The hydrothermal reaction of kaolinite in the system Na2O-K2O-(MgO)-Al2O3-SiO2-H2O has been investigated at 200 and 300ºC.The study
of the solid products indicates that, at 200ºC, disordered illite was the first phase formed, which evolved at increasing run times towards well ordered illite and analcime. After longer reaction times
illite and kaolinite were the sole phases identified by XRD. The b parameter of the illites increased at longer reaction times, from 9.05 to 9.08 Å. Study by TEM/AEM revealed the presence of
surface chlorite-like layers, covering the illite packets, however. The trend observed in the products of the reaction at 300ºC was similar, except for the presence of boehmite or pseudoboehmite at the
higher temperature. The values of the b parameter of illite, of the order of 9.12-9.16 Å, indicate that illites formed at 300ºC have a more trioctahedral character. The results obtained
also indicate that Mg determines which product will be formed, K- and Na-zeolites and Al-rich illites being the phases formed in the absence of Mg.