The effect of synthesis temperature on the FT-Raman and FT-IR spectra of saponites

Saponites hydrothermally synthesised between 125 degrees C and 280 degrees C were studied by Fourier transform (FT)-Raman and FT-infrared (IR) spectroscopy and compared to natural saponite. FT-IR spectra with increasing synthesis temperature showed saponite crystallisation and disappearance of amorphous material. The 457 cm(-1) gel band developed at higher temperatures in two components at 347 cm(-1) and 465 cm(-1) ascribed to e(1)(2) of SiO2 and Al-OH translation modes of saponite. Above 150 degrees C, minor octahedral sheet vibrations appeared at +/-530 cm(-1) (Mg-OH perpendicular mode) followed by a band around 750 cm(-1) (Al-OH deformation mode) above 200 degrees C. At this temperature, two bands due to the a(1)(2) SiO2 and apical Al-O bond of AlO4 in the tetrahedral sheet became visible at 691 and 837 cm(-1), respectively. The strongest SiO2 band showed a continuous shift from 1043 to 1013 cm(-1) with increasing synthesis temperature. Two overlapping bands around 660 and 690 cm(-1) appeared almost simultaneously before the wide amorphous material band around 1240 cm-L had disappeared, indicating that in the saponite structure, the octahedral and tetrahedral layers form almost simultaneously. sharpening of other bands associated with various Si-O modes was not observed up to 200 degrees C, The FT-Raman spectra only revealed the formation of internal tetrahedral vibrational modes. Upon increasing temperature, the first band observed was at 683 cm(-1) at 150 degrees C followed by the 360 cm(-1) band at 175 degrees C. Around 240-280 degrees C, two bands around 430-465 cm(-1) were observed. The increase in intensity of these bands with temperature confirmed the concept that the silica in the gel rapidly changes its configuration. (C) 2000 Elsevier Science B.V. All rights reserved.