A series of pure Ga-Fe3+ smectites were synthesized at 180 degrees 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 nu OH, delta OH and (nu + delta) OH band positions did not follow the relation: nu OH (cm(-1))=(1/2 pi c)*(k(OH)/mu(OH))(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. (C) 2016 Elsevier B.V. All rights reserved.