The clay size (particles 0.002 mm and smaller) overlaps the silt sizes somewhat. The essential difference between the two, however, is that a clay mineral is not inert. It is a complex hydroaluminum silicate,
Al2O3 . nSiO2 . kH2O
where n and k are numerical values of attached molecules and vary for the same mass. The clay mineral has a high affinity for water, and individual particles may absorb 100+ times the particle volume. The presence or absence (during drying) of water can produce very large volume and strength changes. Clay particles also have very strong interparticle attractive forces, which account in part for the very high strength of a dry lump (or a clay brick). Water absorption and interparticle attraction collectively give the activity and cohesion to clay (and to soils containing clay minerals).