The clay mineral content of the Oligocene sediments in the Linxia Basin has been investigated using X-ray diffraction, scanning electron microscopy, and high-resolution transmission electron microscopy. The clay mineral assemblages are mainly mixed-layer illite-smectite (I-S), illite, kaolinite and minor palygorskite in the early-Middle Oligocene deposits, mixed-layer I-S, illite and kaolinite in the Middle Oligocene deposits, mainly illite and chlorite (usually >50–70 vol.%), mixed-layer I-S, and trace to minor palygorskite in the late Oligocene sediments, respectively. The mineral assemblage indicates a warm and seasonally humid climate in the Middle Oligocene, with an episode of warm and dry conditions in the early stage of the Middle Oligocene, and a trend of temperature decrease and more arid conditions in the late Oligocene. Climate evolution in the Oligocene corresponds with the significant elevation change in central Tibet since late Oligocene, and therefore, suggests that tectonic-forced cooling of climate took place in Linxia in the northeast margin of the Tibetan Plateau. The ubiquitous mixed-layer I-S and carbonates throughout the Oligocene sediments reflect relatively small fluctuations in climate conditions during the epoch. The changes in clay mineral components and feldspars in the late Oligocene suggest a variation in the source of clastic materials, which probably reflects an increase in erosion of soils and poorly weathered parent rock in more elevated or high-relief source areas during this period of tectonic uplift.

Amino acids are ubiquitous in the Earth’s surface environments as reactive biological molecules produced by every living thing including bacteria. To evaluate the effects of amino acids on mineral dissolution and to reveal the mechanism by which they interact with the mineral surface, we performed dissolution experiments of X-ray amorphous silica in solution containing 0.1 mmol Na with 10.0 mmol amino acids such as cysteine, asparagine, serine, tryptophan, alanine, threonine, histidine, lysine and arginine in near-neutral solutions. Dissolution experiments in solutions of 0.1, 1.0 and 10.0 mmol NaCl without amino acids were also conducted as amino acid-free controls. The results of this study indicate that basic amino acids such as histidine, lysine and arginine can interact more strongly with the negatively charged surface of amorphous silica than other non-basic amino acids due to their greater dissociation, thus forming cationic species. This electrostatical interaction enhanced dissolution rates of amorphous silica by approximately one order of magnitude compared with amino acid-free controls. In contrast, no significant effect on the dissolution rates of amorphous silica was observed in solutions containing cysteine, asparagine, serine, tryptophan, alanine and threonine because of lesser interaction with the surface of amorphous silica.

Rapid dissolution of partly amorphized kaolinite in the systems kaolinite + NiCl2, kaolinite + Ni(OH)2, and kaolinite + NiCl2 + Ni(OH)2, at a temperature of 200°C and at pH between 5.3 and 7.4, leads to the precipitation of Ni-poor kaolinite, Ni-rich kaolinite and Al-Ni-serpentine. Identification of the phases was carried out using a combination of X-ray diffraction and transmission/analytical electron microscopy. Ni-bearing kaolinite shows variable morphologies in the systems studied: stacks of kaolinite with relatively small Ni contents and fine-grained curved particles of Ni-rich kaolinite dominate in the Cl-bearing system; spherical particles with a disordered structure and relatively uniform Ni contents (in the order of 0.15 atoms per formula unit (a.p.f.u.)) and platy particles of Al-Ni-serpentine characterize the products formed in the Ni(OH)2-richest systems. The presence of Ni(OH)2 in the systems (with and without Cl) favors the dissolution process as well as rapid precipitation of spherical particles, and the formation of serpentine. A difference from Mg systems studied previously is a well defined phase intermediate in composition between kaolinite and serpentine which originated in the Ni-bearing systems. Increasing Ni content is clearly reflected in the parallel increase in the b cell parameter of kaolinite. The average composition of the coexisting Al-Ni-serpentine is: (Al1.24Ti0.01Fe0.02Ni1.31) (Si1.58Al0.42)O5(OH,Cl)2.

