The solid-liquid interface formed by single terminated muscovite mica in contact with two different ionic solutions is determined using surface x-ray diffraction. Specular and non-specular crystal truncation rods of freshly cleaved mica immersed in CsCl or RbBr aqueous solution were measured. The half monolayer of surface potassium ions present after the cleavage is completely replaced by the positive ions (Cs+ or Rb+) from the solution. These ions are located in the ditrigonal surface cavities with small outward relaxations with respect to the bulk potassium position. We find evidence for the presence of a partly ordered hydration shell around the surface Cs+ or Rb+ ions as well as partly ordered negative ions in the solution. The liquid ordering induced by the crystalline surface is vanishing at distances larger than 5 Å from the surface.
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