Showing papers in "Clays and Clay Minerals in 2016"

Investigation of the interlayer organization of water and ions in smectite from the combined use of diffraction experiments and molecular simulations. a review of methodology, applications, and perspectives

Abstract: Investigation of the organization of interlayer water and cations in smectite is a permanent topic in clay science for environmental science, civil engineering, materials science, and industrial applications. Experimental X-ray (or neutron) diffraction methods and molecular simulations are key techniques to probe the organization of the smectite structure at a similar molecular length scale. The combination of both of these experimental and numerical methods represents a complementary approach to reveal the structural heterogeneity of real samples, design and model a wide range of smectite structures, and validate the simulation results through comparison with experimental data. This paper first revisits establishment of the original interlayer model as developed in the 1930s for the organization of water and ions in the smectite structure using X-ray diffraction (XRD) techniques. Then, based on a simplified approach, key theoretical tools are provided to calculate XRD pattern 00 l reflections for a periodic smectite structure with a wide range of interlayer compositions and organizations using conventional spreadsheet software. In addition to educational purposes, this theoretical description is used to describe the principal parameters governing the positions and intensities of experimental XRD 00 l reflections. This calculation toolbox is also used to determine better the layer-to-layer distances considered in molecular simulations and to validate these simulations through a detailed collation procedure using experimental data. Recent examples of the application of such a procedure to collate experimental diffraction data and molecular simulations are presented for the specific case of deciphering the molecular organization of interlayer water and cations in the different smectite hydrates (mono-, bi-, and tri-hydrated layers). The extension of this approach to the interlayer refinement of organo-clays is also detailed, and perspectives regarding the characterization of other lamellar compounds are discussed.

Review of Clay-Drug Hybrid Materials for Biomedical Applications: Administration Routes.

Abstract: Focus here is placed on the pharmaceutical and biomedical applications of novel clay-drug hybrid materials categorized by methods of administration. Clay minerals have been used for many years as pharmaceutical and medicinal ingredients for therapeutic purposes. A number of studies have attempted to explore clay-drug hybrid materials for biomedical applications with desired functions, such as sustained release, increased solubility, enhanced adsorption, mucoadhesion, biocompatibility, targeting, etc. The present review attempts not only to summarize the state-of-the-art of clay-drug hybrid materials and their advantages, depending on the methods of administration, but also to deal with challenges and future perspectives of clay mineral-based hybrids for biomedical applications.

Molecular dynamics simulations of anion exclusion in clay interlayer nanopores

Abstract: The aqueous chemistry of water films confined between clay mineral surfaces remains an important unknown in predictions of radioelement migration from radioactive waste repositories. This issue is ...

Ion Adsorption at Clay-Mineral Surfaces: The Hofmeister Series for Hydrated Smectite Minerals

Abstract: Many important properties of clay minerals are defined by the species of charge-balancing cation. Phenomena such as clay swelling and cation exchange depend on the cation species present, and understanding how the cations bind with the mineral surface at a fundamental level is important. In the present study the binding affinities of several different charge-balancing cations with the basal surface of the smectite mineral, montmorillonite, have been calculated using molecular dynamics in conjunction with the well-tempered metadynamics algorithm. The results follow a Hofmeister series of preferred ion adsorption to the smectite basal surfaces of the form: K+ > Na+ > Ca2+ > Cs+ > Ba2+ The results also revealed the energetically favorable position of the ions above the clay basal surfaces. Key features of the free-energy profiles are illustrated by Boltzmann population inversions and analyses of the water structures surrounding the ion and clay surface. The results show that weakly hydrated cations (K+ and Cs+) preferentially form inner-sphere surface complexes (ISSC) above the ditrigonal siloxane cavities of the clay, while the more strongly hydrated cations (Na+) are able to form ISSCs above the basal O atoms of the clay surface. The strongly hydrated cations (Na+, Ca2+, and Ba2+), however, preferentially form outer-sphere surface complexes. The results provide insight into the adsorption mechanisms of several ionic species on montmorillonite and are relevant to many phenomena thought to be affected by cation exchange, such as nuclear waste disposal, herbicide/pesticide-soil interactions, and enhanced oil recovery.

Adsorption of soil-derived humic acid by seven clay minerals: a systematic study

Abstract: Humic acid (HA)-clay complexes are well known for their contribution to soil structure and environmental processes. Despite extensive research, the mechanisms governing HA adsorption are yet to be resolved. A systematic study was conducted to characterize the adsorption of a soil-derived HA to seven clay minerals. Clay surfaces affected HA adsorption directly due to structural differences and indirectly by altering solution pH. The following order of HA removal was obtained for the clay minerals at their natural pH: illite ≫ palygorskite > kaolinite > sepiolite > montmorillonite = hectorite ≫ talc. Removal of HA (precipitation and adsorption) by kaolinite and illite was attributed to the low pH they induce, resulting in protonation of the clay and HA surfaces. In spite of the low pH, the zeta potential for HA remained negative, which promoted HA adsorption to the protonated clay surfaces by ligand exchange. Ionic strength did not affect HA adsorption to clay minerals with low zeta potentials, indicating that charge screening is not a major mechanism of HA adsorption for these minerals, and supporting the suggestion that ligand exchange is the main adsorption mechanism to pH-dependent sites. The increase in ionic strength did, however, promote HA adsorption to clay minerals with high zeta potentials. At pH 8–9 the order of HA affinity for clay minerals was: palygorskite >>sepiolite > montmorillonite = hectorite > kaolinite > illite > talc, emphasizing strong HA interactions with the fibrous clays. This strong affinity was attributed to their large surface areas and to strong interactions with OH groups on these clay surfaces. Results indicated that HA did not enter the intracrystalline channels of the fibrous clays but suggested that their macro-fiber structure facilitates HA adsorption. The sorption of HA to kaolinite further increased in the presence of Cu2+, and the sorption of Cu2+ increased in the presence of HA, due to a number of synergistic effects. This study emphasizes the diverse effects of clay structure and solution chemistry on HA adsorption.

