A size control for quartz silt (making particulates at planetary surfaces)

Quartz silt is widespread in terrestrial sediments 1]. Its ubiquity has led to its neglect as a geomaterial, and studies of silt as such are relatively rare, but it presents an interesting and continuing petrological problem. Is silt a specific geological material, is it defined by a formation process, and a set of size parameters? In the world of clastic quartz sedimentology there are obvious mode sizes; there is a sand mode at around 300–500 μm and a silt mode, an order of magnitude smaller, at 30–50 μm. Are these both defined by specific geological processes?