Abstract: Bottom sediment clay mineral assemblages from the Rappahannock River and estuary contain kaolinite, illite, unorganized illite, dioctahedral vermiculite, 12.4 Å montmorillonite, 14.2 Å montmorillonite, chlorite, feldspar and quartz. The x-ray diffraction characteristics of these species and their thermal modifications at 25–500°C are described and illustrated. Progressive changes in the mineralogical composition of bottom sediments occur between the fresh-water and saline-water portions of the Rappahannock system. Chlorite and feldspar occur exclusively in the saline portions of the estuary. Illite shows progressive increase in crystallization quality. There appears to be less kaolinite, proportionately, in the estuarine sediments of the most saline zone. x-Ray diffraction diagrams showing these progressive changes are illustrated.