Surface crystal chemistry of phyllosilicates using x-ray photoelectron spectroscopy: a review

Abstract: The characterization of freshly cleaved mica surfaces for surface structure and chemical composition was briefly reviewed and focused on surface crystal chemistry using X-ray photoelectron spectroscopy (XPS) and other surface-sensitive techniques. This paper considers micas, which are useful as a first approximation for the behavior of many clay surfaces. Emphasis was given to phyllosilicate XPS binding energies (“chemical shift”), which were described and used to obtain oxidation state, layer charge, and chemical bonding information from the chemical shifts of different peaks. The chemical shift of the Si2p binding-energy to lower values can result from a negative charge increase because of Si4+ replacement by Al3+ and/or Fe3+. The apparent interlayer coordination number reduction from twelve to eight at muscovite and tetraferri-phlogopite (001) surfaces was indicated by the XPS measured K2p binding-energy and is consistent with bond relaxation. Although chemical shifts are valuable to distinguish chemical bonding and oxidation state, chemical shifts usually cannot distinguish between different Al coordination environments where Al is in both tetrahedral and octahedral sites.

Structure and Dynamics of Water-Smectite Interfaces: Hydrogen Bonding and the Origin of the Sharp O-Dw/O-Hw Infrared Band From Molecular Simulations

Abstract: Experimental studies have shown that a sharp, high-frequency IR band at ~3615 cm−1 (in H2O form) and at ~2685 cm−1 (in D2O form) is a common feature for all smectites, and its position correlates with layer charge. In order to explain the molecular origin of this band in terms of total layer charge, charge localization, as well as nature of interlayer cations influencing the position and intensity of this peak, a series of classical molecular dynamics (MD) simulations was performed for several smectite models. The smectite layers were described using a modified CLAYFF force field, where the intramolecular vibrations of H2O were described more accurately by the Toukan-Rahman potential. The power spectra of molecular vibrations of hydrogens were calculated for selected sub-sets of interlayer H2O to analyze quantitatively their contribution to the observed spectral features. The statistics of hydrogen bonds in the smectite interlayers were also analyzed to support the spectral calculations. The simulation results demonstrated clearly that only the H2O molecules in close proximity to the smectite surface are responsible for the sharp vibrational band observed. Other hypotheses for the possible origins of this band were considered carefully and eventually rejected. Two orientations of H2O molecules donating one or two H bonds to the basal oxygens of the smectite surface (monodentate and bidentate orientations, respectively) were observed. In both orientations, these H bonds are quite weak, pointing to a generally hydrophobic character of the smectite surface. Both orientations contributed to the high-frequency band, but the monodentate orientation provided the predominant contribution because surface H2O molecules in this orientation were much more abundant. In good agreement with experiment, only a small difference in the peak position was observed between smectites with different charge localization. The effect of the total layer charge, i.e. the red-shift for higher-charge smectites, was also confirmed. This shift arose from the decrease in the H-bonding distances of H2O in monodentate and bidentate orientation.

Intercalation of ethylene glycol in smectites: several molecular simulation models verified by x-ray diffraction data

Abstract: Organo-clays represent a special challenge for molecular simulations because they require accurate representation of the clay and the organic/aqueous sections of the model system and accurate representation of the interactions between them. Due to the broad range of force-field models available, an important question to ask is which sets of parameters will best suit the molecular modeling of the organo-intercalated smectites? To answer this question, the structure of the ethylene glycol (EG)-smectite complex is used here as a testing model because the intercalation of EG in smectites provides a stable interlayer complex with relatively constant basal spacing. Three smectite samples with substantially different layer charge and charge localization were selected for X-ray diffraction (XRD) measurements. Their molecular models were built and molecular-dynamics simulations performed using various combinations of the organic force fields (CGenFF, GAFF, CVFF, and OPLS-aa) with ClayFF and INTERFACE force fields used to describe smectites. The simulations covered a range of different EG and water contents. For every structure, the density distribution of interlayer species along the direction perpendicular to the layer plane was calculated and then used to optimize the XRD patterns for these simulated models. A comparison of these results with experimental XRD patterns shows very large discrepancies in the structures and basal spacings obtained for different layer charges as well as for different force fields and their combinations. The most significant factor affecting the accuracy of the calculated XRD patterns was the selection of the clay-mineral force-field parameters. The second important conclusion is that a slight modification of the basal oxygen parameters for non-electrostatic interactions (increase of their effective atomic diameters) may be a simple and straightforward way to improve significantly the agreement between the modeled XRD patterns with experiments, especially for high-charge smectites. Generally, among organic force fields, the least accurate results were obtained with CGenFF. For unmodified ClayFF, its combination with GAFF gave the best results, while the two other sets (OPLS-aa and CVFF) gave the best results in combination with ClayFFmod. The INTERFACE and INTERFACEmod produced much better results for low-charge montmorillonite than for high-charge smectites.

Molecular simulation of cesium adsorption at the basal surface of phyllosilicate minerals

Abstract: A better understanding of the thermodynamics of radioactive cesium uptake at the surfaces of phyllosilicate minerals is needed to understand the mechanisms of selective adsorption and help guide the development of practical and inexpensive decontamination techniques. In this work, molecular dynamics simulations were carried out to determine the thermodynamics of Cs+ adsorption at the basal surface of six 2:1 phyllosilicate minerals, namely pyrophyllite, illite, muscovite, phlogopite, celadonite, and margarite. These minerals were selected to isolate the effects of the magnitude of the permanent layer charge (⩽2), its location (tetrahedral vs. octahedral sheet), and the octahedral sheet structure (dioctahedral vs. trioctahedral). Good agreement was obtained with the experiments in terms of the hydration free energy of Cs+ and the structure and thermodynamics of Cs+ adsorption at the muscovite basal surface, for which published data were available for comparison. With the exception of pyrophyllite, which did not exhibit an inner-sphere free energy minimum, all phyllosilicate minerals showed similar behavior with respect to Cs+ adsorption; notably, Cs+ adsorption was predominantly inner-sphere, whereas outer-sphere adsorption was very weak with the simulations predicting the formation of an extended outer-sphere complex. For a given location of the layer charge, the free energy of adsorption as an inner-sphere complex varied linearly with the magnitude of the layer charge. For a given layer charge location and magnitude, adsorption at phlogopite (trioctahedral sheet structure) was much less favorable than at muscovite (dioctahedral sheet structure) due to electrostatic repulsion between adsorbed Cs+ and the H atom of the OH- ion directly below the six-membered siloxane ring cavity. For a given layer charge magnitude and octahedral sheet structure, adsorption to celadonite (octahedral sheet layer charge) was favored over adsorption to muscovite (tetrahedral sheet layer charge) due to the increased distance to the surface K+ ions and the decreased distance to the O atom of the OH- ion directly below the surface cavity.

