In this paper, we presented data obtained with Induced Polarization (IP) measurements on artificial mixtures of spherical metallic particles and sand. The particles made of nickelized steel were fully or partially covered by colored varnish to model natural semi-conducting grains isolated from pore water by carbonate or quartz shells. We measured IP effect in time domain with four models where different portions of the particle surface were isolated. We characterized IP by the total chargeability and relaxation time. We found that the total chargeability and the relaxation time strongly decreased with increase of the isolated part of the particle surface, which means that isolated parts of the surface is passivated. For the mixture included fully isolated particles the IP response was found to be the same as the sand response. These findings suggest that part of semi-conducting minerals, which is isolated from pore water, does not polarize, and, therefore, cannot be detected by IP survey.