The interfacial structure between the muscovite (001) surface and aqueous solutions containing monovalent cations (3 x 10(-3) m Li+, Na+, H3O+, K+, Rb+, or Cs+, or 3 x 10(-2) m Li+ or Na+) was measured using in situ specular X-ray reflectivity. The element-specific distribution of Rb+ was also obtained with resonant anomalous X-ray reflectivity. The results demonstrate complex interdependencies among adsorbed cation coverage and speciation, interfacial hydration structure, and muscovite surface relaxation. Electron-density profiles of the solution near the surface varied systematically and distinctly with each adsorbed cation. Observations include a broad profile for H3O+, a more structured profile for Li+ and Na+, and increasing electron density near the surface because of the inner-sphere adsorption of K+, Rb+, and Cs+ at 1.91 +/- 0.12, 1.97 +/- 0.01, and 2.26 +/- 0.01 angstrom, respectively. Estimated inner-sphere coverages increased from similar to 0.6 to 0.78 +/- 0.01 to similar to 0.9 per unit cell area with decreasing cation hydration strength for K+, Rb+, and Cs+, respectively. Between 7 and 12% of the Rb+ coverage occurred as an outer-sphere species. Systematic trends in the vertical displacement of the muscovite lattice were observed within similar to 40 angstrom of the surface. These include a <0.1 angstrom shift of the interlayer K+ toward the interface that decays into the crystal and an expansion of the tetrahedral octahedral tetrahedral layers except for the top layer in contact with solution. The distortion of the top tetrahedral sheet depends on the adsorbed cation, ranging from an expansion (by similar to 0.05 angstrom vertically) in 3 x 10(-3) m H3O+ to a contraction (by similar to 0.1 angstrom) in 3 x 10(-3) m Cs+. The tetrahedral tilting angle in the top sheet increases by 1 to 4 degrees in 3 x 10(-3) m Li+ or Na+, which is similar to that in deionized water where the adsorbed cation coverages are insufficient for full charge compensation.