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

Abstract

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 Fe⁰ and Fe²⁺ 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 akaganéite and hematite in the more oxidised environments. Magnetite was found only starting from Fe⁰, i.e. in the less oxidative condition, whereas sporadic hematite was detected just in Fe²⁺ synthetic gleys at 8.5 pH. Vis-NIR reflectance spectroscopy fully confirmed XRPD results. The features of reflectance spectra allowed (i) to discriminate Fe²⁺ from Fe⁰ synthetic gleys; (ii) to detect goethite and akaganéite in the high-reflectance spectra of Fe2+  synthetic gleys; (iii) to correlate the very poor reflectance of Fe0  synthetic gley spectra to magnetite; and, (iv) to identify some goethite and lepidocrocite features in Fe⁰ synthetic gleys spectra. The analysis of second-derivative spectra also addressed to goethite, lepidocrocite and magnetite in Fe⁰ synthetic gleys; and (ii) goethite, akaganéite and - at pH 8.5 - hematite in Fe²⁺ 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.