboratory tests such as the wet analysis, liquid limit and plastic limit. Clays containing kaolinite will have relatively low activity and those containing montmorillonite will have high activity.

A qualitative classification based on activity is given in Table 1:

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