MODELING POWDER X-RAY DIFFRACTION PATTERNS OF THE CLAY MINERALS SOCIETY KAOLINITE STANDARDS: KGA-1, KGA-1b, AND KGa-2

Abstract: Three kaolinite reference samples identified as KGa-1, KGa-1b, and KGa-2 from the Source Clays Repository of The Clay Mineral Society (CMS) are used widely in diverse fields, but the defect structures have still not been determined with certainty. To solve this problem, powder diffraction patterns of the KGa-1, KGa-1b, and KGa-2 samples were modeled. In a kaolinite layer among three symmetrically independent octahedral sites named as A, B, and C and separated from each other by b /3 along the b parameter, the A and B sites are occupied by Al cations, whereas, the C sites located along the long diagonal of the oblique kaolinite unit cell are vacant. The layer displacement vectors t1 and t2 are related by a pseudo-mirror plane from defect-free 1 T c kaolinite enantiomorphs, whereas, the random interstratification within individual kaolinite crystallites creates right-hand and left-hand layer sub-sequences producing structural disorder. A third layer displacement vector, t , located along the long diagonal of the oblique layer unit cell that contains the vacant octahedral site and coincides with the layer pseudo-mirror plane may exist. Thus, a structural model should be defined by the probability of t 1, t 2, and t layer displacement translations W t 1, W t 2, and W t , respectively, determined by simulated experimental X-ray diffraction (XRD) patterns. X-ray diffraction patterns were calculated for structures with a given content of randomly interstratified displacement vectors, and other XRD patterns were calculated for a physical mixture of crystallites having contrasting structural order with only C-vacant layers. The samples differ from each other by the content of high- and low-ordered phases referred to as HOK and LOK. The HOK phase has an almost defect-free structure in which 97% of the layer pairs are related by just the layer displacement vector t 1 and only 3% of the layer pairs form the enantiomorphic fragments. In contrast, the LOK phases in the KGa-1, KGa-1b, and KGa-2 samples differ from HOK phases by the occurrence probabilities for the t 1, t 2, and t layer displacements. In addition, the LOK phases contain stacking faults that displace adjacent layers in arbitrary lengths and directions. Low XRD profile factors (Rp = 8–11%) support the defect structure models. Additional structural defects and previously published models are discussed.

Cadmium(ii) complexes adsorbed on clay edge surfaces: insight from first principles molecular dynamics simulation

Abstract: Aiming to identify the complexing mechanisms of heavy metal cations on edge surfaces of 2:1-type clay minerals, systemic first-principles molecular dynamics (FPMD) simulations were conducted and the microscopic structures and complex free energies were obtained. Taking Cd(II) as a model cation, the structures on both (010) and (110) edges of the complexes were derived for the three possible binding sites (≡SiO, ≡Al(OH)2/≡AlOH≡AlSiO, and vacant sites). The stable complexes adsorbed on the three binding sites on both terminations had similar structures. The free energies of the complexes on (010) edges were calculated by using the constrained FPMD method. The free energies of complexes on the ≡SiO and ≡Al(OH)2 sites were similar and they were both significantly lower than the free energy of the complex on the octahedral vacant site. In association with the concept of high energy site (HES) and low energy site (LES) in the 2 Site Protolysis Non Electrostatic Surface Complexation and Cation Exchange (2SPNE SC/CE) sorption model, the vacant site was assigned to HES and the other two sites to LES, respectively.

Illite-Smectite-Rich Clay Parageneses from Quaternary Tunnel Valley Sediments of the Dutch Southern North Sea — Mineral Origin and Paleoenvironment Implications

Abstract: The Pleistocene sediment infill of elongated glacial incisions of the southern North Sea (SNS) is often referred to as tunnel valleys (TVs). The depositional environment is not yet fully understood and the present study addresses this challenge from the perspective of clay-mineral transformation (illite to illite-smectite) reported from the largest Elsterian TV of the SNS. Material acquired from the K14-12 borehole in the Dutch offshore was analyzed by X-ray diffraction (XRD), electron microscopy, electron microprobe, and laser particle-size analysis. Illite and illite-smectite (I-S) appeared as dominant clays along with minor amounts of kaolinite, kaolinite-smectite, and chlorite. The largest amount of I-S is recognized in the main TV portion, while in pre-glacial and uppermost deposits, I-S is less abundant. The XRD peak fitting and deconvolution suggest that I-S consists of several intermediates — ordered (well crystallized illite + R3 I-S) and disordered (R0 I-S + R0 I-SS). Given the average particle sizes (>2 µm) and Kubler index values (0.415–0.341°Δ2θ), illite as well as chlorite and kaolinite were interpreted as detrital. On the basis of the distinctive distribution, grain sizes, and compositional variations of I-S, formation by means of early diagenetic in situ smectitization of illite under a cold climate is proposed. The process operated via a series of mixed-layer intermediates derived from an illite component being converted progressively to low-charged smectite. The reaction is marked by a significant net loss of K and Al with replacement by Si in a tetrahedral coordination. Layer-charge imbalance is accommodated by Fe(III) and Mg entering an octahedral sheet, whereas Ca partly fills the interlayer sites. Smectitization rates were controlled by illite grain sizes. The results of the present study support strongly the existence of an ice-marginal freshwater depositional environment at the glacial maximum in the SNS in which early diagenesis at low temperatures resulted in incomplete conversion of illite to smectite.

Xrd and tem studies on nanophase manganese oxides in freshwater ferromanganese nodules from green bay, lake michigan

