The experimental phase diagram for aqueous mixtures of charged gibbsite platelets and silica spheres is presented. The platelets are 95 nm in diameter, and the diameter ratio between the spheres and the platelets is 0.18. Here the spheres are acting as depletants in the mixtures perturbing the phase behavior of the pure platelet suspensions. An important finding is that a large isotropic/columnar coexistence region has been identified in the phase diagram, which appeared already at low concentrations of the platelets. Microradian X-ray diffraction measurements revealed the structure of the liquid crystalline phases and the orientational order of platelets. An interesting observation is that in the columnar phase the silica spheres are located between the columnar stacks. All samples were in equilibrium because sedimentation did not affect the system because of the relatively small size of the colloidal particles and the charges present at their surfaces.