Dominique Righi

Geochemical differences in an Oxisol-Spodosol toposequence of Amazonia, Brazil

The morphology and geochemical characteristics of the soils of a toposequence near Manaus, Brazil, have been determined. Such toposequences occur on slopes leading down into valleys cut in a nearly level plateau. Oxisols form the upper and Spodosols the lower members of toposequences. The transition from the Oxisols down the slope through a band of Ultisols to the Spodosols is very gradual. Lowest members of the toposequences, Spodosols do not occupy the valley floors. n nGreat contrasts as well as shared characteristics occur among members of the toposequence chosen for study. Proportions of clay in the Oxisols are 80–90% as contrasted to 2–5% in the Spodosols. Changes in amounts of clay are gradual, continuing down the slope. The clay fraction is kaolinitic. All soils are strongly acid and have very low base saturation. n nThe characteristics of the soils imply that several processes of weathering and pedogenesis have been operative. The small amount of SiO2 in the Oxisols is attributed to dissolution of quartz without corresponding hydrolysis of kaolinite. The decreases in clay contents and increases in quartz in the soils from the upper to the lower end of the toposequence are attributed to a combination of hydrolytic weathering and either or both of clay eluviation and selective erosion of that fraction. The sequence of soils within the toposequences further suggests that a podzolization process begins when a critical stage in impoverishment is reached. Destruction of clay minerals continues but the migration of organo-mineral complexes within profiles becomes important. A part of those complexes moves out of the Spodosols to form black waters in the streams.

Characterization of hydroxy-interlayered vermiculite and illite/smectite interstatified minerals from the weathering of chlorite in a cryorthod

X-ray diffraction, FTIR, and chemical analyses were performed on clay fractions (1–2 µm, <0.1 µm), separated by means of size fractionations and high-gradient magnetic separation techniques, from a Cryorthod developed in a chlorite-mica schist saprolite. Weathering of large phyllosilicates pre-existing in the saprolite involves physical fragmentation and mineralogical transformations. Chloritic minerals in the coarse fractions were the most affected by physical breakdown, while micas were generally preserved. As a consequence, a concentration of mica layers occurred in the coarse clay fraction, while chloritic residues accumulated in the fine clays. These residues exhibited the typical XRD pattern of hydroxy-interlayered intergrade minerals, but the interlayered contaminants were found to be mainly hydroxy-Mg cations. Further mineralogical transformations of the intergrade minerals involved the progressive removal of the hydroxide interlayered sheet and dissolution of chloritic layers. Illite/smectite mixed-layers were formed in the surface horizon of the soil profile. These processes were associated with a strong decrease in Fe and Mg contents in the clay fractions.

MIXED-LAYER KAOLINITE-SMECTITE MINERALS IN A RED-BLACK SOIL SEQUENCE FROM BASALT IN SARDINIA (ITALY)

Clay minerals from soils of a red-black soil complex developed from basaltic parent material in Sardinia are formed along a short toposequence (200 m). At the foot of the sequence, a clay-rich, black Vertisol forms, whereas at the summit, the soil is a dark reddish-brown Inceptisol. X-ray diffraction, infrared spectroscopy (FTIR), cation exchange capacity (CEC) and permanent and variable charges analyses were used, and the data show that clay minerals varied according to soil horizon and topographic position of the soil. Clay minerals in the Inceptisol are dominated by kaolinite and mixed-layer kaolinitesmectite (K-S, K:S >0.5), whereas the Vertisol contains smectites and K-S with K:S proportions <0.5. In the Vertisol, the proportion of kaolinitic layers in the K-S increases from the C horizon (K:S ∼0.35–0.40) to the Ap horizon (K:S ∼0.40–0.45). This soil clay-mineral distribution, in relation to topography, is similar to that reported for other (kaolinitic) red-black (smectitic) soil associations in subtropical and tropical areas. The sequence forms by downward drainage on summits and slopes, and buildup of ions in ‘lows’ produces smectites. Fourier transform infrared spectra indicate that two types of smectite are formed in the C horizon of the Vertisol; one is more ferric (Fe-beidellite, nontronite), the other more aluminous. Mineralogical evolution in the soil profile (from C to Ap horizon) shows a decreasing proportion of ferric smectite layers (compared to the more aluminous smectite layers). This would indicate that ferric smectite layers are preferentially transformed (or dissolved) to give kaolinite layers, with Fe precipitating as oxides and/or oxy-hydroxides or retained partly in kaolinite layers. Because the surface properties of clay minerals are related to mineralogy, the CEC (33–41 cmol kg−1) in the brown Inceptisol is ∼50% pH-dependent charge while in the Vertisol up to ∼75% of the CEC (48–61 cmol kg−1) comes from accessible permanent charges.

