Clays and Clay Minerals, Vol.46. No. 5. 528-536, 1998. USE AND LIMITATIONS OF SECOND-DERIVATIVE DIFFUSE REFLECTANCE SPECTROSCOPY IN THE VISIBLE TO NEAR-INFRARED RANGE TO IDENTIFY AND QUANTIFY Fe OXIDE MINERALS IN SOILS A. C. SCHEINOST,]~ " A. CHAVERNAS, 2 V. BARRON 2 AND J. TORRENT 2 ~Agronomy Department, Purdue University, West Lafayette, Indiana 47907, USA 2Departamento de Ciencias y Recursos Agrfcolas y Forestales, Universidad de C6rdoba, Apdo. 3048, 14080 C6rdoba, Spain Abstract--We measured the visible to near-infrared (IR) spectra of 176 synthetic and natural samples of Fe oxides, oxyhydroxides and an oxyhydroxysulfate (here collectively called "Fe oxides"), and of 56 soil samples ranging widely in goethite/hematite and goethite/lepidocrocite ratios. The positions of the second-derivative minima, corresponding to crystal-field bands, varied substantially within each group of the Fe oxide minerals. Because of overlapping band positions, goethite, maghemite and schwertmannite could not be discriminated. Using the positions of the 4Tl<----6AI, 4T2<----6AI, (4E;4AI)4---6AI and the electron pair transition (4T~ h-4Ti)<----(6Ai q-6ml), at least 80% of the pure akaganeite, feroxyhite, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by discriminant functions. In soils containing mixtures of Fe oxides, however, only hematite and magnetite could be unequivocally discriminated from other Fe oxides. The characteristic features of hematite are the lower wavelengths of the 4"171 transition (848-906 nm) and the higher wavelengths of the electron pair transition (521-565 nm) as compared to the other Fe oxides (909-1022 nm and 479-499 nm, resp.). Magnetite could be identified by a unique band at 1500 nm due to Fe(II) to Fe(III) intervalence charge transfer. As the bands of goethite and hematite are well separated, the goethite/hematite ratio of soils not containing other Fe oxides could be reasonably predicted from the amplitude of the second-derivative bands. The detection limit of these 2 minerals in soils was below 5 g kg t, which is about 1 order of magnitude lower than the detection limit for routine X-ray diffraction (XRD) analysis. This low detection limit, and the little time and effort involved in the measurements, make second-derivative diffuse reflectance spectroscopy a practical means of routinely determining goethite and hematite contents in soils. The identification of other accessory Fe oxide min- erals in soils is, however, very restricted. Key Words---Crystal Field Bands, Diffuse Reflectance Spectroscopy, Goethite, Hematite, Intervalence Charge Transfer, Iron Oxides, Magnetite. INTRODUCTION Diffuse reflectance spectroscopy in the visible (400-700 nm) and extended visible range (400-1200 nm) has been used as an ancillary method to identify and semiquantitatively estimate Fe oxides in clays, soils and sediments (Cornell and Schwertmann 1996, and references therein). Although spectra of samples with unknown mineralogy may simply be compared with spectra of reference minerals (Singer 1982; Mor- ris et al. 1989), the parametrization of these spectra allows for a quantitative approach (Torrent and Barr6n 1993). The most frequently used procedures for the parametrization are: 1) Calculation of the tristimulus values (X, Y, Z) of the CIE color system (CIE 1978). The tristimulus val- ues are computed from the spectral reflectance and en- ergy of the light source for each wavelength, and can be converted, by means of appropriate formulas or graphs, to the parameters of other color systems, such I Present address: Department of Plant and Soil Science, University of Delaware, Newark, Delaware 19717-1303 USA. as Munsell or L*a*b* (Wyszecki and Stiles 1982). Several indices based on such parameters have been correlated, for instance, with the hematite content, pro- viding a semiquantitative estimation of this mineral in soils (Torrent et al. 1980; Barr6n and Torrent 1986; Nagano et al. 1992, 1994). 2) Application of the Kubelka-Munk theory. With this method, the reflectance values, R, are used to ob- tain the remission function F(R) = (1 - R)V(2R), which allows, in turn, calculation of the coefficients of absorption (K) and scattering (S) corresponding to the 2 phenomena involved in the sample-light inter- action process. It has been shown that K and S are additive in a mixture of pigments. Therefore, one can estimate the concentration of an Fe oxide in the mix- ture by knowing the color characteristics of each of the components of the mixture, and assuming a stan- dara r color for the Fe oxides investigated (Barr6n and Torrent 1986; Fernfmdez and Schulze 1992). 3) Calculation of derivatives of the reflectance or the remission function to derive the position and in- tensity of the absorption bands. These bands are pro- duced by crystal-field transitions of Fe(III) in an oc- Copyright 9 1998, The Clay Minerals Society 528