Hydroxy-interlayered Minerals Research Articles

Review Article. What are the nature and formation conditions of hydroxy‐interlayered minerals (HIMs) in soil?

AbstractPrimary minerals of the parent material undergo weathering during the formation of terrestrial soils to varying extent. As a result, secondary minerals develop, which comprise, among many others, hydroxy‐interlayered minerals (HIMs). These minerals have formed by interlayering of hydroxy‐metal complexes (especially of Al3+, also Mg2+, Fe2+/3+) into micas, expansible 2:1 phyllosilicates and forming oligomers, or by weathering of primary chlorite. The degree of interlayer filling and the stability of these fillings affect several physico‐chemical soil properties, for instance the cation exchange capacity. Although many studies have been conducted on formation, occurrence, and properties of HIMs in soil during the last decades, several challenges still exist. These challenges include analytical identification and quantification of HIMs in soil, the nature of the interlayer filling and the identification of favorable conditions in soil for the formation of HIMs. In order to deepen the understanding of formation, properties, and fate of HIMs in soil, we critically reviewed the available literature. Based on the review, we recommend using a new structural model that enables quantification of hydroxy‐interlayered smectite in soil by X‐ray diffractometry, laboratory experiments on the formation and preservation of different types of interlayers and considering the temporal and spatial dimension of the formation of HIMs in soil in more detail.

Crystal structure model development for soil clay minerals – I. Hydroxy-interlayered smectite (HIS) synthesized from bentonite. A multi-analytical study

During the acidic weathering of soils, aluminum (Al) is dissolved from primary silicates such as feldspars or chlorites and intercalated in layer silicates. Part of this Al polymerizes in the interlayer of former smectites or vermiculites and the resulting phases are called hydroxy-interlayered minerals. These phases are known for a long time but are still insufficiently understood in terms of chemical composition and crystallographical structure. In the present study a sample set consisting of a continuous series of hydroxy-interlayered smectite samples with different degrees of hydroxy-interlayering ranging from 0 to 96% mol/mol smectite/HIS was prepared from a bentonite sample by adding different amounts of a 0.1 mol/L AlCl3 solution and different reaction times. The products were investigated by a multi-analytical approach using X-ray diffraction, Infrared spectroscopy, X-ray Fluorescence spectrometry, Fe2+:Fetotal determination, Nuclear Magnetic Resonance spectroscopy, Simultaneous Thermal Analysis coupled with mass spectrometry and Cation Exchange Capacity to provide an in-depth characterization. X-ray diffraction indicated that the turbostratic disorder typical for smectite did not change during hydroxy-interlayering. Nuclear Magnetic Resonance spectroscopy proved the exclusive six-fold coordination of interlayer Al and based on chemical calculations the average size and charge of the interlayer structures were characterized. Hydroxy-interlayers evolved in-situ in the smectite interlayers by polymerization. These interlayer “polymers” were small oligomers, the average size of which increased from monomers to pentamers with increasing degree of hydroxy-interlayering while the average net charge of each oligomer decreased. An average structural formula of each interstratified smectite-hydroxy-interlayered smectite was calculated from X-ray Fluorescence data including Fe2+:Fetotal ratios. If all exchangeable cations are attributed to smectite layers and all hydroxy-interlayers are attributed to hydroxy-interlayered smectite layers, the degree of hydroxy-interlayering ranged from 0 to 96% mol/mol smectite/HIS. The analysis provides an improved structural model of hydroxy-interlayered smectite suitable for Rietveld refinement which can be used to quantify the amounts of hydroxy-interlayered smectite in natural samples.

Crystal structure model development for soil clay minerals – II. Quantification and characterization of hydroxy-interlayered smectite (HIS) using the Rietveld refinement technique

