Raman spectra and vibrational analysis of the trioctahedral mica phlogopite

Raman spectra were measured and normal mode calculations were performed for phlogopite. Calculated fundamental mode frequencies were fit to observed f requencies assigned to features in the two polarized Raman spectra collecte d, so that unambiguous assignment of eigenmodes could be made to nearly all Raman spectral features. Average force constant values determined for Si-O and O-Si-O environments as well as for the non-silicate environments in cy closilicates and gillespite provided an excellent starting point for the ph logopite calculations that quickly converged to a best fit of theoretical m ode frequencies to the observed Raman fundamental mode frequencies. Final v alues of the force constants for T-O stretch and O-T-O bend are within the range of force constant values determined for Si-O and O-Si-O environments in cyclosilicates and gillespite. The force constant similarities extend to the K-O, M(1,2)-O bond stretch and M(1,2)-O-T bond bend environments. Rama n-active modes at frequencies greater than 850 cm(-1) are localized intra-s heet modes dominated by T-O stretch and O-T-O bend motions. Raman-active mo des between 350 and 850 cm(-1) are dominated by mixtures of internal sheer modes with displacements from the M2 sites. Raman-active lattice modes are calculated at frequencies less than 350 cm-l. Calculated frequencies for th e IR-active A(u) and B-u, modes generally fall within the frequency ranges of broad bands in the IR spectra for phlogopite presented elsewhere. Eigenm odes calculated fur phlogopite are generally unique to that mica structure and differ from those determined fur the cyclosilicates and gillespite. The few eigenmodes calculated for phlogopite that resemble those calculated fo r the cyclosilicates are the most localized T-O stretch motions and are ass igned to the highest frequency modes.