In the Nuussuaq Basin, West Greenland, a thick succession of Tertiary dolerites has penetrated Upper Cretaceous mudstone. The mixed-layer minerals of mudstone core samples have been analyzed by X-ray diffraction, solid-state 29Si and 27A1 magic-angle spinning nuclear magnetic resonance, Mössbauer and infrared spectroscopies, thermal analysis, chemical analysis, stable isotopes (18O/16O), and K/Ar dating. The mixed-layer minerals include for each sample two mixed-layer phases consisting of pyrophyllite, margarite, paragonite, tobelite, illite, smectite and vermiculite layers. The main, 80 m thick intrusion resulted in the formation of pyrophyllite, margarite, paragonite and tobelite layers. However, the tobelite layers are absent in samples <21 m from this intrusion. Furthermore, chlorite was formed and kaolinite destroyed in samples adjacent to minor intrusions and at distances <60 m from the large intrusion. For the first time, the detailed, complex mixed-layer structures formed during contact metamorphism of kaolinitic, oil-forming mudstones have been investigated accurately. The formation of tobelite layers reveals that oil formation has taken place during contact metamorphism. Furthermore, K/Ar dating of mixed-layer minerals from shale indicates that the intrusives are of early Eocene age. The 80 m thick intrusive is responsible for the main mixed-layer transformations, whereas two thin (3 m and 0.5 m thick) intrusions contribute little. Thus, the detailed mixed-layer investigation has contributed significantly to the understanding of the regional geology and the contact metamorphic processes.

Temperature-dependent clay mineral assemblages and vitrinite reflectance data have been used to investigate levels of diagenesis from the Messinian Laga Basin in the Central Apennines developed at the footwall of the Sibillini Mts. and the Gran Sasso Massif. Data are from stratigraphic units forming the main siliciclastic basin fill up to Middle Messinian gypsum-arenites and its pre-orogenic substratum. Specifically, the largest Rom% values and percentages of illite layers in illite-smectie (I-S) are found in the basin depocenter and at the footwall of the main carbonate thrust sheets. Smaller Rom% values, and percentage of illite layers in I-S characterize less subsided sectors surrounding the depocenter.

The X-ray diffraction data were treated using decomposition methods and the peaks identified were rationalized in terms of discrete and/or mixed-layer phases. Complex clay mineral assemblages were found in the Laga Fm. including three sub-populations of illitic material corresponding to authigenic and detrital components. I-S mixed layers record the maximum paleotemperature the Laga Fm. experienced, which is directly related to its burial history. Kübler index (KI) data, however, suggest higher temperatures related to detrital K-micas inherited from the uplift of the Alpine-Apennines chain.

A tentative calculation of paleotemperatures from selected data of organic and inorganic parameters is also proposed and compared with recent sedimentological, stratigraphic and structural data. We conclude that the Laga Basin fill never experienced temperatures of >100–110°C, generally due to variable sedimentary loading, whereas localized anomalous heating is due to the effect of the tectonic emplacement and subsequent local erosion of the Sibillini and Gran Sasso thrust sheets.

A new occurrence of berthierine has been found in the Pulsifer granitic pegmatite, near Auburn, Maine. Berthierine was found in miarolitic cavities (‘pockets’) as aggregates of radial platy crystals with albite, as fracture-fillings in microcline and as fine-grained anhedral flakes with muscovite. Berthierine samples from all associations are depleted in Mg, but show variable enrichment in Mn. The textural relationships of the berthierine assemblage indicate that its formation is probably related to the dissolution of microcline in the presence of a low-temperature, neutral to alkaline, Fe-rich hydrothermal solution. This mode of occurrence is in direct contrast to the typical formation of berthierine in granitic pegmatites that is generally related to the alteration of cordierite-group minerals.

