Effect of pressure on kaolinite illitization

The illitization of kaolinite at increasing pressures was followed by hydrothermal experiments of kaolinite in KOH solution at 300 °C for 12 h and pressures between 500 and 3000 bar. XRD indicated a direct transformation of kaolinite into muscovite/illite with increasing pressure. However, the 27Al MAS NMR spectra showed, in addition to the muscovite/illite resonances, the presence of a signal at 61 ppm that should correspond to a secondary phase, not detected by XRD. A second series of experiments at 300 °C and 1000 bar for 1, 3 and 6 h was carried out to show direct evidence of such phase. The XRD patterns of the products clearly showed the crystallization of K–F zeolite, while the 27Al MAS NMR spectra of these samples displayed a signal at 61 ppm that must correspond, therefore, to Al in the K–F zeolite structure. In conclusion, kaolinite transformed into muscovite/illite when submitted to hydrothermal reaction in KOH solution with increasing pressure, with the formation of a secondary metastable phase called K–F zeolite, whose coherent diffraction domains were too small as to be detected by XRD.