Laurent Caner

Unraveling complex <2 μm clay mineralogy from soils using X-ray diffraction profile modeling on particle-size sub-fractions: Implications for soil pedogenesis and reactivity

Abstract A specific methodology was developed to refine the complex clay mineralogy commonly encountered in soil environments. The soil examined was a Cambisol developed into a ferralitic paleosol. The sample was split into four sub-fractions of different particle sizes (<0.05, 0.05-0.1, 0.1-0.2, and 0.2-2 μm), and their respective mass contributions to the overall <2 μm clay fraction were determined. For each sub-fraction, X-ray diffraction (XRD) patterns were modeled using a trial-and-error approach based on the direct comparison of experimental and calculated profiles. Quantitative information derived from the fitting procedure for the different sub-fractions allowed for the determination of the complex mineralogy of the <2 μm clay fraction through the identification and quantification of eight clay phases. The results show that the finest and most reactive clay fraction (<0.05 μm) was totally hidden in the XRD pattern of the <2 μm fraction, the fraction commonly considered in soil mineralogical analyses. Similarly, this procedure revealed the presence of illite-smectite-chlorite and kaolinite-illite mixed-layer minerals seldom described in soil literature using classical methods. The use of this methodology improved our understanding of the pedogenesis of this soil through the identification and quantification of clay phases structural properties. The analysis of the evolution of structural parameters with particle size allowed for the detection of local modifications in the interlayer composition of expandable and hydroxy-interlayered vermiculite layers. Following this approach, key information can be derived to determine subtle changes in clay mineralogical composition that are related to microorganism and/or plant activity.

Combining visible-based-color parameters and geochemical tracers to improve sediment source discrimination and apportionment.

Parameter selection in fingerprinting studies are often time-consuming and costly because successful fingerprint properties are generally highly site-specific. Recently, spectroscopy has been applied to trace sediment origin as a rapid, less expensive, non-destructive and straightforward alternative. We show in this study the first attempt to combine both geochemical tracers and color parameters derived from the visible (VIS) spectrum in a single estimate of sediment source contribution. Moreover, we compared the discrimination power and source apportionment using VIS-based-color parameters and using the whole ultra-violet-visible (UV-VIS) spectrum in partial last square regression (PLSR) models. This study was carried out in a small (1.19 km(2)) rural catchment from southern Brazil. The sediment sources evaluated were crop fields, unpaved roads, and stream channels. Color parameters were only able to discriminate unpaved roads from the other sources, disabling its use to fingerprint sediment sources itself. Nonetheless, there was a great improvement in source discrimination combining geochemical tracers and color parameters. Unlike VIS-based-color parameters, the distances between sediment sources were always significantly different using the whole UV-VIS-spectrum. It indicates a loss of information and, consequently, loss of discriminating power when using VIS-based-color parameters instead of the whole UV-VIS spectrum. Overall, there was good agreement in source ascription obtained with geochemical tracers alone, geochemical tracers coupled with color parameters, and UV-VIS-PLSR models, and all of them indicate clearly that the main sediment source was the crop fields, corresponding to 57 ± 14, 48 ± 13, and 62 ± 18%, respectively. Prediction errors for UV-VIS-PLSR models (6.6 ± 1.1%) were very similar to those generated in a mixed linear model using geochemical tracers alone (6.4 ± 3.6%), but the combination of color parameters and geochemical tracers decreases the prediction error (5.4 ± 2.0%). Therefore, the use of VIS-based-color parameters combined to geochemical tracers can be a rapid and inexpensive way to improve source discrimination and precision of sediment source apportionment.

Occurrence of sombric-like subsurface A horizons in some andic soils of the Nilgiri Hills (Southern India) and their palaeoecological significance

This paper deals with four pedons (three Fulvudands and one Andic Haplorthox) located at high elevations (2000-2400 m a.s.l.) in the Nilgiri Hills on an old laterized surface of South India, and characterized by thick (50- 100 cm) epipedons whose colour tends to darken with depth. In two of them, the colour contrast between the top and the bottom A horizons was so well expressed that they were morphologically very similar to the African Oxisols reported to have sombric subsurface horizons. Soil organic matter (SOM) of the different selected A horizons was characterized by its mean residence time (MRT) and its delta(13)C value, whereas their humic acids (HAs) were examined by techniques that made it possible to determine their chemical composition and their chromatic properties. It was found that the darker colour of the subsurface A horizons of these soils was related to three joined characteristics: a SOM delta(13)C signature indicating a C4-type vegetation (i.e. a grassland-type) origin, the presence of humic acids belonging to the A-type (i.e. melanic) category of Kumada (Kurnada, K., 1987. Chemistry of soil organic matter. Developments in Soil Science, Elsevier, Tokyo), and large mean residence time values. In contrast, the top A horizons had a more recent SOM, mainly inherited from a C3-type vegetation, and humic acids (HAs) that were less condensed than the A-type HAs. It was also found that where the presence of sombric-like subsurface A horizons was the most evident, the vegetation change recorded by the soils (i.e. the change from a grassland-type vegetation towards a C3-type vegetation) was expressed on the thickest part of their epipedon. These observations led us to propose that the occurrence of sombric-like subsurface A horizons resulted from the following sequence of events: (i) an exceptionally thick accumulation of organic matter deriving from grassland followed by (ii) its progressive replacement by less dark organic matter derived from a C3-type vegetation. This proposal was found to be in reasonable agreement with the history of the vegetation changes experienced by the Nilgiri Hills since the end of the Pleistocene. On the other hand, it is also very similar to one of the hypotheses proposed by Van Wambeke (Van Wambeke, A., 1992. Soils of the Tropics-Properties and Appraisal. McGraw-Hill, New York) to explain the frequent occurrence of sombric horizons in the Oxisols and Ultisols of the high-altitude areas of Central Africa