The formation of hard hematite in steam generators with relatively high levels (5–10 µg/L) of dissolved oxygen at temperatures around 280–290°C and pressures around 6–8 MPa can serve as an analog for the formation of hard hematite in sedimentary processes. Furthermore, in steam generators, as well as in nature, hematite is an effective cementing agent, capable of incorporating as much as twice its own weight of other solids to form a hard composite material. Laboratory simulations showed ferrihydrite to be the likely starting material for the formation of hard, dense hematite at temperatures much lower than those required for sintering of anhydrous hematite. These laboratory simulations, performed at temperatures around 260°C and pressures of ∼500 MPa, resulted in the formation of hard hematite or hematite-based composite solids over periods of 3–5 h, compared with several months in steam generators and many years in nature. The amount of water present during the synthesis (10–15% of the weight of dry ferrihydrite) and the gradual removal of water proved to be key parameters in the formation of hard, dense hematite. The mechanism, studied by means of X-ray diffractometry, Mössbauer spectroscopy and infrared spectroscopy, appeared to involve build-up, then gradual condensation of OH bridges, leading to the conversion of ferrihydrite to hydrohematite with approximately 4–5% of residual water. The presence of other solids, such as copper and its oxides, alumina and silica, in large quantities, resulted in smaller grain size of the hydrohematite product but did not affect its mechanical properties. On the other hand, the use of hydrazine to provide a reducing environment produced goethite during the precursor synthesis stage and soft magnetite during the pressing stage. However, whenever hematite was produced, it could not be subsequently reduced to magnetite by hydrazine under the reaction conditions specified above. The mechanical properties as well as the spectroscopic characteristics of the product of pressing agreed with observations on sedimentary hematite-cemented rocks.

Naturally deformed biotite in contact-metamorphosed slates affected by a shear zone of the Southern Iberian Massif near Jaén (SE Spain) were studied by X-ray diffraction, optical microscopy, scanning electron microscopy, electron probe microanalysis and high-resolution transmission and analytical electron microscopy. Biotite is found in the contact metamorphism aureole produced by the intrusion of a granodioritic stock, but shear strain caused its deformation. The southern part of the shear band is strongly deformed, containing thick clay gouge zones. The northern part is less deformed and develops weaker planar-linear fabrics. X-ray diffraction data reveal the predominance of the 2M1 biotite in the undeformed samples whereas the 1M polytype is predominant in the sheared samples. Chemical data and electron images of the biotite from unsheared slates do not show the presence of intercalated phases. This biotite is almost defect-free and electron diffraction patterns have sharp reflections indicating a two-layer polytype (probably 2M1). Back-scattered electron images from the deformed biotite in the moderate deformation part of the shear zone do not reveal intergrown minerals, but the electron microprobe analyses show some Fe- and Mg-enriched compositions. Transmission electron microscopy indicates that disordered polytype packets are predominant (probably 1Md). Their electron diffraction patterns have diffuse streaking along c*. These packets have high dislocation densities, microcavities with ∼5 Å latticefringe regions (probably brucite-like sheets) and interlayering of chlorite-berthierine. Kaolinized biotite can be observed in the clay gouges from the strongly deformed south part of the shear zone. The degree of streaking, as an indication of the intensity of deformation, revealed that the disordered polytype packets are more deformed than the two-layer polytype packets. The microcavities of the disordered polytype packets could act as potential channels for transport of fluids during the shearing stage and serve as sites for chloritization of biotite, producing chlorite-berthierine domains within biotite. Berthierine is an intermediate metastable phase replaced by chlorite with along-layer transitions.

