does silt form from sand ; does clay form from silt
Sand to silt is a physical process, silt to clay is most often a chemical process. 

Sand and larger rocks are physically weathered to form silt, everything from frost wedging to simple abrasion. 

Clay is a different creature entirely, for a start there is a disagreement about the definition of clay across disciplines. Several disciplines of geology use different size based definitions.  Clays can for from physical or chemical processes depending on the type of clay. Most often clay form from the hydrolysis of tiny mineral particles.