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---6A I 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.

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

Active sand seas and the formation of desert loess

https://horizon.documentation.ird.fr/exl-doc/pleins_textes/pleins_textes_7/b_fdi_51-52/010015957.pdf

Relationships between Satellite-Based Radiometric Indices Simulated Using Laboratory Reflectance Data and Typic Soil Color of an Arid Environment

Soil and environmental issues in sandy soils

Previo us investigations of goethite revealed a substantial variation of color and diffuse reflec- tance spectxa (DRS) in the extended visible range (350-2200 nm). To better understand the causes of this variability and to assess the potential of DRS as a mineralogical tool, we investigated the DRS of pure and Al-substituted goethite, c~-Fej_~AI~OOH with x from 0 to 0.33, and mean crystal lengths (MCL) from 170 to 1800 nm. The strongly overlapping ligand field bands were extracted by fitting the single-electron transitions 6A l --~ 4Tt, 6A 1 ----) 4T2, 6A 1 ~ (4E; hA1), and 6A I --~ 4E(~D) as functions of the ligand field splitting energy, 10 Dq, and the interelectronic repulsion parameters, Racah-B and -C. With x increasing from 0 to 0.33, 6A 1 ---)' aT / decreased from 10,590 to 10,150 cm -1 (944 to 958 nm), and 6A l ~ 4T 2 decreased from 15,310 to 14,880 cm 1 (653 to 672 nm), while 10 Dq increased from 15,770 to 16,220 cm -1. From the change of 10 Dq we calculated a decrease of the Fe-(O,OH) distances from 202.0 to 200.9 pm (-0.5%). This decrease is smaller than the average decrease of all (A1,Fe)-(O,OH) distances (- 1.8%) calculated from the change of the unit-cell lengths (UCL). That is, there remains a substantial difference in size between the larger Fe- and the smaller Al-occupied octahedra in the solid solution which may indicate the existence of diaspore clusters within the goethite structure. The increasing strain in the crystal structure due to the size mismatch and limited contractibility of the oxygen cage around Fe may be the primary reason for AI substitution being restricted to x < 0.33. The bands 6A l ~ (4E; 4Aa) and 6A 1 ~ 4E(4D) did not shift, indicating a constant covalency of the Fe-(O,OH) bonds with B = 628 cm ~ and C = 5.5B. Whereas variation of band energies could be explained in terms of the Fe-(O,OH) ligand field, the variation of color and band intensities was mainly determined by crystal size. Although our study confirmed the potential of DRS for mineralogical investigations, there is still a gap between the fundamental theory and the explanation of some spectral features.

Diffuse reflectance spectra of Al substituted goethite; a ligand field approach

Summary

In semi-arid tropics soil hardening may reduce both water infiltrability and biological activity, thus inducing the development of large almost bare areas. A sandy-loam soil with contrasting loose and underlying hard horizons was studied in the southern plain of the Chad basin. The fabrics of these horizons were studied using combined sieving and sedimentation techniques, mercury porosimetry and scanning electron microscopy. The horizons had similar particle size distributions of the skeleton grains. The hard horizon differs by a small increase in its fine clay (< 0·2 μm) content. The hardness is closely related to a fabric with clay coatings on the skeleton grains and clay wall-shaped bridges linking the latter. This induces a strong continuity of the solid phase. This fabric requires a minimum of clay content (6%) to make the coatings and the wail-shaped bridges, and it can be 30% less porous than the loose horizon, without any change in the packing of the skeleton grains. These characteristics of the fabric of the hard horizon are like those of fragipans elsewhere. The continuity of the solid phase, from the microscopic to the macroscopic scale, as well as the absence of a network of cracks explains the considerable strength of the hard horizon, and consequently the difficulties for water infiltration, root penetration and tillage.


Resume

Dans les regions tropicales semi-arides, le durcissement des sols limite I'infiltration de I'eau et I'activite biologique, provoquant ainsi le developpement de grandes etendues presque desertiques. Un sol sablo-limoneux presentant un horizon meuble au-dessus d'un horizon a forte cohksion a ete selectionne dans le bassin tchadien. L'etude de ces horizons a ete realisee par granulometrie, porosimktrie au mercure et microscopie electronique 2 balayage. Les deux horizons posskdent une meme distribution de taille des grains du squelette. En revanche, I'horizon a forte cohesion se distingue par une faible augmentation de la teneur en argile fine (<0·2 pm) et par le mode d'assemblage de ses constituants eIementaires. Les grains du squelette sont revetus d'argile et relies par des cloisons argileuses. Une forte continuite de la phase solide est ainsi obtenue avec un minimum d'argile (6%). La porositC de I'horizon a forte cohesion est de 30% infkrieure celle de I'horizon meuble, sans diffkrence dans I'assemblage des grains du squelette. Les caractkristiques de I'assemblage des constituants elementaires des sols etudies prksentent de nombreuses similarite avec celles decrites pour des fragipans. Enfin, en I'absence de reseau fissural, la continuite de la phase solide explique la forte cohesion, et consecutivement les difficultks d'infiltration de I'eau, de penetration racinaire et de travail du sol.

https://www.researchgate.net/profile/Bruand_A/publication/227966974_A_hard_sandy-loam_soil_from_semi-arid_Northern_Cameroon_I_Fabric_of_the_groundmass/links/00b49525575fa1c338000000.pdf

A hard sandy‐loam soil from semi‐arid Northern Cameroon: I. Fabric of the groundmass

Summary
Second derivative diffuse reflectance spectroscopy (DRS) in the visible range has been used to characterize changes in colour and identify the nature of Fe oxides which withstand reduction during experimental yellowing of reddish materials. It is accepted that haematite dissolves preferentially and faster than goethite, and Al-substitution controls the dissolution kinetics of Fe oxides. However, DRS has shown that haematite is more resistant than predicted and that some Fe-oxides, probably trapped within kaolinite particles, are inaccessible to solvents. DRS allows the nature of dissolved phases at each deferration step to be determined and changes in Al-content of residual phases throughout deferration to be followed. It also demonstrated that Helmholtz coordinates correlate very well with changes in Fe-oxide mineralogy and are preferable to redness ratings when monitoring differential dissolution of Fe oxides through colour measurements. DRS is a powerful and sensitive technique for monitoring the dissolution of Fe oxides in soils.

A. J. Herbillon

Papers

A hard sandy‐loam soil from semi‐arid Northern Cameroon: I. Fabric of the groundmass

Spectroscopic control of iron oxide dissolution in two ferralitic soils