Why are most sand dunes made of sand-sized particles instead of clay?

Most sand dunes are composed of sand-sized particles rather than clay primarily because of the physical properties of clay particles. Clay particles exhibit mutual coherence, which prevents them from being easily picked up by the wind. If they are lifted, they tend to be carried high into the atmosphere rather than settling to form dune structures.1

In contrast, sand-sized particles are ideal for dune formation because they are heavy enough to be moved by wind through processes like saltation (jumping) and creep, yet light enough to be deposited when wind velocity decreases. While clay can occasionally form dunes, this only occurs when the particles are aggregated into larger, sand-sized clumps. Conversely, silt is easily picked up by the wind but is typically transported away too rapidly to accumulate into stable, dunelike bed forms.231