Abstract The cordierite is commonly prepared by reaction of talc, clay and gibbsite within the range of 1200–1350 °C. This study deals with the effect of the amorphization by grinding of that mixture and its influence on the cordierite formation. The mixture previously mentioned underwent six different treatments: mixing without grinding (A) (only mixing); non-amorphizing grinding (A M ) and amorphizing grinding in oscillating mill at four different times (H samples). The phases formed by thermal treatment were studied using differential thermal analysis (DTA)–thermogravimetric analysis (TG)–DTG, dilatometries and X-ray diffraction (XRD) techniques in certain combinations. The thermal analysis of the A and A M series were compared and they do not show significant differences, whereas the H samples present remarkable alterations in the DTA peaks as well as in the weight losses (TG). Thus, a great number of DTA peaks tend to decrease the temperature of the maximum and to lower the intensity as the amorphization time increases. Calcination tests performed within the range 900–1200 °C show important differences in the diffractograms obtained from the intermediate products. While at 1350 °C the A and H samples reach the same final phases, within the range 1200–1360 °C they present important differences in the DTA indicating that the sequence and direction of reaction are different. The same behavior can be observed by dilatometric analysis.