Near-infrared spectroscopic study of synthetic and natural pyrophyllite

The near-infrared spectrum of hydrothermally synthesised pyrophyllite (400 degrees and 450 degrees C at 1 kbar for 7 days) has been compared to that o f natural pyrophyllite. The near-infrared spectra are very similar and only very minor differences are observed in the band positions and intensities. The bands >7,000 cm-' are assigned to the first overtones of the Al2OH str etching infrared and Raman vibrations and the combination bands of the Al2O H and adsorbed H2O stretching infrared and Raman vibrations. The 6,000-7,00 0 cm(-1) region is dominated by the first overtone of the adsorbed H2O stre tching mode and the 3(rd) overtone of the adsorbed H2O bending mode. The 5, 000-6,000 cm-' region is characterised by combination bands of the Al2OH st retch and adsorbed H2O stretching modes with the adsorbed H2O bending mode. Furthermore, the combination modes of the Al2OH and adsorbed H2O vibration s are considerably broader as expected based on the broad nature of the H2O bands in the mid-infrared region. Bands in the 4,000-5,500 cm-' region are attributed to the combination bands of the stretching and libration modes of the Al2OH in the pyrophyllite structure and of adsorbed H2O. The bands a round 4,800-4,850cm(-1) represent overtones of the H2O bending vibration ar ound 1,632cm(-1).