Abstract: Freshwater ferromanganese nodules (FFN) from Green Bay, Lake Michigan have been investigated by X-ray powder diffraction (XRD), micro X-ray fluorescence (XRF), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM). The samples can be divided into three types: Mn-rich nodules, Fe-Mn nodules, and Fe-rich nodules. The manganese-bearing phases are todorokite, birnessite, and buserite. The iron-bearing phases are feroxyhyte, goethite, 2-line ferrihydrite, and proto-goethite (intermediate phase between feroxyhyte and goethite). The XRD patterns from nodule cross sections suggest the transformation of birnessite to todorokite. The TEM-EDS spectra show that todorokite is associated with Ba, Co, Ni, and Zn; birnessite is associated with Ca and Na; and buserite is associated with Ca. The todorokite has an average chemical formula of Ba 0.28 p2+ (Zn 0.14 p+2 Co 0.05 p3+ Ni 0.02 p2+ ) (Mn 4.99 p4+ Mn 0.82 p3+ Fe 0.12 p3+ Co 0.05 p3+ Ni 0.02 p2+ ) O12·nH2O. Barium is the main cation in the structural tunnels of the todorokite. The average chemical formula of birnessite and Ca-buserite are: Na 0.14 p+ Ca 0.19 p2+ (Mn 1.48 p4+ Mn 0.52 p3+ 0.52) O4·nH2O and Ca 0.27 p2+ (Mn 1.46 p4+ Mn 0.54 p3+ )O4·nH2O, respectively. Most nodules have a concretionary structure of alternating Fe/Mn layers, commonly with a core of reddish feldspar containing phyllosilicates and hematite micro-crystals. Other cores consist of goethite, cristobalite, tridymite, hercynite, hematite-bearing quartz, coal, and chlorite-bearing rock fragments. The hexagonal or hexagonal-like structures of hematite micro-crystals and clay minerals in the cores may serve as heterogeneous nucleation sites for the Mn-oxides and Fe-(oxyhydr)oxides. The alternating Fe/Mn layers in FFNs might be caused by oscillatory redox condition fluctuations at the sediment-water interface due to changes in water level.

INFLUENCE OF AQUEOUS Si AND Fe SPECIATION ON TETRAHEDRAL Fe(III) SUBSTITUTIONS IN NONTRONITES: A CLAY SYNTHESIS APPROACH

Abstract: Most dioctahedral 2:1 swelling clays in natural systems contain ferric iron, Fe(III), which can be located in both the tetrahedral and the octahedral sheets. The distribution of Fe(III) between octahedral and tetrahedral sites in nontronite depends on the Fe and Si speciation during nontronite synthesis. The role played by the chemical properties of solutions in the Fe(III) distribution between structural sites was studied through nontronite syntheses. A chemical series of Fe(III)-nontronites with variable tetrahedral [4]Fe(III) content ( x ) ([Si4− x Fe(III) x ]Fe(III)2O10(OH)2Na x ) was synthesized at 150°C across a range of initial aqueous pH values between 11 and 14. The permanent layer charge, due to Fe(III)-for-Si(IV) tetrahedral substitutions only, ranged from 0.43 to as high as 1.54 per half-unit cell. A ![Formula][1] value of 1.562 A was measured by X-ray diffraction (XRD) for the highest charged nontronite ( x = 1.54). This high ![Formula][2] value has not been reported in the literature for a dioctahedral smectite until now. The [4]Fe(III) content ( x ) of the synthetic nontronites, estimated using Fourier-transform infrared spectroscopy (FTIR) through the wavenumber of the main stretching νSi–O band, was correlated with synthesis pH and its influence on calculated aqueous Si speciation. The increase in synthesis pH induced the increase in anionic aqueous Si species ratios ( i.e. ![Formula][3] and ![Formula][4] ), and favored the incorporation of Fe(III) in tetrahedral sites of synthesized nontronites. During nontronite formation in natural systems, the level of tetrahedral Fe(III)-for-Si(IV) substitutions may, therefore, be partly linked to the aqueous Si speciation and thus strongly dependent on the pH of the crystallization fluids. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /embed/mml-math-4.gif

Clay minerals in deeply buried paleoregolith profiles, Norwegian North Sea.

Abstract: Recent discoveries of oil in deeply buried paleoregolith profiles on the Utsira High, Norwegian North Sea, was the first time basement rocks had been demonstrated to be petroleum reservoirs on the Norwegian continental shelf. The present study aimed to establish the processes responsible for the primary weathering sequence, distinguish them from other phases of alteration, and create a model for the development of reservoir properties in crystalline basement rocks. Hand-specimen and laboratory tests revealed a link between reservoir properties in weathered granitic rocks and alteration facies. Samples were obtained from two distinct paleoregolith profiles on the Utsira High. The core samples were studied in detail by optical microscopy, X-ray powder diffraction, scanning electron microscopy, and X-ray fluorescence. In the altered coherent rock facies, porosity and permeability were mainly created by joints and fractures prior to subaerial exposure. In the altered compact rock and altered incoherent rock facies, the development of reservoir properties was increasingly affected by physicochemical interactions between the rock and percolating fluids during subaerial exposure and early diagenesis. In well 16/3-4, the altered coherent rock facies contained R0 illite-smectite (I-S), well ordered kaolinite, and a mixture of fine-grained mica and illite, produced in semi-open and closed microsystems. In the altered compact rock and altered incoherent rock facies, disordered kaolinite became more abundant at the expense of R0 I-S, well ordered kaolinite, plagioclase, and biotite, suggesting alteration in semi-open microsystems. The collapse of the rock structure and clogging of mesofractures by clays contributed to reduced permeability in the clay-rich upper part of the altered incoherent rock. In contrast, well 16/1-15 represented a more deeply truncated weathering profile compared to 16/3-4, characterized by open and interconnected mesofractures and moderate formation of clay. R0 I-S was present and kaolinite was rare throughout the profile, suggesting stagnant conditions. During burial, a porosity-reducing serpentine-chlorite Ib β = 90° polytype formed in the overlying sandstone and the regolith. Application of these results should improve the success of exploration and production efforts related to hydrocarbon reservoirs in the altered crystalline basement.

KAOLINITE AND HALLOYSITE DERIVED FROM SEQUENTIAL TRANSFORMATION OF PEDOGENIC SMECTITE AND KAOLINITE-SMECTITE IN A 120 ka TROPICAL SOIL CHRONOSEQUENCE

