Vibrational analysis of palygorskite and sepiolite

ÐLattice dynamic calculations for the sepiolite and palygorskite structures using polarized Raman and FTIR spectra provide a fundamental basis for interpreting spectral features by assigning vibrational modes. The SiÿO stretch and O-SiÿO bond bending force constants determined for palygorskite are similar to equivalent values calculated previously for the other phyllosilicates. The MgÿO bond stretch values, on the other hand, are about half of those determined for the equivalent AlÿO and MgÿO bond stretch environments in other phyllosilicates, suggesting that the bonding within the octahedral ribbons in palygorskite and sepiolite is weaker than that in the continuous octahedral sheets in micas. The weaker bonding allows more flexible octahedral environments in palygorskite and sepiolite, giving rise to higher probabilities for cation substitutions and vacancies relative to the micas. Above ~700 cmÿ1 in the IR and 750 cmÿ1 in the Raman spectra, the eigenmodes are dominated by atomic displacements within the silicate sheets. Below 700 cmÿ1 the eigenmodes become mixed with motions among the Mg octahedra and the silicate sheets; the eigenmodes assigned to the most prominent peaks in the Raman spectra (near 700 cmÿ1)belong to this group. As mode frequencies decrease, the correspondingigenmodes evolve from more localized Mg-O stretch, O-Mg-O bend, and O-Si-O bend motions to longer range motions such as silicate sheet deformations caused by silicate tetrahedra rotation, and silicate sheet hearing around the Mg-octahedral sheets.