The influence of organics on the crystallization of Al precipitates has been well documented. However, the effects of organics and ageing on the transformation and structural configuration of Al precipitates in relation to their surface and charge properties are not fully understood. This study investigated the structural, microporous and surface properties of Al precipitates formed under the influence of tannate and ageing. The Al precipitates were synthesized at an initial Al concentration of 7 × 10−3 M, an OH/Al molar ratio (MR) of 3.0, and initial tannate/Al MRs of 0, 0.001, 0.01 and 0.1, and aged for 1, 10 and 40 days. As indicated by a decrease in gibbsite and bayerite and an increase in the oxalate-extractable Al contents, the non-crystalline precipitates increased with the increase of the initial tannate/Al MR. This observation is in accord with the X-ray diffraction and Fourier transform infrared (FTIR) data. The impact of tannic acid on the nature of the Al precipitates is also reflected in the increase of the contents of the pyrophosphate-extractable Al, which is indicative of organically bound Al. This observation is in agreement with the increase in the intensity of characteristic FTIR absorption bands of tannate and the organic C and adsorbed water contents. The decrease in the crystallinity of Al precipitates with increase in the tannate/Al MR resulted in the development of microporosity, increase in BET specific surface area and decrease of the average pore diameter and point of zero salt effect (PZSE). The FTIR absorption bands characteristic of tannate of the Al precipitates became weaker with ageing, in accord with the ageing-induced decrease in the contents of organic C and pyrophosphate-extractable Al. Ageing drastically decreased the BET specific surface area of the Al precipitates formed in the absence of tannate but this effect was less conspicuous for the products formed at the tannate/Al molar ratio of 0.1. The ageing-induced change in the PZSE of the Al precipitates formed both in the absence and presence of tannate was not significant. The results accomplished in this study are of fundamental significance to our understanding of the combined effects of organics and ageing on structural configuration of hydrolytic precipitates of Al in relation to their microporosity, surface and charge properties in the environment.

X-ray diffraction has been used to study the sorption of water by multilayer films containing ordered stacks of alternating layers of an organic polyelectrolyte and individual platelets of a smectite. The position and shape of the 001 reflections due to the ordered polyelectrolyte/platelet stacks were essentially the same when in air at low relative humidity (23–25% RH) and under water. These data indicate that swelling occurs exclusively in X-ray amorphous regions within the film, and that the ordered polyelectrolyte/clay domains themselves are not ‘swellable’ by water.

As the water content is increased, the consistency of a fine-grained soil changes from a semi-solid state to a plastic state and finally to a liquid state. The plastic limit (PL) is the point at which the consistency, caused by the soil water content, is transformed from a semi-solid state to a plastic state. The liquid limit (LL) is the point at which the consistency is transformed from a plastic state to a liquid state. The plastic limit and liquid limit are often collectively referred to as the Atterberg (or consistency) Limits. Although the liquid and plastic limits are easily determined, fundamental interpretations of the limits and quantitative relationships between their values and compositional factors are more complex. Previous studies examined artificially-prepared soil samples that contained monomineralic clays and a non-clay substance(quartz sand). These studies have shown that in soils without expandable clays the PL and LL water contents were mostly related to surface area and clay content. For soils that contain expandable clays, the PL and LL values are also dependent on interlayer water content. Hence, expandable clay mineral contents are needed to calculate PL and LL values. These relationships have been presented in a general analytical form. The aim of these investigations was to identify practical applications. Mineral compositions and surface areas of five randomly selected natural soil samples were used to estimate PL and LL values. The estimated values were compared to experimentally measured liquid limits (by the ‘fall-cone’ test) and plastic limit (by the ‘rolling thread’ test) values. The measured PL values ranged from 18.77 to 44.92% and the LL values from 31.19 to 82.10%. The differences between estimated and measured Atterberg Limits were 3.0–7.1% for thePL and 2.7–7.8% for the LL. Minor differences in measured and estimated Atterberg Limits were probably due to soil organic matter (1.2–2.7%).