Abstract: Tropical soils range from nutrient-depleted lateritic soils rich in halloysite or kaolinite to Inceptisols rich in interstratified kaolinite-smectite (K-S), smectite, or related 2:1 clays. Given the strong influence of clay minerals on tropical soil quality, better understanding of factors influencing their occurrence is important for modeling and managing tropical environments. This study examines the alteration of smectite to kaolinite by way of intermediate K-S and halloysite in a 120 ka moist tropical chronosequence. Iron-rich smectite (11.6 ± 2.2% Fe2O3) is the dominant mineral in Holocene soils (1–8 ka) originating from sediments rich in plagioclase and clinopyroxene. The cation exchange capacity (CEC) of smectite is 54–84 cmolc/kg and pH is 6.1 to 7.4. Within 50 ka, smectite fixes Al-hydroxy complexes into interlayers, K+ is retained preferentially over Ca2+, and 2:1 layers are stripped of tetrahedral sheets; the resulting K-S inherits flaky smectite crystal habit and the 2:1 layers — which only expand partially — include Al-hydroxy smectite and some illite-like layers. After 50 ka, the dominant mineral is K-S, the CEC is 18–28 cmolc/kg, and the pH is 5.3. Flaky Fe-kaolinite with ~10% residual smectite layers and halloysite (7.4% Fe2O3) also occur in 50 ka soil. The 120 ka soils are dominated by flaky Fe-kaolinite (<10% residual smectite layers) and halloysite (4.9% Fe2O3), and Fe-poor hexagonal kaolinite also occurs (5–10% of soil). The CEC is 11–16 cmolc/kg and the pH is 4.7–5.3. Changes in crystal chemistry of the soil clays (decreasing Fe, Mg, Ca, and K; increasing Al) over time reflects two reaction mechanisms: (1) cell-preserved transformation of smectite layers to kaolinite layers that accompanies conversion of smectite to K-S and eventually kaolinite; this results in the formation of flaky Fe-rich kaolinites after 50 ka; and (2) dissolution of K-S followed by crystallization of halloysite. Neoformation of hexagonal kaolinite and/or halloysite with low Fe (<3% Fe2O3) follows dissolution of Fe-kaolinite or halloysite after 100 ka. This sequence is probably common in moist tropical soils and these findings may inform modeling of soil composition in tropical landscapes where tectonic, volcanic, or geomorphic activity periodically exposes unweathered parent material, producing a range of soil ages.

Structural and Spectroscopic Characterization of Montmorillonite Intercalated with N-Butylammonium Cations (N = 1-4) — Modeling and Experimental Study

Abstract: A detailed structural characterization of organo-clays is a key in understanding their properties. In this work, mono-, di-, tri-, and tetra-butylammonium (nBA; n = 1–4) cations intercalated in the layered clay mineral montmorillonite (Mnt) have been studied for the first time by combining a theoretical approach based on density functional theory (DFT) and infrared spectroscopy. The DFT calculations revealed the detailed structure and position of nBA cations in the interlayer space. A relation between the basal spacing (d001 parameter) and the cation size and structure was found, and explained with respect to the structure, composition, and size of the organic cations. Hydrogen bonds between -NHx/-CH3/-CH2 groups of the nBA cations and oxygen atoms of the basal planes of the Mnt layers were found to be an important factor for the arrangement and energetic stabilization of cations in the interlayer space. The N–H…O hydrogen bonds are stronger than C–H…O hydrogen bonds and the stabilization decreases with decreased number of bands. Analysis of DFT-calculated vibrational modes helped in understanding a problematic region of the experimental infrared spectra (4000–3000 cm-1), in which assignment of all vibrational modes unambiguously was not possible because of a significant overlap of broad bands.

Tolerance of clay minerals by cement: effect of side-chain density in polyethylene oxide (PEO) superplasticizer additives

Abstract: Polycarboxylate superplasticizer (PCE) is a widely used water-reducing agent that can reduce significantly the water demand of concrete, which reduces the porosity and enhances the strength and durability of the concrete. (The PCE consists of a single backbone with many long PEO side chains.) Generally, aggregate occupies >70 wt.% of concrete; clay minerals are ubiquitous in nature and are difficult to avoid in mined aggregates. Clay minerals in aggregate often render the PCE ineffective and give rise to rapid loss of the fluidity of the concrete; this phenomenon is referred to as ‘poor clay tolerance of PCE.’ Though the poor clay tolerance of PCE is known widely, the relationship between the clay tolerance and the molecular structure of the PCE, in particular the effect of the side-chain structures, on clay tolerance is not understood completely. The objective of the present study was to determine the effect of different grafting densities of polyethylene oxide (PEO) side chains on the clay tolerance of PCE. The raw materials included mainly PCE, which was synthesized using acrylic acid and isopentenol polyoxyethylene ether, and a natural montmorillonite (Mnt), one of the most common clay minerals. The loss of fluidity of the cement paste was tested to assess the clay tolerance; total organic carbon was used to measure the amount of PCE adsorbed; X-ray diffraction, transmission electron microscopy, Fourier-transform infrared spectroscopy, and thermogravimetric analysis were used to investigate the microstructure of the intercalated Mnt. The results showed that preventing the superficially adsorbed PCE from being intercalated into Mnt was of great importance in terms of the improvement in clay tolerance of PCE, which increased with greater grafting density of PEO in the side chain of the PCE. The results also suggested the possibility that polymers which intercalate preferentially into the Mnt could improve significantly the clay tolerance of the PCE system.

Differentiating Styles of Alteration within Kaolin-Alunite Hydrothermal Deposits of Çanakkale, NW Turkey

Abstract: The Biga Peninsula of NW Turkey is host to many kaolin and halloysite deposits with mineralization occurring at the intersections of fault zones in contact with Late Eocene.Miocene calc-alkaline volcanic rocks. Distinguishing between the relative overprinting of hypogene by supergene processes in these deposits is a challenge and important because they affect the physical-chemical properties of minerals and their potential for industrial applications. This study examines the Saribeyli-Sigirli and Bodurlar kaolin deposits in NW Turkey, which were formed from similar volcanics as evidenced by 40Ar/39Ar. Late Eocene (34.2 ± 0.20 Ma) to Early Oligocene (32.7 ± 0.17 Ma) ages for both primary volcanic rocks and alunites are consistent with surrounding rocks in the Canakkale region. Criteria used to distinguish hypogene alteration from supergene alteration processes come from X-ray diffraction (XRD), Fourier-transform infrared (FTIR) and Raman spectroscopies, thermal gravimetric analysis (TGA), scanning and transmission electron microscopy (SEM, TEM), and elemental analyses. Isotopic δ18O depletion and δD enrichment of the Saribeyli-Sigirli deposit suggests that it was more influenced by magmatic waters than was the Bodurlar deposit. The Bodurlar deposit contains a paucity of dickite compared to the Saribeyli-Sigirli deposit, which is evidenced by lower TGA endotherms, higher ratios of XRD intensities for reflections at 1.316 A and 1.307 A, distinctive FTIR absorbance bands at 3620 cm− 1 and 3652 cm−1, and relative Raman intensities of the γ1 and γ5 vibrational modes. A genetic model is proposed whereby these deposits are mainly formed through an acid-sulfate hydrothermal alteration, in what appears to be a volcanic-hydrothermal system. The extent of hydrothermal alteration was controlled by fault density and the initial texture of the volcanic rocks. These steam-heated environments included sulfide-enriched vapors and groundwater mixed to varying degrees in the vadose zone. The Saribeyli-Sigrli and Bodurlar deposits, respectively, contain mineral assemblages that reflect both hypogene (kaolinite, alunite, dickite) and supergene (kaolinite, halloysite, jarosite) processes. These observations offer a basis for comparing and discriminating the relative influence of these two important alteration processes responsible for the formation of kaolin deposits in NW Turkey and around the world.