The acidic weathering process of soils is a worldwide problem. In this process, Al3+ is dissolved from silicates and intercalated into swellable clay minerals by replacing exchangeable cations such as Na+ or Ca2+. Al3+ polymerizes in the interlayer regions of the clay minerals to form fixed oligomers, which significantly reduces the clay minerals swellability and cation exchange, and hence, the soil fertility. The layer silicates with fixed (Al) oligomers in the interlayer are known as hydroxy-interlayered minerals, such as hydroxy-interlayered smectite (HIS), or hydroxy-interlayered vermiculite (HIV). These phases have been recognized for many years, but it is still problematic to adequately characterize the details of their structure and to determine their proportion in soils, based on X-ray diffraction (XRD), which is the most common method for mineral quantification. In the present study, two crystal structure models are used to allow characterization and quantitative determination of HIS in artificial and natural soils, using the Rietveld refinement technique based on XRD. Both models are identical regarding the 2:1 layer and the description of the hydroxy-interlayering, but differ in the effect imposed by environmental conditions. The first model describes the smectite in the ethylene glycol saturated state (≈17 Å), commonly used by clay scientists as a diagnostic intercalate for swellable smectitic or vermiculitic layers in pure smectite or vermiculite or interstratifications. The second model describes the smectite in the air-dried (AD) state including the collapsed state (≈10 Å), one-water layer state (≈12.5 Å), and two-water layer state (≈15 Å). Both models are used to fit XRD patterns obtained from samples with preferred orientation and from XRD random powder specimens, each including structural disorder effects. The models provide plausible refinement results with respect to crystallographic parameters and quantitative proportions. The degree of hydroxy-interlayering gives an estimate of how many exchangeable cations of the smectite were replaced by fixed Al oligomers that inhibit expandability. This value corresponds to calculated values within a range of precision of ≤15% mol/mol. Physical mixtures of HIS with clinochlore, muscovite, quartz, and albite were quantified using the proposed crystal structure model. Results showed that the HIS model delivers reliable quantification results (deviations from actual values ≤3% w/w) which may be applied also for natural soil.

Hydroxy-interlayered minerals in the Holocene paleosol on the southernmost Loess Plateau, China

The presence and formation of hydroxy-interlayered minerals in the Holocene paleosol at the Wugong profile on the southernmost Chinese Loess Plateau were investigated by examining the < 1 μm and 1–2 μm clay fractions of the 50–150 cm horizon by Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). Prior to the analyses free Al and Fe phases were removed by dithionite-citric-bicarbonate (DCB) extraction. XRD patterns were obtained with air-dried, Mg-saturated, Mg-saturated with glycerol solvation (Mg-glycerol) and K-saturated and heated at 110, 250, 350, 450 and 550 °C samples both before and after hydroxy-interlayered materials removal by sodium citrate extraction. The absorption bands at 3699 and 3430 cm− 1 in the FTIR spectra of both sub-fractions indicated the presence of hydroxy-interlayered minerals. The K-saturated and heated XRD patterns confirmed this by the progressive collapse of the 1.4 nm reflection to a 1.0 nm reflection. After removal of the hydroxy-interlayered materials no obvious increase of the relative XRD intensity of the 1.8 nm reflection in Mg-glycerol pattern was observed and the 1.4 nm reflection exhibited a stronger collapse to 1.0 nm after K-saturation and increasing temperature treatment. This indicated that the hydroxy-interlayered minerals in the Holocene paleosol were mainly composed of hydroxyl-interlayer vermiculite. The hydroxyl-interlayer vermiculite was formed by the polymerization of Al/Fe-hydroxy materials within the interlayer space of vermiculite. Shoulder-like reflection at 1.2 nm in the K-550 °C pattern of the 1–2 μm fraction indicated a higher degree of Al/Fe-hydroxylation in the coarse fraction than in the fine fraction, which was consistent with the lower extractable Al and Fe in the coarse fraction. The reflection at 1.38 nm in the K-550 °C pattern could be interpreted as pedogenic chlorite. Thus, transformation of vermiculite into hydroxy-interlayered vermiculite and finally into a chlorite-like mineral (pedogenic chlorite) has occurred in the Holocene paleosol on the southernmost Chinese Loess Plateau.

