Revisiting the cation mass-valence sum approach to assigning infrared OH-bands in dioctahedral smectites in the light of new data from synthetic Ga-Fe3+ smectites

A series of pure Ga-Fe3+ smectites were synthesized at 180°C in order to complete IR data in the Al-Ga-Fe3+ system. MIR and NIR data from natural and synthetic smectites in the Al-Fe3+, Al-Ga and Fe3+-Ga chemical series exemplified a case where some ν OH, δ OH and (ν+δ) OH band positions did not follow the relation: ν OH (cm−1)=(1/2π c)*(kOH/μOH)1/2. This relation is often used to approximate the position of the OH vibration bands from the reduced masses of octahedral cations linked to hydroxyls in clay minerals. In this study, better relations were established between the wavenumbers of the OH absorption band of smectites and the average ionic radius of neighboring octahedral cations. Similar trends were established previously for M-(OH)2 hydroxides (G.W. Brindley, C. Kao, Phys. Chem. Miner. 10 (1984) 187-191) suggesting a possible generalization of a link between OH frequencies in IR spectra and the average ionic radii of neighboring octahedral cations for hydroxylated minerals.