Abstract: Mixed-layer clays composed of randomly interstratified kerolite/stevensite occur as lake and/or spring deposits of probable Pliocene and Pleistocene age in the Amargosa Desert of southern Nevada, U.S.A. The percentage of expandable layers of these clays, determined from computer-simulated X-ray diffractograms, ranges from almost 0 to about 80%. This range in expandabilities most likely results from differences in solution chemistry and/or temperature at the time of formation. An average structural formula for the purest clay (sample P-7), a clay with about 70% expandable layers, is: [ ( Mg 2.72 Al 0.07 Fe 0.03 Li 0.09 )( Si 3.96 Al 0.04 ) O 10 ( OH ) 2 ] −0.21 [ X + ] +0.21 . The data suggest that talc, kerolite, and stevensite form a continuous structural series based on layer charge.