Frictional experiments were carried out on powder and single crystal specimens of muscovite. Friction coefficients for powder samples were found to be about twice as large as those for single crystals. We found that whereas in single crystal samples sliding occurs along the (001) plane, the situation is more complicated in powder samples. Experiments under wet conditions showed an similar to 30% reduction in friction coefficient for both powder and single crystal samples compared with dry conditions. The degree of the reduction depended on the fluid chemistry of NaCl or CsCl solutions, indicating that the presence of adsorbed water reduced the friction coefficients. Surface flatness of gauge particles and high affinity of the surface to water were crucial for realizing the effective lubrication due to adsorbed water.