Near-infrared and mid-infrared spectroscopic study of sepiolites and palygorskites

Abstract

Near-IR spectroscopy has been used to distinguish between palygorskites and sepiolites. Three near-IR spectral regions contain bands due to (a) the high frequency region between 6400 and 7400 cm−1 attributed to the first overtone of the hydroxyl stretching mode, (b) the 4800–5400 cm−1 region attributed to water combination modes and (c) the 4000–4800 cm−1 region attributed to the combination of the stretching and deformation modes of the M–MgOH units of palygorskites and sepiolites. Near-IR bands are observed in the first region and are assigned to the first overtone of the hydroxyl stretching frequency observed at 3620 and 3410 cm−1 in the mid-IR spectra. The near-IR bands observed in the second region are assigned to the combination of the water OH stretching and deformation vibrational modes. A complex set of low intensity bands are observed in the 4100–4600 cm−1 region and are attributed to the combination of the cation hydroxyl stretching, deformation and translation modes. The difference between the near-IR spectra of palygorskites and sepiolites depends on the dioctahedral nature of the palygorskites and the trioctahedral structure of the sepiolites. Changes in the near-IR spectra are therefore related to the Mg3(OH) and Mg2(OH) units in the palygorskites.