Calibration of digital autoradiograph technique for quantifying rock porosity using 14C-PMMA method

Digital autoradiography (DA) based on phosphor plate imaging was tested for porosity quantification using 14C-PMMA method, and was compared to the classical film autoradiography (FA) method. Validation was undertaken by characterizing porosity evolution of moraine deposits of Lamarck granodiorite (eastern Sierra Nevada, USA). The connected porosities obtained with FA and DA are comparable to density measurements. Using adapted exposure times, DA is more accurate than FA for analysing large variations of porosity within a given sample, and is faster than FA.

Development of a fracture network in crystalline rocks during weathering: Study of Bishop Creek chronosequence using X-ray computed tomography and 14C-PMMA impregnation method

The progressive development of porosity during subsoil weathering of granodiorite clasts was studied at the Bishop Creek moraine chronosequence in east-central California. Fractures and other pores were examined using two complementary imaging techniques, X-ray computed tomography (XRCT) and a 14C-PMMA (14C-polymethylmethacrylate) method. The well-known XRCT method allows the investigation of three-dimensional (3-D) pore space. 14C-PMMA is a less-known method based on the complete impregnation of pore space with 14C-doped PMMA, and subsequent autoradiograph of a rock section. These imaging methods allow us to decipher the evolution of pore space in the granodiorite during the 120 k.y. weathering period. The 14C-PMMA imaging technique was found to be more suitable for following the evolution of the whole sequence, from “intact” bedrock to saprock, in terms of crack density, porosity, and aperture. Working with hand specimens, this method was adapted to detect both the low-aperture fractures (microcracks) and macrocracks. Only a slight and progressive increase in total fracture density was observed during the whole weathering period. However, this trend does not hold if macrocracks and microcracks are separated: Microcrack density slightly decreases, whereas macrocrack density increases due to a progressive expansion of microcracks. The total porosity of the rock increases during weathering and is correlated to the progressive aperture increase of all types of cracks. This evolution is accompanied by a change of crack morphology and connectivity, and an overall increase in intragranular porosity of biotite and plagioclase aggregates.

Pretreatment of Soil Samples Rich in Short-Range-Order Minerals Before Particle-Size Analysis by the Pipette Method

Abstract The possibilities of combining the dissolution of short-range-order minerals (SROMs) like allophane and imogolite, by ammonium oxalate and a particle size distribution analysis performed by the pipette method were investigated by tests on a soil sample from Reunion, a volcanic island located in the Indian Ocean, having a large SROMs content. The need to work with moist soil samples was again emphasized because the microaggregates formed during air-drying are resistant to the reagent. The SROM content increased, but irregularly, with the number of dissolutions by ammonium oxalate: 334 and 470 mg g −1 of SROMs were dissolved after one and three dissolutions respectively. Six successive dissolutions with ammonium oxalate on the same soil sample showed that 89% of the sum of oxides extracted by the 6 dissolutions were extracted by the first dissolution (mean 304 mg g −1 ). A compromise needs to be found between the total removal of SROMs by large quantities of ammonium oxalate and the preservation of clay minerals, which were unexpectedly dissolved by this reagent. These tests enabled a description of the clay assemblage of the soil (gibbsite, smectite, and traces of kaolinite) in an area where such information was lacking due to the difficulties encountered in recuperation of the clay fraction.

MINERALOGICAL CHARACTERIZATION OF Ni-BEARING SMECTITES FROM NIQUELÂNDIA, BRAZIL

Nickel-lateritic ore is the most common source of nickel in Brazil. The Niquelândia deposit, located in State of Goias, is one of the most famous deposits due to the large amounts of nickel associated with both oxidized and mainly silicated ores. The terms oxidized and silicated ores are used to specify two different ores formed exclusively by oxides and silicate (clay) minerals, respectively. The aim of the present study was to characterize thoroughly the silicated ore to identify the Ni-bearing clay minerals and their crystal chemistry in support of developing a better mineral-processing method or optimizing the current one to improve Ni recoveries. X-ray diffraction, chemical analyses, scanning electron microscopy, and Fourier transform infrared (FTIR) spectroscopy demonstrated that nickel is associated with Ni-rich stevensite and to a lesser extent with Fe-rich montmorillonite. The crystal chemistry performed by FTIR spectroscopy revealed that Ni is present in the octahedral positions, substituting for Mg or Fe, which results in significant chemical and layer-charge heterogeneity in the samples. This heterogeneity seems to be responsible for reduction in Ni recoveries during the hydrometallurgical process.