Hypothesis: The adsorption mechanisms of fatty acids on minerals are largely debated from years, and their understanding is now required to improve flotation processing in the critical context of raw materials. Three wavenumbers have been observed in the literature for the asymmetric stretching vibration of COOafter the adsorption of fatty acids on mineral surfaces. They have been interpreted as different adsorbed forms, such as a precipitate formation, an adsorption of sole or bridged carboxylates, an anion exchange, or adsorbed modes, such as monodentate or bidentate configurations. Experiments/theory: Diffuse reflectance infrared Fourier transform spectroscopy was combined with ab initio molecular dynamics simulations and simulation of infrared spectra. Fluorite and sodium octanoate - or longer-chain fatty acids - were used as prototypical materials for all the investigations. Findings: At low fatty acids concentration, the asymmetric stretching vibration of COO peaks at 1560 cm(-1) while, at higher concentration, this infrared band converts into a doublet peaking at 1535 and 1575 cm(-1). Using simulations, we assign the band at 1560 cm(-1) to the adsorption of a carboxylate molecule bridged on a sodium counter-cation and the doublet at 1535 and 1575 cm(-1) to the adsorption of the sole carboxylate anion under a monodentate or a bidentate binuclear configuration, respectively. The formation of an adsorbed layer on the mineral surface is initiated by the adsorption of a sodium carboxylate and followed by the adsorption of mixed sole anionic forms. The role of the carboxylate countercation is highlighted for the first time, which was totally ignored in the literature beforehand. This particularly opens the path to the development of innovative strategies to enhance the separation contrast between minerals, which is of uttermost importance for the recovery of critical raw materials. (c) 2020 Elsevier Inc. All rights reserved.