Potential of vis-NIR reflectance spectroscopy for the mineralogical characterization of synthetic gleys: a preliminary investigation

An investigation was carried out aiming at assessing the potential of vis-NIR reflectance spectroscopy, through comparison with the conventional X-ray powder diffraction (XRPD) technique, for the characterization of synthetic gleys, obtained in laboratory starting from Fe 0 and Fe 2+ under different conditions of initial pH (5.5, 7.0 and 8.5). XRPD analysis showed that in any case goethite formed, whereas magnetite and lepidocrocite developed in the less oxidised, and akaganeite and hematite in the more oxidised environments. Magnetite was found only starting from Fe 0 , i.e. in the less oxidative condition, whereas sporadic hematite was detected just in Fe 2+ synthetic gleys at 8.5 pH. Vis-NIR reflectance spectroscopy fully confirmed XRPD results. The features of reflectance spectra allowed (i) to discriminate Fe 2+ from Fe 0 synthetic gleys; (ii) to detect goethite and akaganeite in the high-reflectance spectra of Fe 2+ synthetic gleys; (iii) to correlate the very poor reflectance of Fe 0 synthetic gley spectra to magnetite; and, (iv) to identify some goethite and lepidocrocite features in Fe 0 synthetic gleys spectra. The analysis of second-derivative spectra also addressed to goethite, lepidocrocite and magnetite in Fe 0 synthetic gleys; and (ii) goethite, akaganeite and - at pH 8.5 - hematite in Fe 2+ synthetic gleys. The results obtained appear to be as original as significant, introducing vis-NIR reflectance spectroscopy as an innovative and promising effective method for the characterization of the soil gley forms, and highly stimulating for further investigations.