Experimental aluminization of vermiculite interlayers: An X-ray diffraction perspective on crystal chemistry and structural mechanisms

Natural aluminization of swelling clay minerals is ubiquitous in acidic soils leading to the formation of hydroxy-interlayered (HI) minerals. This process has drawn special attention over the last 4–5 decades owing to the negative impact on soil fertility of the induced reduction of cation exchange capacity. Combination of chemical analyses and of X-ray diffraction profile modeling on a series of experimentally self-aluminized samples aimed at an improved description of HI minerals and more especially of their interlayer crystal chemistry. Both exchangeable alkali cations and hydroxy (Al, Fe) cations coexist within HI vermiculite interlayers. The latter form discontinuous gibbsite-like interlayer sheets with ~15% completeness, despite layer-to-layer distances similar to that of chlorite. From charge compensation considerations, the isolated hydroxy (Al, Fe) cation clusters bear ~1.5 positive charges per cation. Analysis of X-ray diffraction data indicates that aluminization of initially swelling interlayers is a layer-by-layer process leading to mixed layers composed of randomly interstratified swelling and aluminized layers, all aluminized layers likely hosting a similar number of hydroxy (Al, Fe) cations along the aluminization process. This model contradicts the widely accepted description of HI minerals as a solid solution between expandable 2:1 clay and aluminous chlorite end-members. As a consequence, the proportion of HI layers in the mixed layer is a robust estimate of aluminization progress and both the amount of extractible (Al, Fe) and the extent of CEC decrease are positively correlated to this essential parameter.

Mineral composition vs. soil forming processes in loess soils — A case study from Kraków (Southern Poland)

The mineral composition of soil parent material has a clear impact on: soil formation, soil properties (especially chemical properties), and mineral transformation in soil environment. Despite many studies concerning loess soils, detailed, quantitative studies on the mineral composition and transformation of clay minerals in loess soils formed during Holocene and the influence of the mineral composition of parent material on the properties of loess soils are rare. The main aims of the present study were (1) to determine the influence of loess mineral composition on soil genesis and (2) to determine layer silicate transformations and clay mineral formation in loess soils in relation to soil-forming processes using qualitative and quantitative X-ray diffraction analyses (XRD and QXRD). The obtained results show that the occurrence of carbonate minerals (i.e. calcite) and their influence on chemical properties of soil play a crucial role in the evolution of loess soils. In loess that does not contain carbonates, dispersion and translocation of clay particles (including clay minerals) down the profile occur. This leads to the formation of Luvisols and Albeluvisols. In carbonate loess, clay minerals and soil organic matter form stable humus–mineral complexes leading to the accumulation of humus and the formation of Chernozems. The transformation of mica and/or smectite and/or vermiculite to hydroxy-interlayered minerals (HIMs) is the dominant process in the studied loess soils developed in a temperate climate because intense weathering of layer silicates is possible after removal of carbonates from soil profile. This indicates that the occurrence of carbonates which maintain soil pH on a level of about 7 or higher is most likely the main factor controlling the rate of weathering of layer silicates in a loess soil environment. In carbonate loess, the dissolution and leaching of carbonates prevail over the weathering of silicates. It is highly likely that the continuous leaching of carbonates – assuming that the climate will not change – will lead to more intense weathering of layer silicates.

Fast but spatially scattered smectite-formation in the proglacial area Morteratsch: An evaluation using GIS

Proglacial areas in the Alps usually cover a time span of deglaciation of about 150 years (time since the end of the ‘Little Ice Age’ in the 1850s). In these proglacial areas soils have started to develop. Due to the continuous retreat of the Morteratsch glacier (Swiss Alps), the corresponding proglacial area offers a continuous time sequence from 0 to 150 year-old surfaces. Furthermore, an optimal digital dataset about the development of vegetation and soils is available for this area. The soils have been developing on glacial till having a granitoidic character. We investigated the clay mineral assemblage at 35 sites within the glacier forefield. Smectite can be used as a proxy for weathering intensity in these environments. In the proglacial area, the smectite concentration in the topsoil steadily increased with time of weathering; however, this development displayed a strongly scattered trend. The complex interplay of biological, physical, and chemical processes in pedogenesis and clay mineral evolution limits our ability to predict and interpret landscape dynamics. We consequently tried to analyse the role of topographic and vegetation modifications on the smectite content. Sites not or only slightly prone to erosion (flattening slope ridge, steepening slope ridge) or flat morphology promoted the formation of smectite. In addition, the texture of the soil material influenced soil moisture and hence the degree of weathering and the development of vegetation. Although vegetation is not a fully independent factor, because it is interrelated to the surrounding environmental conditions, it seemed to exert its influence on weathering and, consequently, the formation of smectite. Green alder stands and grass heath, where moister soils develop that have a finer texture or where more organic acids are produced, were correlated with a higher smectite content. Humus forms serve as a proxy for the soil biota and soil organic matter composition. At sites having a Eumoder and a higher soil organic matter content, the smectite concentration was elevated. At these sites, the production of chelating compounds or organic acids in the soil is believed to promote the development of smectites via an intermediate stage of hydroxy-interlayered minerals and the subsequent removal of the hydroxide polymers. In this work we have demonstrated that the topographic signature and the effect of vegetation on the formation of smectite and consequent weathering are pervasive. Our results will serve as a basis for further spatio-temporal modelling.

