The natural Mg-vermiculite (VB) from Brazil and its Na-form (Na-VB) was studied after milling in ball, jet and vibrational mills. The morphology of vermiculite samples was studied using scanning electron microscopy (SEM) and the changes of vermiculite structures were monitored using X-ray diffraction (XRD) analysis. The particle size (PS) changes were characterized by the median particle size (d(50)), volume-weighted mean diameter (d(43)), mode diameter (d(m)) and span value. The surface changes of vermiculite samples were evaluated using xi-potential measurements as a function of particle electrophoretic mobility (mu). The electrophoretic mobility (mu) of the VB and its milled samples was in the range of 1.16 to 1.96 x 10(-4) m(2)/Vs and of the Na-VB and its milled samples were in range of 3.44 to 4.25 x 10(-4) m(2)/Vs. The range of the difference in the xi-potential of the VB was from -15.1 to -25.4 mV and Na-VB was from 55 mV to the -44.5 mV. Samples containing small particles and homogeneous particle size distribution showed a low value of xi-potential. (C) 2013 Elsevier B.V. All rights reserved.