Effect of pressure on kaolinite illitization

摘要

The illitization of kaolinite at increasing pressures was followed by hydrothermal experiments of kaolinite in KOH solution at 300 ℃ 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 ~(27)Al 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 ℃ 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 ~(27)Al 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.