Characterization of zinc in contaminated soils: complementary insights from isotopic exchange, batch extractions and XAFS spectroscopy

Isotopic exchange (IE) of trace metals is an established method for characterizing metal reactivity in soils, but it is still unclear which metal species are isotopically exchangeable. In this study, we used IE to quantify 'labile' zinc (Zn) in 51 contaminated soils that were previously studied by Zn K-edge X-ray absorption fine structure (XAFS) spectroscopy and sequential extraction (SE). All soils had been contaminated by runoff water from 17- to 74-year-old galvanized power-line towers. They covered a wide range in pH (4.0―7.7), organic carbon (0.9―10.2%), clay (3.8―45.1%) and Zn concentrations (251―30 090 mg kg ―1 ). Isotopic exchange was also performed on selected Zn minerals used as references for linear combination fitting of XAFS spectra. The isotopically exchangeable fraction (%E) of Zn generally decreased with increasing pH, but small %E values were also observed for acidic soils with a large fraction of Zn in hydroxy-interlayered minerals (Zn-HIM). The fraction of Zn identified by XAFS spectroscopy as (tetrahedrally and octahedrally coordinated) 'sorbed Zn' agreed reasonably well with the isotopically exchangeable fraction but was in many cases larger than the %E, indicating that some 'sorbed Zn' may be isotopically non-exchangeable, such as Zn sorbed in micropores of Fe oxyhydroxides. Zinc identified by XAFS spectroscopy as Zn precipitates (Zn phyllosilicates, Zn-layered double hydroxide (Zn-LDH) or hydrozincite) or as Zn-HIM was largely isotopically non-exchangeable ('non-labile'). Comparison between IE and extraction results suggested that the isotopically exchangeable Zn was mainly extracted in the first two fractions of the SE. However, non-labile Zn was also extracted in these first two fractions for some soils, including a hydrozincite-containing soil. Despite the presence of Zn-LDH and/or Zn phyllosilicates in almost all soils, the Zn concentrations in solution and labile Zn increased with increasing soil total Zn at a given pH, which contradicts the concept of precipitation control by a single phase. Solution Zn was well predicted from the labile Zn following a sorption model.

Aluminum hydroxy-interlayered minerals and chemical properties of a subtropical Brazilian Oxisol under no-tillage and conventional tillage

No tillage systems significantly influence the soil system, but knowledge about the effects on the mineralogy of tropical and subtropical soils is limited. This study evaluated the long-term effects (26 years) of no-tillage (NT) on aluminum hydroxy-interlayered minerals of a subtropical Oxisol in Southern Brazil (Guarapuava, PR), compared to the same soil under conventional tillage (CT). The clay fraction (&lt; 2 µm) in soil samples of the surface horizons of a field experiment under both management systems was analyzed by X-ray diffraction (XRD) to identify and characterize Al hydroxy-interlayered minerals before and after treatment with sodium citrate to remove intra-layer material. Soil liquid (solution) and solid phases were also characterized. The contents of total organic C, exchangeable cations, P, and the values of extractable acidity and cation exchange capacity as well as electrical conductivity and levels of dissolved organic C, basic cations, aluminum, Si, and sulfur in the soil solution were higher in the NT soil. Under both soil management systems, more than 90 % of the total soluble Al was complexed with organic compounds, with similar Al activity. No significant changes were detected by 2:1 clay mineral XRD analyses in terms of extension or intercalation of Al-hydroxy-polymers in the no-tilled in comparison to the conventionally tilled soil. In both soil management systems, Al and Si activities in the soil solution indicated thermodynamic stability of 2:1 clay minerals with partially occupied by hydroxy-Al, suggesting deceleration in the intercalation process and a tendency of transforming clay minerals from extensive into partial intercalation.

