Low-Frequency (OH)− Motions in Layer Silicate Minerals
Neutron inelastic scattering spectra for kaolinite, dickite, pyrophyllite, and muscovite show a characteristic peak between 850 and 910 cm−1, while those for chrysotile, antigorite, talc, phlogopite, and amphibole minerals show characteristic peaks at 620–650 cm−1 and 460–510 cm−1. These peaks correspond to localized torsional oscillations of (OH)− groups. Lower-frequency peaks are also observed and are associated with optical and acoustic modes involving hindered translations. Within a series, the similarity in the shapes and the positions of the peaks indicates that the motions of the (OH)− groups are determined primarily by nearest-neighbor cation coordination. Differences between the two series can be attributed to the different environments when the octahedral layer of the lattice is populated either by two or by three cations.
The spectra of the hydrated minerals, montmorillonite, hectorite, and halloysite, show lines characteristic of liquid water. Upon dehydration, peaks corresponding to the motions of structural (OH)− units are observed.
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