The alteration of spodumene to cookeite has been identified in a Li-bearing aplite-pegmatite from northern Portugal. Optical microscopy and scanning and transmission electron microscopy (SEM/TEM) were used to characterize the alteration products in both cookeite + quartz and cookeite + kaolinite ± mica assemblages. Mutual relationships between the minerals were assessed using back-scattered electron imaging. The first assemblage occurs along the cleavage planes of spodumene, whereas the second forms as the result of spodumene breakdown. Fine mica grains surrounded the cookeite aggregates in the second assemblage only. Precipitations of cookeite and quartz in open pore spaces and parallel packets of cookeite and kaolinite were identified by TEM. Selected area electron diffraction carried out on cookeite identified a layer-stacking sequence from highly disordered to one-layer order-disorder with a lesser participation of two-layer polytypes. The thickness of one-layer cookeite packets averaged ∼85 −100 nm. Disordered cookeite has a mean thickness of 450 nm. The chemistry of cookeite was analyzed by both electron and ionmicroprobe techniques. The structural formula of cookeite from the first assemblage corresponds to: Al2.0(Si3.23Al0.766)O10(OH)2(Al1.80Li1.166Mg0.004Fe0.09)(OH)6. Two distinct compositional varieties were found: one corresponding to ideal cookeite, whereas the second belongs to the cookeite-donbassite join.

The thermodynamic stability field for the observed minerals was calculated using the SUPCRT® programme. Two distinct stages of cookeite crystallization are discussed. The lower pressure and temperature stability of the reaction of spodumene to cookeite + quartz were estimated at ∼2.4 kbar and 240°C. The cookeite + kaolinite ± mica assemblage is still in equilibrium with quartz at ∼2.2 kbar and 220°C. The lower limit for the stability of cookeite was found at 205°C and 2 kbar.

Two major species were identified in Fe-treated montmorillonite: monomeric or dimeric hydroxoaqua cations $\text{Fe}{(\text{OH})}_x^{(3-x)+}$${\rm{Fe}}({\rm{OH}})_x^{(3 - x) + }$ (form I), and polymeric structures with edge-shared Fe(O,OH)6 (form II). These species have different electron spectra (absorption maximum is 29,600 cm−1 in form I, and 26,000 and 28,000 cm−1 in form II), chemical and thermal stability, and electrochemical behavior. Form I behaves as a partly exchangeable cation in interaction with Cu2+ from Cu-trien solution and Ni2+ from Ni-EDTA, that can be used for selective quantitative analysis. On heating above the dehydration temperature (∼100–150°C) montmorillonite with Fe3+ in form I is converted to a mica-like structure and Fe3+ ions are fixed more strongly in the montmorillonite structure. Form II behaves similarly to hydrous ferric oxides, but its thermal crystallization to hematite is postponed to ∼500–600°C. The Fe3+ cations in the interlayer space are much less thermally stable than Al pillars in pillared interlayered clays (PILCs). Form I is more active in oxidative dehydrogenation of propane, while form II is the active species in sorption of As and the non-specific combustion of propane. To produce only form II by the treatment of montmorillonite with Fe3+, its load must be kept below ∼20 wt.%; otherwise the usual hydrous ferric oxides are formed.

The formation of hydroxylated phases was investigated using K-depleted biotite (Na-biotite) and K-depleted muscovite (Na-muscovite) under hydrothermal treatment with alkali (Li+, K+, NH4+, Rb+ and Cs+), alkaline earth (Mg2+, Ca2+, Sr2+ and Ba2+), and aluminum (Al3+) cations at 200°C for 1 and/or 3 days. The K-depleted biotite treated with alkali cations produced anhydrous hydroxylated phases, while the K-depleted muscovite did not significantly exchange alkali cations but dehydrated to form Na-muscovite in all cases. The alkaline earth cations, however, produced hydrous hydroxylated phases with both K-depleted micas. The degree of hydration energy of cations and the charge density of micas were found to influence the formation of anhydrous and hydrous phases from the K-depleted micas. This type of topotactic cation exchange potentially could be used for fixation and immobilization of radioactive species such as Cs, Sr, Ra, etc. in the transformed micas. The K-depleted biotite and muscovite treated with Al3+ were transformed to hydroxy-Al interlayered vermiculites (HIV) because of hydrolysis and polymerization of Al3+. These HIV phases could also serve as useful adsorbents for soil and groundwater contaminants.

