The chlorine effect on different samples of refractory clays has been studied within a wide temperature interval. The isothermal and non isothermal calcination assays were carried out in currents of N(2) and a mixture of Cl(2)/N(2) (1:1), using a thermogravimetric system designed in our laboratory and masses of approximately 300 mg. Isothermal assays were also performed with masses of 2 g, using a fixed bed reactor with horizontal dynamic flow. The initial samples of refractory clays and the residues resulting from thermal treatments were characterized by X-ray fluorescence (XRF), X-ray diffraction (XRD), scanning electron microscopy (SEM), electron probe microanalysis (EPMA), specific surface area analysis (BET) and CIELAB colorimetry. The thermograms in N(2) and Cl(2)/N(2) showed two mass losses, between room temperature and 650 degrees C, due to the elimination of the mineral hydration water and the transformation of kaolinite into metakaolinite. A third mass loss was observed over 750 degrees C in Cl(2)/N(2) due to the chlorination and elimination of the iron contained in the samples. The X-ray diffractograms of the residues from the different thermal treatments showed that the calcination produced the total transformation of kaolinite and other phases contained in the mineral. The XRF analysis of the residues resulting from the isothermal assays in Cl(2)/N(2) permitted to determine that the iron contained in the samples might be completely removed by chlorination at 900 degrees C for 2 h. The colorimetric analysis of the chlorination residues indicated that the discoloration and bleaching of samples were caused by the deferrification of the mineral. The results of the characterizations using SEM and EPMA supported the previous observations. (C) 2010 Elsevier B.V. All rights reserved.