Organic and inorganic phosphate interactions with soil hydroxy-interlayered minerals

Even though extensive work has been done on P interactions with free Al- and Fe-hydroxide minerals, limited or no information exists on sorption/desorption processes of organic and inorganic phosphate forms with soil hydroxy-interlayered minerals. The purpose of this study was to assess the potential of soil clays rich in hydroxy-interlayered vermiculite (HIV) and smectite (HIS) minerals to adsorb and release inorganic and organic P forms as compared to geological reference gibbsite and goethite minerals. Seven-day P adsorption/desorption experiments were conducted involving six soil clays containing >40% HIV, two soil clays containing >55% HIS, and two reference goethite and gibbsite clays equilibrated with P pools consisting of 10 mg P g–1 clay from KH2PO4 or inositol hexakisphosphate. The results indicated a strong tendency for HIV and HIS minerals to sorb P at amounts equivalent to or even higher than those sorbed by gibbsite or goethite minerals. Organic P was sorbed at higher quantities and was retained with greater energy than inorganic P forms. Ammonium oxalate-treated samples produced a clearer picture of the net contribution of hydroxy-interlayered minerals to P sorption capacity compared to free Al/Fe hydroxide components, but the degree of Al-hydroxy-interlayering somewhat obscured P-sorption mineral quantity correlations. Increasing P-sorption equilibration times generally increased total inorganic and organic-sorbed P loads only of the ammonium oxalate treated samples, suggesting increased P interaction with less accessible interlayer reactive sites. Reaction times had no significant effect on P desorption characteristics, although some samples released less P at longer equilibration times probably due to readsorption processes. The findings suggest that soils containing high amounts of hydroxy-interlayered minerals may act as good P sinks and tolerate higher organic P loads than free Al or Fe hydroxides without increasing risks of P contamination.

Soil mineralogy trends in California landscapes

California's diverse environmental gradients serve as natural experiments for examining controls on soil mineral distribution in landscapes. In this paper we use example soilscapes from throughout California to examine how lithology, climate, topography, and duration of pedogenesis interact to produce distinctive weathering environments and characteristic suites of soil minerals. Seven soil-geomorphic sequences were assembled from the literature to illustrate major soil mineralogical trends: 1) granitic terrain of the Peninsular Ranges, 2) granitic terrain of the central Sierra Nevada, 3) andesitic terrain of the northern Sierra Nevada, 4) fluvial terraces on the east side of the Great Valley, 5) marine terraces of the central coast, 6) ultramafic terrain of the Klamath Mountains, and 7) an alluvial fan in the Mojave Desert. Results of this analysis show that kaolin is present in virtually all pedons, irrespective of climate, parent material, age, or topographic position. Kaolin does not form in ultramafic soils due to insufficient aluminum. Many secondary clay minerals reflect the state's strong climatic influence, with palygorskite, smectite, and vermiculite in the dry, hot environments; hydroxy-interlayered minerals, gibbsite, and short range-ordered minerals in the cool, moist environments. In arid and semi-arid regions, the distribution of calcite, gypsum, and soluble salts is strongly related to patterns of eolian dust deposition and water infiltration and leaching.

Soil properties controlling Zn speciation and fractionation in contaminated soils