Characterization of fulvic and humic acids from an Oxisol-Spodosol toposequence of Amazonia, Brazil

Fulvic and humic acids (FA and HA) extracted from the A1 and Bh horizons of an Oxisol-Spodosol of Amazonia were characterized by elemental analysis, nitrogen fractionation, and visible and IR spectroscopies. In Oxisols, FA are very aliphatic and have large contents of nitrogen, which is present essentially in labile forms (ammonia and amino acids). The corresponding HA are poorly aromatic, and are also rich in labile nitrogen forms. Going down the sequence to Spodosols, FA become less aliphatic and nitrogen rich. The degree of aromaticity of HA increases from Oxisols to Spodosols and total nitrogen tends to decrease; however, the proportion of non-hydrolysable nitrogen increases. These changes may be related to differences in clay content of the soils, vegetative cover and migration of organic compounds.

WEATHERING SEQUENCES OF ROCK-FORMING MINERALS IN A SERPENTINITE: INFLUENCE OF MICROSYSTEMS ON CLAY MINERALOGY

Under closed geochemical conditions, the weathering of a serpentinite rock composed of serpentine (70–85%) and magnesian chlorite (10–15%) associated with magnetite and chromite leads to the complete replacement of serpentine and chlorite by 2:1 layer silicates and produces new Fe oxides. The serpentine minerals crystallize under different habits issued from the serpentinization processes: mesh and hourglass pseudomorphic textures were formed from olivine, and thin-bladed pseudomorphic textures from pyroxene and amphibole crystals. Serpentine veins crosscut the whole rock with locally non-pseudomorphic interpenetrating and interlocking serpentines.Specific weathering microsystem habits with specific clay mineral crystallizations originate from these different habits: a poorly aluminous saponite in thin-bladed textures, two Fe-rich montmorillonites in mesh and hourglass (MH) textures, and in veins (V) which differentiate on Al, Mg and Fe contents. Magnesian chlorites, isolated from serpentine by hand-picking under a stereomicroscope, are found to weather to trioctahedral vermiculite. Magnetite and chromite extracted from the bulk samples are replaced by newly formed Fe oxides, maghemite, goethite and hematite, which give way to specific Fe accumulation habits in the regolith zone of the weathering profile.

Study of current soil-forming processes using bags of vermiculite and resins placed within soil horizons

Abstract We have tested an in situ experimental method in which a cationic exchange resin, a chelating resin and a test-mineral (vermiculite) were placed in permeable bags and inserted into various soil horizons. Later, the bags were removed and the contents analysed to characterize the currently active geochemical processes in forest acid soils (Humods, Orthods, Ultisols, Oxisols, Dystrochrepts). Two main geochemical and soil-forming processes were distinguished: simple acidolysis and acidocomplexolysis (podzolization) involving complexing organic acids. Simple acidolysis was revealed by the transformation of the test-mineral into an intergrade hydroxy-Al vermiculite, whereas adsorption of Al or Fe on the chelating resin was indicative of the presence of Al- or Fe-organic complexes, which is typical of podzolization. The processes observed in the in situ experiment varied from being completely consistent with the observed soil morphology to showing major inconsistencies with it. The active phase of acidocomplexolysis characteristic of podzolization was found to be active although the morphological results of that process were only slightly developed in the soil morphology. Conversely, although a strongly expressed spodic morphology was found, it was not accompanied by a currently active acidocomplexolysis process. Thus the spodic horizon appeared to be a relict feature. Seasonal interruptions of podzolization were also revealed.

Characterization of soil clay minerals; decomposition of X-ray diffraction diagrams and high-resolution electron microscopy

Fine clays (<0.1 μm) extracted from an acid soil developed in a granite saprolite from the Massif Central, France, were characterized by X-ray diffraction (XRD) using a curve decomposition program, and high-resolution transmission electron microscopy (HRTEM) associated with a method of impregnation of moist samples. Direct measurement of d-spacings were performed on HRTEM photographs. Decomposition of XRD patterns indicated 5 to 6 different clay phases including chlorite (and/or hydroxy-interlayered vermiculite), vermiculite/smectite, illite/vermiculite and illite/smectite mixed layers. Expandable phases with decreasing layer charge (vermiculite, high- and low-charge smectite) were shown in the clay assemblage. When performed on K-saturated samples subjected to wetting and drying cycles, HRTEM observations were consistent with the XRD results. The major clay mineral phases identified by the decomposition of XRD patterns were also found by direct measurement of d-spacings on HRTEM images. Vermiculite and high-charge smectite appeared to be impregnated with preservation of their initial hydration state, whereas low-charge smectite interlayers were penetrated by the resin molecules during the impregnation procedure. It was concluded that the decomposition of XRD patterns gave a realistic analysis of the clay phases present in a complex soil clay sample, as well as the direct measurement of a limited number (50) of clay crystals on HRTEM images.