The citrate-induced desorption kinetics of pre-adsorbed Se from montmorillonite (Mt) and its complexes with hydroxyaluminum (HyA-Mt) and hydroxyaluminosilicate (HAS-Mt) were studied. The mole fraction of pre-adsorbed Se released in 24 h from different clays followed the trends of Mt>> HAS-Mt> HyA-Mt with a significant increase with elevated citrate concentration in the desorbing solution. In contrast, the amount of Se adsorbed per unit mass of different clays followed a clear-cut opposite trend of HyA-Mt>HAS-Mt>>Mt. The Se desorption kinetics in different systems indicated multiple rate characteristics; where an initial fast reaction (0.25–2 h) was followed by a slow reaction (2–16 h). Of the six different kinetic models tested (zero-, first-, and second-order; power function, Elovich, and parabolic diffusion), the second-order rate equation showed the overall best fit to the fast and slow desorption kinetic data from the clays. Based on second-order rate constants, the rates of the mole fraction of Se desorption by citrate from different clay systems at 298 K followed the order Mt > HAS-Mt >> HyA-Mt. For both fast and slow reaction, the rates of desorption increased proportionally with the level of citrate. Replicate experiments conducted across a range of temperature (288–318 K) yielded Arrhenius parameters that followed the order HyA-Mt > HAS-Mt >> Mt. Considering that a lower mole fraction of Se desorption after a particular reaction period, slower desorption kinetics, and a greater activation energy of desorption are the indices of increased adsorption bond strength, the results clearly indicate that HyA- and HAS-interlayering and coatings on Mt not only augmented its Se adsorption affinity and capacity, but also increased the adsorption bond strength. Silication in HyA not only reduced the Se adsorption capacity, but also weakened the adsorption bond strength. This establishes a significant role of HyA/HAS-interlayering and coating on Mt in influencing the rate of citrate-induced release of Se. Reduction in surface-positive potential following citrate adsorption on the clay surface, a direct ligand exchange between Se and citrate, and structural dissolution are possible mechanisms responsible for citrate-induced Se desorption in the present study.

Hydroxy-interlayered minerals (HIMs) are typical of moderately acidic soils. Barnhisel and Bertsch (1989) defined the hydroxy-interlayered clay minerals as a solid-solution series between smectite, vermiculite and pedogenic or aluminous chlorite end-members. Their experimental data for the relationship between the decrease in cation exchange capacity (CEC) and the amount of Al fixed in the interlayers of smectites and vermiculites is reinterpreted using calculated structural, chemical and X-ray diffraction (XRD) evidence. The adsorbed Al ions are in a 6-fold coordination state: [Al(OH)x(H2O)y](3−x)+ with x+y = 6. The polymerization process occurs before saturation of the exchange sites by Al ions. Some of the adsorbed Al ions form polynuclear cations keeping a constant positive charge.

X-ray diffraction patterns of oriented preparations in the ethylene glycol-solvated state suggest that HIMs consist of randomly interstratified expandable and chlorite-like layers (17 and 14.2 Å). Chlorite-like layers result from the selective adsorption of Al complex ions in specific interlayer zones that behave similarly to Al-chlorite (donbassite-like) with incomplete (60%) ‘gibbsite-like’ sheets (chlorite60). Using this framework, HIM XRD patterns can be interpreted by comparison with calculated chlorite60-dismectite mixed-layer mineral patternss using the NEWMOD software.