Modified method for bentonite purification and characterization; a case study using bentonite from tsunagi mine, niigata, japan

Abstract: A modified procedure for bentonite purification and a new method for the quantitative characterization of bentonite using smectite content are reported. Bentonite found in a drill core of Tsunagi Mine, Niigata, Japan was evaluated by the new method to demonstrate the substantial increase in smectite content from 40% in the original bentonite to 75% after purification using a new procedure. Powder samples were prepared by putting blocks of bentonite into acetone to remove water without mechanical crushing. The powdered, acetone-dried bentonite was purified by a dispersion-sedimentation method in water after cation exchange of the interlayer Ca2+ ion with Na+ ion by the reaction of raw bentonite with aqueous NaCl. The purification was evaluated using X-ray diffraction and thermogravimetric analyses (TG). The raw bentonite contained feldspar, quartz, and cristobalite, and feldspar and quartz were removed by the new purification procedure. The purification was evaluated quantitatively by comparing the TG data before and after the purification. The purified bentonite swelled in water to give a stable aqueous suspension and 3 g of purified bentonite dispersed in 60 mL of water was stable for several days. The replacement of interlayer sodium with dibehenyldimethylammonium gave an organophilic clay, which swelled in toluene. The bentonite has potential practical uses as a purified bentonite and an organophilic bentonite.

Effect of polydispersity of clay platelets on the aggregation and mechanical properties of clay at the mesoscale

Abstract: The results from mesoscale simulations of the formation and evolution of microstructure for assemblies of Na-smectite particles based on assumed size distributions of individual clay platelets are presented here. The analyses predicted particle arrangements and aggregation (i.e. platelets linked in face-face configurations) and are used to link geometric properties of the microstructure and mechanical properties of the particle assemblies. Interactions between individual ellipsoidal clay platelets are represented using the Gay-Berne potential based on atomistic simulations of the free energy between two Na-smectite clay-platelets in liquid water, following a novel coarse-graining method developed previously. The current study describes the geometric (aggregate thickness, orientation, and porosity) and elastic properties in the ‘jammed states’ from the mesoscale simulations for selected ranges of clay particle sizes and confining pressures. The thickness of clay aggregates for monodisperse assemblies increases (with average stack thickness consisting of n = 3–8 platelets) with the diameter of the individualclay platelets and with the level of confining pressure. Aggregates break down at high confining pressures (50–300 atm) due to slippage between the platelets. Polydisperse simulations generate smaller aggregates (n = 2) and show much smaller effects of confining pressure. All assemblies show increased order with confining pressure, implying more anisotropic microstructure. The mesoscale simulations are also in good agreement with macroscopic compression behavior measured in conventional 1-D laboratory compression tests. The mesoscale assemblies exhibit cubic symmetry in elastic properties. The results for larger platelets (D = 1000 A) are in good agreement with nano-indentation measurements on natural clays and shale samples.

Stability of the hydronium cation in the structure of illite

Abstract: Some aspects of the crystal structure of illite are not understood properly yet, in spite of its abundance and significance as a component of soils, sediments, and low-grade metamorphic rocks. The present study aimed to explore the role of hydronium cations in the interlayer space of illite in a theoreticalexperimental approach in order to clarify previous controversial reports. The infrared spectroscopy of this mineral has been studied experimentally and by means of atomistic calculations at the quantum mechanical level. The tetrahedral charge is critical for the stability of the hydronium cations, the presence of which has probably been underestimated in previous studies. In the present study, computational studies have shown that the hydronium cations in aqueous solutions are likely to be intercalated in the interlayer space of illite, exchanging for K cations. During the drying process these cations are stabilized by hydrogen bonds in the interlayer space of illite.

Characterization of Natural Clays from Italian Deposits with Focus on Elemental Composition and Exchange Estimated by EDX Analysis: Potential Pharmaceutical and Cosmetic Uses

Abstract: Purification processes performed on natural clays to select specific clay minerals are complex and expensive and can lead to over-exploitation of some deposits. The present study aimed to examine physicochemical (mineralogy, morphology, size, surface charge, chemical composition, cation exchange capacity [CEC], and pH) and hydration (swelling, wettability, water sorption, and rheological behavior) properties of three native clays from Italian deposits for potential pharmaceutical and cosmetic uses due to the presence of phyllosilicate minerals. Particular emphasis was placed on energy dispersive X-ray (EDX) microanalysis coupled with the ‘cesium method’ to assay clay elemental composition and CEC. One bentonite of volcanic origin (BNT) and two kaolins, one of hydrothermal origin (K-H) and another of lacustrine-fluvial origin (K-L), were evaluated in comparison with a commercial, purified bentonite. The CEC assay revealed the complete substitution of exchangeable cations (Na+ and Ca2+) by Cs+ in BNT samples and CEC values consistent with those of typical smectites (100.64 ± 7.33 meq/100 g). For kaolins, partial substitution of Na+ cations occurred only in the K-L samples because of the interstratified mineral component which has small CEC values (11.13 ± 5.46 meq/100 g for the K-H sample and 14.75 ± 6.58 meq/100 g for the K-L sample). The degree of isomorphous substitution of Al3+ by Mg2+ affected the hydration properties of BNT in terms of swelling, water sorption, and rheology, whereas both of the poorly expandable kaolins exhibited significant water-adsorption properties. The EDX microanalysis has proved to be of considerable interest in terms of providing more information about clay properties in comparison with other commonly used methods and to identify the role played by both chemical and mineralogical composition of natural clays for their appropriate use in pharmaceutical and cosmetic fields.