We determined the speciation of Zn in 49 field soils differing widely in pH (4.1–7.7) and total Zn content (251–30,090 mg/kg) by using extended X-ray absorption fine structure (EXAFS) spectroscopy. All soils had been contaminated since several decades by inputs of aqueous Zn with runoff-water from galvanized power line towers. Pedogenic Zn species identified by EXAFS spectroscopy included Zn in hydroxy-interlayered minerals (Zn-HIM), Zn-rich phyllosilicates, Zn-layered double hydroxide (Zn-LDH), hydrozincite, and octahedrally and tetrahedrally coordinated sorbed or complexed Zn. Zn-HIM was only observed in (mostly acidic) soils containing less than 2000 mg/kg of Zn, reflecting the high affinity but limited sorption capacity of HIM. Zn-bearing precipitates, such as Zn-LDH and Zn-rich trioctahedral phyllosilicates, became more dominant with increasing pH and increasing total Zn content relative to available adsorption sites. Zn-LDH was the most abundant Zn-precipitate and was detected in soils with pH > 5.2. Zn-rich phyllosilicates were detected even at lower soil pH, but were generally less abundant than Zn-LDH. Hydrozincite was only identified in two calcareous soils with extremely high Zn contents. In addition to Zn-LDH, large amounts of Zn in highly contaminated soils were mainly accumulated as sorbed/complexed Zn in tetrahedral coordination. Soils grouped according to their Zn speciation inferred from EXAFS spectroscopy mainly differed with respect to soil pH and total Zn content. Clear differences were observed with respect to Zn fractionation by sequential extraction: From Zn-HIM containing soils, most of the total Zn was recovered in the exchangeable and the most recalcitrant fractions. In contrast, from soils containing the highest percentage of Zn-precipitates, Zn was mainly extracted in intermediate extraction steps. The results of this study demonstrate that soil pH and Zn contamination level relative to available adsorption sites are the most important factors controlling the formation of pedogenic Zn-species in aerobic soils and, consequently, Zn fractionation by sequential extraction.

Local coordination of Zn in hydroxy-interlayered minerals and implications for Zn retention in soils

The objective of this study was to determine the local coordination of Zn in hydroxy-interlayered smectite (HIS) as a function of Zn loading and synthesis conditions and to assess the importance of hydroxy-interlayered minerals (HIM) for Zn retention in contaminated soils. Published and newly collected extended X-ray absorption fine structure (EXAFS) spectra of HIS reacted with Zn at molar Zn/hydroxy-Al ratios from 0.013 to 0.087 (corresponding to final Zn contents of 1615–8600 mg/kg Zn) were evaluated by shell fitting. In Zn–HIS, Zn was octahedrally coordinated to oxygen at 2.06–2.08 Å and surrounded by Al atoms at 3.03–3.06 Å in the second-shell. With increasing molar Zn/hydroxy-Al ratio, the coordination number of second-shell Al decreased from 6.6 to 2.1. These results were interpreted as a progressive shift from Zn incorporation in the vacancies of gibbsitic Al-polymers to Zn adsorption to incomplete Al-polymers and finally uptake by cation exchange in the polymer-free interlayer space of HIS with increasing Zn loadings. In a second part, we determined the speciation of Zn in eight contaminated soils (251–1039 mg/kg Zn) with acidic to neutral pH (pH 4.1–6.9) using EXAFS spectroscopy. All soils contained hydroxy-Al interlayered vermiculite (HIV). The analysis of EXAFS spectra by linear combination fitting (LCF) showed that a substantial fraction of total Zn (29–84%) was contained in HIM with high Zn loading. The remaining Zn was adsorbed to organic and inorganic soil components and incorporated into phyllosilicates. In sequential extractions of Zn–HIS spiked into quartz powder and the Zn contaminated soils, Zn was mainly released in the two most resistant fractions, in qualitative agreement with the findings from LCF. Our results suggest that formation of Zn–HIM may strongly retain Zn in pristine and moderately contaminated acidic to neutral soils. Due to their limited sorption capacity, however, HIM do not allow for the accumulation of high levels of Zn in response to continued Zn input into soils.

Soil Hydroxy-interlayered minerals: A re-interpretation of their crystallochemical properties

AbstractHydroxy-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.

Hydroxy-interlayered minerals of forest soils in A-Li Mountain, Taiwan

Two forest soils (Typic Hapludult and Typic Haplorthod), on A-Li Mountain, central Taiwan were selected for this study. Soil physical and chemical properties, clay mineral compositions, and characteristics of layer charge were investigated. The amount and composition of their interlayer materials were studied by sodium citrate extraction in combination with X-ray diffraction (XRD) analysis. High soil acidity and strong leaching processes resulted in low soil pH (3.5–5.2) and low base saturation. Illite and kaolinite were the major clay minerals in surface horizons, no hydroxy-interlayered vermiculite (HIV) was found in O, A, and E horizons. The greatest extent of interlayering phenomena was present in the Bt2 horizon of Ultisol and the Bs2 horizon of Spodosol. Iron (Fe) incorporated with Al is the major component of the interlayer materials because the hydroxy-interlayered materials partially blocked the interlayer space and neutralized layer charge. A possible explanation for the occurrence of hydroxy-interlayered vermiculite (HIV) in forest soils was suggested. The limited interlayering in surface horizons was interpreted to be caused by a low pH, preventing the precipitation of hydroxy-interlayered materials. In addition, interlayer material from 2:1 clay minerals may be removed by abundant organic matter, through its marked chelating properties relative to Fe and Al cations. The downward leaching of Fe and Al were logically necessary for the development of extensive interlayering behavior in the B horizon where soil pH increased and organic matter contents decreased.