Differences in the dehydration-rehydration behavior of halloysites : New evidence and interpretations

Two reference halloysites from New Zealand (Te Puke and Opotiki) were studied by X-ray diffraction under (1) various levels of relative humidity (RH) from 95 to 0% (dehydration), and (2) various temperatures increasing from 25 to 120°C (dehydration). They were also studied by differential thermal and thermogravimetric analyses at 40 and 0.2% RH. The impact of freeze drying along with the influence of cation saturation (Ca and K) on halloysite hydration were studied. The dehydration of the two halloysite samples upon decrease in RH started below 70% RH. However, the dehydration of Opotiki was still incomplete at ∼0% RH regardless of the saturation cation whereas Te Puke was completely dehydrated at ∼10% RH. For each sample, the decrease in RH and the increase in temperature induce similar dehydration behavior, but the dehydration processes of the Opotiki and Te Puke samples are different. The dehydration of Te Puke proceeds with one intermediate hydration state reacting as a separate phase due to the presence of ‘hole’ water molecules. The dehydration of the fully hydrated Opotiki halloysite gives a dehydrated phase and no 8.6 Å phase. The results suggest the presence of different types of water molecule, the ‘associated’ and the ‘hole’ water, controlling the dehydration behavior of halloysites. Freeze-dried halloysite samples are essentially dehydrated and the size of their coherent scattering domains is strongly reduced. Rehydration experiments performed after dehydration either at 95% RH or by immersing the sample in water for 3 months result in their partial rehydration. Calcium saturation promotes the rehydration process. The results suggest the presence of interlayer cations in the Opotiki sample, Ca ions being associated with the strongly held ‘hole’ water. As a result of this study, we assert that the (de)hydration behavior of halloysite is highly heterogeneous and cannot be generalized a priori.

Podzols in Amazonia

Summary In the area near Manaus, Amazonia (Brazil), soil sequences including Oxisols, Ultisols and Spodosols (podzols) are widely distributed. Such toposequences occur on slopes leading down into valleys cut in a nearly level plateau. The geochemistry of the organic and mineral components of one of these sequences has been investigated. Spodosols develop on quartzose sands which may be the results of an intense hydrolysis of clays and/or an impoverishment of clays through eluviation or selective erosion. Large analogies with podzolization in the temperate zone were found, especially in the organic matter dynamics: as the clay content decreases more organic compounds with increased aromaticity and more polycondensed structures were allowed to move, forming the Bh horizons or reaching the water streams. An in situ experimentation was also performed to characterize presently active soil forming processes. It was found that podzolization in Amazonia was mainly active during the rainy season.

PEDOGENIC SMECTITES IN PODZOLS FROM CENTRAL FINLAND: AN ANALYTICAL ELECTRON MICROSCOPY STUDY

Transmission electron microscopy (TEM) including high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) were used to study the fine clay fraction (<0.1 μm) from the eluvial E horizon of podzols located in central Finland that had developed from till materials. Soils of increasing age (6500-9850 y BP) were selected to represent a chronosequence of soil development. Expandable phyllosilicates (vermiculite, smectites) are formed in the eluvial E horizon of podzols in a short time (6500 y). TEM observations show that dissolution and physical-breakdown processes affect the clay particles. As the age of the soils increases, fragmentation and exfoliation of large precursor minerals lead to thinner clay particles of two to three layers thick. The chemical compositions of individual particles obtained by AEM indicate that expandable phyllosilicates from the E horizon of podzols are heterogeneous, involving a mixture of vermiculite, Mg-bearing smectites, and aluminous beidellite. Results suggest that heterogeneity is related to the nature of their precursors. Vermiculite and Mg-bearing smectites are derived from biotite and chlorite weathering whereas phengitic micas alter to aluminous beidellite. Because the transformation of biotite and chlorite is more rapid than phengitic micas, biotite and chlorite contributes predominantly to smectites in the younger soils, as long as ferromagnesian phyllosilicates are present in the E horizons. If not, a larger proportion of smectites is derived from phengitic micas in the older soils. Direct measurement of d(001 ) values on lattice fringe images from alkylammonium-saturated samples shows that interlayer charge varies from high-charge expandable minerals (0.6–0.75 per half unit cell) in the younger soils to 0.5–0.6 per half unit cell in the oldest soils. Thus, the proportion of the components in the clay assemblage, as well as their chemistry and interlayer charge, change over time with soil evolution.