​Index properties such as grain size distribution, specific gravity, and plasticity index, and measurements of complex permittivity between 10 MHz and 1.3 GHz showed that pyrometamorphosed bentonites had larger grain size, lower specific surface, high electrolyte concentration, decreased double layer phenomena, increase in the ratio of free to adsorbed water for a given moisture content, and reduced plasticity. These transformations have important implications for phenomena such as swelling, permeability, diffusion and adsorption, which affect the performance of bentonite as an engineering and industrial material.