Evaluation of the Clay Fraction in some Danish Soils, Making Use of High-Gradrent Magnetic Separation (HGMS)

Abstract After removal of the ‘free Fe-oxyhydroxides’, a high-gradient magnetic separation (HGMS) procedure was used to separate the minerals of the clay fraction in soil samples from different horizons of two Alfisols, a Mollisol, an Ultisol and a Spodosol into a magnetic and a non-magnetic fraction. The magnetic fraction constituted 12–19% of the total clay fraction in the Alfisols, 28–35% in the Mollisol, 9–10% in the Ultisol and 11–16% in the Spodosol. X-Ray diffraction (XRD), Mossbauer spectroscopy and total elemental analysis (TEA) were used for the characterization of the different fractions. The CBD-treated magnetic clay contained mainly paramagnetic 2:1 layer silicate clay minerals, including hydroxy-interlayered clay minerals (HICM) high in Fe. The nonmagnetic clay was composed of diamagnetic minerals such as quartz, feldspars, gibbsite, kaolinite and the diamagnetic 2:1 layer silicate clay minerals.

Pedon Zonation of Hydroxy‐interlayered Minerals in Ultic Haplaquods

AbstractThe distribution of hydroxy‐interlayered minerals (HIM) was studied in Ultic Haplaquods with respect to horizon and location within the soil matrix. Mineralogy of the clay fraction and of tenacious grain coatings was determined for selected horizons by means of x‐ray diffraction and differential scanning calorimetry. Smectite and kaolinite were the dominant layer silicates in A and E horizons, where sand grains were stripped of coatings and clay occurred as floccules. Hydroxy‐interlayered minerals were the dominant layer silicates in Bh and E′ horizons where grains were coated. Tenacious coatings removed by ultrasound from sands occurring in the Bh and E′ horizons contained more HIM and less quartz than any other portion of the soil matrix. Podzolic stripping of coatings in A and E horizons and transformation of HIM to smectite via interlayer depolymerization could explain the observed HIM depth distribution. Higher HIM concentrations in tenacious coatings in Bh and E′ horizons suggest that the coatings predated podzolization. Hydroxy‐interlayered minerals in Btg1 horizons dispersed in deionized water more readily than did kaolinite, suggesting that differential dispersion in conjunction with a fluctuating water table may be a factor in pedogenic redistribution of clay minerals.

Clay Mineralogical Relationships in Florida Haplaquods1

The clay mineralogy of 59 Alfic and Ultic Haplaquod pedons was studied with respect to distributional relationships involving horizons, chemical parameters, and geomorphic surfaces. Relative estimates of clay-sized minerals were based on x-ray diffraction (XRD) peak height ratios after calibration with standards and normalization. Mineralogical differences based on horizon, base status, or elevational groupings were tested statistically. Quartz dominated the clay fractions of upper, sandier horizons for all pedons. Kaolinite, smectite, and gibbsite were usually more abundant in deeper, finer textured horizons. Hydroxy-interlayered minerals, relative to other phyllosilicates, were significantly concentrated (p <0.0001) in Bh horizons. Kaolinite tended to be more abundant than other phyllosilicates in A and E horizons. Alfic subgroup pedons usually contained less kaolinite and more smectite than ultic pedons. Most pedons on older (>30 m) geomorphic surfaces: (i) were virtually depleted of bases, (ii) contained little or no smectite, and (iii) were dominated by kaolinite in lower horizons. Soils on younger (>30 m) surfaces contained significantly (p = 0.0054) more smectite than older surfaces on the average, and differences were even greater when only ultic subgroups were considered. The mineralogical-chemical-geomorphic relationships collectively support the concept that smectite stability decreases with the loss of bases which has occurred most thoroughly on the oldest surfaces.