Introduction to a special issue on molecular computer simulations of clays and clay-“water interfaces: recent progress, challenges, and opportunities

Abstract: Over recent years, a steady increase has been noted in the publication of research articles in Clays and Clay Minerals which have included the use of molecular simulation methods. Based on a keyword search of Web of Science , the number of molecular simulation papers has increased from zero during the first four decades of the journal to five during the late 1990s, to 14 in the 2000s, to 36 so far in the 2010s including those published in this special issue. Most research papers using computational molecular modeling approaches to study the properties of clays and clay-related systems and their practical applications are published in the broader materials science and chemical engineering literature. The clay-minerals community has taken quite a long time to embrace the technical insights associated with using a molecular modeling approach. Not only does a molecular basis help to develop and test crystallographic models of clay structures, it provides critical insight into understanding the fundamental mechanisms that control many physical and chemical properties, thermodynamics, kinetic processes, and reactivity. Molecular simulations help in the interpretation of spectroscopies and microscopies, and in many other critical issues associated with the behavior and dynamics of clay mineral systems. Though clearly not a panacea for all technical challenges in clay mineralogy, molecular models and simulations provide a solid theoretical context to enable us to delve deeper into the fundamental …

Mineralogy, geochemistry, and genesis of sepiolite and palygorskite in neogene lacustrine sediments, eski̇şehi̇r province, west central anatolia, turkey

Abstract: Sepiolite and palygorskite are common as layers and nodules in the Neogene lacustrine sediments of the Eskisehir area. This study aims to determine their mineralogical and geochemical characteristics, plus the distribution of these economically important sepiolite and palygorskite deposits within the lacustrine environment. Using these data the research goes on to discuss the environmental conditions for their formation. Sepiolite and palygorskite layers are associated with dolomite, marlstone, and argillaceous limestone. The sepiolite nodules (meerschaum), which are former magnesite gravels, are observed in the Upper Miocene reddish-brown fluvial deposits derived from the ophiolite and its fractureinfills at the northern margin of the basin. Sepiolite and palygorskite are only sparsely associated with dolomite and accessory magnesite, quartz, feldspar, and amphibole. Sepiolite and palygorskite fibers formed as oriented platy fan, interwoven, and knitted aggregates in the absence of dolomite indicated precipitation from supersaturated solution. Sepiolite and palygorskite fibers edging dolomite crystals postdate dolomite and formed through precipitation in a vadose environment under semi-arid to arid climatic conditions. High values of Mg+Fe+Ni and enrichment of light rare earth elements (LREE) relative to middle rare earth elements (MREE) and heavy rare earth elements (HREE), Sr content, depletion of Rb+Ba and K, and negligible negative Eu anomalies all reflect the derivation from the Paleozoic metamorphic and Upper Cretaceous ophiolitic rocks. Locally, Upper Miocene to Lower Pliocene volcanic, volcanoclastic, and fluvio-lacustrine sedimentary rocks supplied the required Si, Mg, Al, and Fe for precipitation of Al-sepiolite and Mg-palygorskite with average structural formulae of Si11.91Al0.09O30Mg6.60Al0.78Fe0.13 (OH)4Na0.12K0.06(OH2)4·nH2O and Si7.74Al0.26O20Mg2.52Al1.13Fe0.38(OH)2(OH2)4Na0.32K0.14 Ca0.12·nH2O, respectively. In contrast to the layered sepiolites, the absence of Al and high Ni content in sepiolite nodules suggest formation through replacement of magnesite gravels at shallow burial in an alkal ine environment. The calculated meerschaum sepiol ite chemical formula i s: Si12.02O30Mg7.87Fe0.01(OH)4Na0.13K0.03(OH2)4·nH2O.

Reflectance Spectroscopy for Organic Detection And Quantification in Clay-bearing Samples: Effects of Albedo, Clay Type, and Water Content

Abstract: Reflectance spectroscopy is a rapid and non-destructive method that can be used to detect organic compounds in geologic samples over a wide range of spatial scales that includes outcrops, hand samples, drill cores, and planetary surfaces. In order to assess the viability of this technique for quantification of organics and aliphatic compounds in particular, the present study examines how clay mineralogy, water content, and albedo influence the strength of organic absorptions in near-infrared (NIR) reflectance spectra. The effects of clay structure and water content are evaluated using kaolinite, smectite (montmorillonite), and a mixed-layer illite-smectite as starting materials. Absorption strengths for C—H absorptions are compared to known total organic carbon (TOC) values using both reflectance spectra and single scattering albedo (SSA) spectra derived from a Hapke radiative transfer model. A linear relationship was observed between band depth and TOC for each sample suite, but strong albedo variation led to nonunique trends when band depths were calculated from reflectance spectra. These effects were minimized by conversion to SSA, for which band depth-TOC trends were similar for all mixture suites regardless of albedo or hydration level, indicating that this approach may be more broadly applicable for clay and organic-bearing samples. Extrapolation of band depth-TOC trends for the synthetic mixtures suggested a very conservative lower limit of detection of <1 wt.% TOC, but preliminary results for natural organicbearing shales indicated that detection limits may be an order of magnitude lower.

Thermal constraints on clay growth in fault gouge and their relationship with fault-zone evolution and hydrothermal alteration: case study of gouges in the kojaku granite, central japan

Abstract: In order to elucidate the process of mineralization of clay minerals in fault gouge and its spatial-temporal relationship with fault-zone evolution and hydrothermal alteration, X-ray diffraction (XRD) analysis and K-Ar dating were performed on clay samples from the Kojaku Granite of central Japan, including fault gouge along an active fault. The area studied is suitable for understanding thermal constraints on clay mineralization because the wall rock is homogeneous and its thermal history well defined. The results from XRD indicated that the clay minerals in the gouge samples are dioctahedral smectite, kaolinite, and 1Md illite, whereas clay fillings in fractures and joints in the intact granite (clay vein) include 2M1 illite in addition to dioctahedral smectite and 1Md illite. The evolution of clay mineralization is reconstructed as follows: (1) high-temperature hydrothermal alteration of feldspar and biotite produced 2M1 illite in clay veins; and (2) alteration accompanied by shearing at a lower temperature resulted in the formation of 1Md illite in the gouges. This scenario is consistent with the cooling history of the granite constrained by fission-track, U-Pb, and K-Ar dating methods. K-Ar dating of the clay samples separated into multiple particle-size fractions indicated that the low-temperature alteration leading to the production of 1Md illite was dated to ~40 Ma. Based on the cooling history of the granite, the 1Md illite formed at temperatures of 60–120°C. This temperature range was at the lower limit of the range reported in previous studies for faults. The spatial and geometrical relation of the faults studied and their K-Ar ages infer evolution which can be described as extensive development of small-scale faults at ~40 Ma followed by coalescence of the small-scale faults to form a larger, recently reactivated, active fault. The K-Ar ages have not been reset by the recent near-surface fault activity.

