Abstract: A study was made of the weathering of muscovite-type mica to dioctahedral vermiculite in soils formed from Wissahickon schist in the Piedmont of Virginia. The trend of weathering as a function of depth was most apparent in the soil fractions from 20 to 0.2 microns. X-ray diffraction and chemical analyses indicate loss of potassium and expansion of the muscovite to form vermiculite as weathering proceeded. The amount of interstratified illite-vermiculite relative to vermiculite varied greatly between three soils and was characteristic for each. The widespread occurrence of dioctahedral vermiculite in highly weathered and acid soils suggests the formation of this mineral from commonly occurring muscovite-type micas.