Monte Carlo simulations were employed to investigate the surface energy of the open tetrahedral surface of 2:1-type phyllosilicate. Argon was selected as the probe molecule. The adsorption isotherm was simulated and the adsorption potential map was calculated. Both the density and energy distributions of adsorbed atoms were derived at different pressures to explore the adsorption mechanism. It is found that there exist two kinds of energetic sites: minima (-15.5 kJ/mol) corresponding to the centers of six-membered rings and platform points (-8.0 kJ/mol) corresponding to the edges and vertexes of hexagons. They are primary and secondary adsorption sites, respectively. The implications for experiments and future studies are discussed. Current results are applicable for understanding surface energy properties of other clay minerals, since they have very similar basal surfaces.