Examining structural and related spectral change in mars-relevant phyllosilicates after experimental impacts between 10–40 gpa

Abstract: Accurate clay mineral identification is key to understanding past aqueous activity on Mars, but martian phyllosilicates are old (>3.5 Ga) and have been heavily bombarded by meteoroid impacts. Meteoroid impacts can alter clay mineral structures and spectral signatures, making accurate remote sensing identifications challenging. This paper uses nuclear magnetic resonance (NMR) spectroscopy to examine the short-range structural deformation induced in clay mineral samples of known composition by artificial impacts and calcination. Structural changes are then related to changes in the visible-near infrared (VNIR) and mid-infrared (MIR) spectra of these clay mineral samples. The susceptibility of phyllosilicates to structural deformation after experimental impacts varies by structure. Experimental results showed that trioctahedral, Mg(II)-rich saponite was structurally resilient up to peak pressures of 39.8 GPa and its unchanged post-impact spectra reflected this. Experimental data on kaolinite showed that this Al(III)-rich, dioctahedral phyllosilicate was susceptible to structural alteration at peak pressures ⩾ 25.1 GPa. This result is similar to previously reported experimental results on the Fe(III)-rich dioctahedral smectite nontronite, suggesting that dioctahedral phyllosilicates may be more susceptible to shock-induced structural deformation than trioctahedral phyllosilicates. The octahedral vacancies present in dioctahedral phyllosilicates may drive this increased susceptibility to deformation relative to trioctahedral phyllosilicates with fully occupied octahedral sheets. Thermal alteration accompanies shock in meteoroid impacts, but shock differs from thermal alteration. NMR spectroscopy showed that structural deformation in thermally altered phyllosilicates differs from that found in shocked phyllosilicates. Similar to shock, dioctahedral phyllosilicates are also more susceptible to thermal alteration. This differential susceptibility to impact-alteration may help explain generic smectite identifications from heavily bombarded terrains on Mars.

High-Resolution Transmission Electron Microscopy (HRTEM) Study of Stacking Irregularity in Fe-Rich Chlorite from Selected Hydrothermal Ore Deposits

Abstract: The structures of Fe-rich chlorite and berthierine and the formation mechanisms of 7 A–14 A interstratified minerals were not previously fully understood owing to the difficulties in analyzing them by X-ray diffraction (XRD). The present study characterizes Fe-rich chlorites in quartz veins of epithermal to xenothermal vein-type ore deposits without later structural modifications, based on high-resolution transmission electron microscopy (HRTEM) along with XRD examination and chemical analysis. Samples have a wide range of Fe/(Fe+Mg) ratios from 0.38 to 0.98 and tetrahedral Al substitution for Si from 0.94 to 1.44 atoms per formula unit (apfu). The variation in Fe content nearly parallels the tetrahedral Al content. The formation temperatures estimated by chlorite geothermometry range from 190°C to 320°C. In HRTEM, most of the samples showed interstratification between 7 A, 14 A, and/or (in some samples) smectite layers. Chlorites with relatively low Fe contents (Fe/(Fe+Mg) ≈ 0.4) were characterized by mostly 14 A periodicity with the polytype IIbb. In contrast, interstratification of 7 A and 14 A layers predominated with increasing Fe content and the proportion of 7 A layers exceeds 80% in Fe-rich samples with Fe/(Fe+Mg) > 0.9. The 7 A component layer approximated Fe-rich berthierine based on the chemical composition. Layer stacking structures in the Fe-rich samples were complex, and characterized by disorder of 7 A and 14 A layers, differences in the polarity of the tetrahedral sheets, variations of the slant of the octahedral sheets, and positional disorder between octahedral and tetrahedral sheets involving the hydrogen bonding, as indicated from HRTEM observations along the Yi directions of the phyllosilicates. The complex stacking structures observed in Fe-rich samples suggest that irregularity was controlled by neither the Fe/(Fe+Mg) ratio nor the formation temperature; stacking was controlled by kinetic factors in the process of mineral precipitation under disequilibrium conditions.

Characteristics of Early Earth’s Critical Zone Based on Middle—Late Devonian Paleosol Properties (Voronezh High, Russia)

Abstract: Land colonization with vascular plants during the late Silurian—early Devonian and then arborescence during the Pragian—Givetian caused the development of new soil types. These true-rooted soils increased the rate of pedogenesis on a global scale. Since that time, soil has become a key component of the biosphere and has given rise to profound development of the Earth’s Critical Zone (CZ). Case studies of Devonian CZs have helped to record the transformation from Precambrian—Lower Paleozoic ‘proto-CZs,’ which had insufficient proto-soil cover, to modern soils with true-rooted pedosphere. Devonian (Givetian—Frasnian) paleosols from the Voronezh region of Russia are combined into pedocomplexes which occupied the top, slope, and bottom positions of a pronounced paleo-relief. Paleosols were developed from terrigenous argillites and volcanigenic-sedimentary deposits. Each pedocomplex consisted of four or more paleosols with different degrees of development and profile preservation. Paleosols exhibited several common characteristics including production and translocation of clay, ferruginization and the presence of siderite nodules, enhanced MnO/Al2O3 and (Fe2O{3}+MnO)/Al2O3 values, and in situ roots and root-system traces. The latter are siderite/goethite substituted. Stable isotope analysis of siderite shows δ13C values of between -6.1 and -13.7% indicating that CO2 had originated from C3 plants. The main mineral component of clay fractions in automorphic paleosols (top and slope of the paleorelief) is kaolinite. The important feature of these paleosols is the red-stained hematite-rich layer in their bases. These horizons developed at different depths and with different thicknesses, and marked the paleo-groundwater tables of each sub-CZ. Evidence of the imprints of vegetation is seen in the abundant in situ roots, plant fragments, and spores of rhyniophytes, lycopsids, progymnosperms, cladoxylalean ferns, and phytoleims of algae-like vascular plants. The near-equatorial location and the overall characteristics of paleosols studied suggest that the aforementioned horizons were formed in a tropically warm and humid climate. The paleo-ecological environments which accompanied pedogenesis were probably controlled by tectonic activity and volcanism.