Vibrational spectroscopy of ferruginous smectite and nontronite

A comparison is made between the Raman and infrared spectra of ferruginous smectite and a nontronite using both absorption and emission techniques. Raman spectra show hydroxyl stretching bands at 3572, 3434, 3362, 3220 and 3102 cm(-1). The infrared emission spectra of the hydroxyl stretching region are significantly different to the absorption spectrum. These differences are attributed to the loss of water, absent in the emission spectrum, the reduction of the samples in the spectrometer and possible phase changes. Dehydroxylation of the two minerals may be followed by the loss of intensity of the hydroxyl stretching and hydroxyl deformation frequencies. Hydroxyl deformation modes are observed at 873 and 801 cm(-1) for the ferruginous smectite, and at 776 and 792 cm(-1) for the nontronite. Raman hydroxyl deformation vibrations are found at 879 cm(-1). Other Raman bands are observed at 1092 and 1032 cm(-1), assigned to the SiO stretching vibrations, at 675 and 587 cm(-1), assigned to the hydroxyl translation vibrations, at 487 and 450 cm(-1), attributed to OSiO bending type vibrations, and at 363, 287 and 239 cm(-1). The differences in the molecular structure of the two minerals are attributed to the Al/Fe ratio in the minerals.