*2.3. ATR-FTIR Measurement*

A Nicolet 6700 Fourier-transform infrared spectrometer (Thermo Electron North America LLC, West Palm Beach, FL, USA) equipped with the Smart iTR accessory was used to collect the mid-infrared spectra. The system was equipped with a liquid-nitrogen-cooled DTGS KBr detector and a diamond ATR crystal with an angle of incidence of 45◦ to ensure the signals were detected. First, the mineral was put on the stage with the freshly polished surface, which was pressed and fastened toward the ATR crystal to collect the background spectra. Second, the same sample was removed from the stage, and a 5 mL xanthate solution of different concentrations was carefully dipped onto the fresh surface and left untouched for a specific time. Third, the mineral sample was tilted to remove most of the solution and gently blown with ultrapure N<sup>2</sup> gas for the removal of residual water. Finally, the mineral sample was again pressed against the ATR crystal, and the ATR-FTIR spectra were collected. The intensities in the spectra were shown in a relative value under the same scale. All spectra were collected at room temperature with no further treatment made toward the spectra except the baseline correction.
