The term 'clay' is common to many disciplines and in geology, they are used in two quite different ways.
In sedimentology, when we described in term of detrital particle size, the term clay refers only to those grains less than 4 microns, thus being microscopic in size. It does not indicate that these particle are of a particular chemical composition. However, the term clay is also used to denote a specific composition, namely a group of silicate minerals related to the micas. Although most of these clay minerals are of clay size, not all clay-size sediment consists of clay minerals!
Particle size based on Wentworth Scale
Kaolinite in thin section
Source: Edward J. Tarbuck. Earth: An Introduction to Physical Geology
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