We have investigated the adsorption of adenine, adenosine, ribose, and adenosine-5′-phosphate (5′-AMP) by allophane at pH 4, 6 and 8. Adenine, adenosine and ribose gave similar isotherms, i.e. adsorption increased regularly with solution concentration and decreased in the order: pH 8 > pH 6 > pH 4. Allophane had a greater affinity for 5′-AMP than for adenine, adenosine or ribose. Further, the extent of adsorption for 5′-AMP increased in the order: pH 8 ≪ pH 6 ≈ pH 4. The adsorption of 5′-AMP at pH 4 and pH 6 was about 60 times greater than at pH 8. The strong adsorption of 5′-AMP accords with the well known high phosphate-retention capacity of allophane and allophane-rich soils. The experimental data may be rationalized in terms of the pH-dependent charge characteristics of the organic solutes and allophane. The large propensity of allophane to retain 5′-AMP is ascribed to ligand exchange between the phosphate of 5′-AMP and the hydroxyl of (HO)Al(OH2) groups, exposed at perforations on the wall of allophane spherules, giving rise to a surface (chelation) complex. The high affinity of nucleotides for allophane has implications for the possible role of allophane in the abiotic formation of RNA-type polynucleotides although nucleotide ‘immobilization’ by surface complexation might hinder RNA oligomerization.

The effect of FeCl3 in acetonic medium on the structure of Na-bentonite was studied using X-ray diffraction (XRD), 57Fe Mössbauer spectroscopy, X-ray fluorescence spectroscopy and infrared spectroscopy to describe the structure of the bentonite before and after treatment. In the samples treated with FeCl3, an increase in the basal spacing was found by XRD, while a new magnetically split component assigned to Fe3+ incorporated within the interlayer regions of montmorillonite showed up in the low-temperature Mössbauer spectra. The Mössbauer parameters observed were close to those of Fe oxyhydroxides, suggesting the presence of some kind of nanoparticles. These results show that the treatment with acetonic FeCl3 solution is an effective method for introducing Fe into montmorillonite in the form of Fe3+ accommodated in the interlayer region. The treated samples proved to be efficient Lewis catalysts in the acylation of aldehydes (benzaldehyde and 4-OH-benzaldehyde) by acetic acid anhydride.

There has been limited research on clay mineral transformation in serpentinitic soils under humid tropical conditions. In this study, four soil pedons were selected along a toposequence from the summit (Entisol), shoulder (Vertisol), backslope (Alfisol) to footslope (Ultisol) positions to explore the contributions and the significance of landscape and weathering status of serpentinitic rock with regard to clay mineral transformations in eastern Taiwan. Experimental results indicated that the large amount of dithionite-citrate-bicarbonate-extractable Fe (Fed) and clay in the subsurface horizon were mainly caused by the strong leaching potential from intensive rainfall and weathering of the fine-grained parent rocks. The clay mineralogy reflected the clear weathering trend of the soils along the toposequence: (1) the soils on the summit and shoulder contained smectite and serpentine, which are predominant in the young soils derived from serpentinitic rocks; and (2) vermiculite gradually increased in the relatively old soils on backslope and footslope. The mineralogical transformations observed along the toposequence indicated that chlorite and serpentine, initially present in the Entisol on the summit, weather into smectite and interstratified chlorite-vermiculite in the intermediate soil on the shoulder under strong leaching and oxidizing conditions. Furthermore, vermiculite formed as the major weathering product of chlorite and smectite in the soil developed on the backslope. In addition to vermiculite, kaolinite and quartz formed in the soils on the footslope with the greatest concentration of Fed along the toposequence.

This work presents the water retention curves obtained for two natural bentonites compacted at different dry densities. The density of the bentonite was kept constant during the determination, for which specific methodologies were developed. The materials tested are the FEBEX and the MX-80 bentonites, in the first of which divalent cations (Ca and Mg) predominate in the exchange complex; MX-80 is mainly sodic. The water retention capacity of the FEBEX bentonite is greater, although the difference between both bentonites becomes smaller towards low suctions. The effect of dry density on the water retention curve is very small or imperceptible for suctions >10 MPa, and below this value the lower the density of the bentonite, the greater its water content. The basal spacing of the samples equilibrated at different suctions has been measured and found to be in the order of those measured by other authors in powder samples equilibrated to the same suctions. The study was performed in the framework of projects concerning the engineered bentonite barrier of high-level radioactive waste repositories.