Proto-imogolite allophane in podzol concretions in Australia: possible relationship to aluminous ferrallitic (lateritic) cementation

Proto-imogolite allophane was found in amounts ranging from 1% to 5% in concretions and ortstein from several sandy podzols in Australia. Micromorphological observations on concretions in which allophane was the dominant clay component indicated that it had been deposited in layers during successive drying episodes to form a yellow, glassy, weakly anisotropic, fluorescent cement around sand grains. Iron oxides and organic matter were minor components of the cement. It is proposed that a proto-imogolite sol is the mobile phase that deposits Al and Si in these concretions, and that the proto-imogolite allophane could be a precursor of kaolinite in older concretions, and of gibbsite in leaching environments. Proto-imogolite sols and allophane could play a similar role in the pedogenesis of kaolinite and gibbsite in concretions in freely drained ferrallitic soils.     

DISSOLUTION KINETICS OF SILICATE MINERALS IN AQUEOUS CATECHOL SOLUTIONS

Fine particle samples of silicate minerals and of gibbsite were shaken with solutions of pyrocatechol, 4-nitro-pyrocatechol or 3,4-dihydroxybenzoic acid at pH 9·3–9·7 and 25°C. In 63 days, from 1·4 per cent (illite) to 17·9 per cent (nepheline) of the silica present was dissolved. Except for kaolinite in solutions of the two substituted catechols, the aluminosilicates dissolved incongruently, leaving residues enriched with aluminium. When accumulated amounts of elements (Si, Al, Na, K) dissolved from nepheline were plotted against time on log-log paper straight lines were obtained. For quartz, kaolinite, and in some cases gibbsite similar plots could be approximated by two straight-line segments. The plots for microcline and oligoclase may be described by a combination of three-line segments. Several possible reaction mechanisms are discussed but a mechanism which explains the similar rate profiles has not yet been formulated.     

THE MINERALOGY OF THE CLAY FRACTION FROM BASALTIC SOILS IN THE GALILEE, ISRAEL

The mineralogy of the clay fraction from basaltic soils in the Galilee, Israel, has been studied by X-ray and differential thermal analysis techniques, supplemented by electron-micrographs and chemical determinations. The mineralogical composition of the clay was greatly influenced by the amount of rainfall. In SE Galilee, where rainfall is 400–550 mm/annum, the dominant clay mineral is montmorillonite, with kaolinite as the second most important component. With increasing rainfall the amount of montmorillonite in the clay falls, and the amount of amorphous oxides of Fe and Al increases. In N and NE Galilee, rainfall is 550–700 mm/annum and the major part of the clay is composed of kaolinite or halloysite, quartz, and amorphous oxides of Fe and Al. The decrease in the amount of montmorillonite with increasing rainfall is explained by decomposition to kaolinite and amorphous oxides.     

Iron-bearing phases in a peat-derived duricrust from Brazil

The formation of aluminium‐substituted iron‐minerals is still subject to debate. We report a case study in a Humic Gleysol soil profile, developed on a sedimentary saprolite, from a basin floor on the Uberaba Plateau (Minas Gerais State, Brazil) where iron‐phases are crystallized by a ferruginization process. The mineralogical and chemical properties were investigated by X‐ray diffraction, thermogravimetric analysis, differential scanning calorimetry and Mössbauer spectroscopy. The specific surface area (BET method) and cation exchange capacity (CEC) were also determined. The soil profile is mainly composed of gibbsite, kaolinite and amorphous alumino‐silicate phases, the latter being more frequent in the H horizon where the organic carbon content is greater. The surface area and CEC values are also greater in the H horizon (58.6 m²g⁻¹ and 4.65 mol_ckg⁻¹, respectively) which indicates an increase in porosity caused by the presence of 20.8% of organic matter and amorphous materials. Goethite occurs as a secondary mineral in the H horizon and as the main mineral in the duricrust in association with haematite. The omnipresence of aluminium in the environment (24.6–46.2% of Al₂O₃) resulted in Al‐substitution in all iron‐bearing phases but the goethite from the H horizon has the greatest Al‐for‐Fe substitution, with at least 20 mole per cent of aluminium. In spite of the greater microporosity and wetness of the H horizon, the immediate contact with the richest Al‐source (gibbsite) favours the precipitation of unusually greatly Al‐substituted goethite instead of haematite in this horizon.     

Speciation and Solubility Control of Aluminium in Soils Developed from Slates of the River Sor Watershed (Galicia,NW Spain)

The Al species in the soid and liquid phases were studied in eight soils developed from slates in a watershed subjected to acid deposition. From soil solution data the mechanisms possibly controlling Al solubility are also discussed. The soils are acidic, organic matter rich and with an exchange complex saturated with Al. In the solid phase, more than 75% of non-crystalline Al was organo-Al complexes, mostly highly stable. In the soil solutions, monomeric inorganic. Al forms were predominant and fluoro-Al complexes were the most abundant species, except in soil solutions of pH<4.8 and Al L/F ratio >3, in which Al³⁺ predominated and sulphato-Al complexes were relatively abundant. The most stable phases were kaolinite, gibbsite and non-crystalline Al hydroxides. In most samples, Al solubility was controlled by Al-hydroxides. Only in a few cases (solutions of pH 4-5, Al³⁺ activity >40 µmol L-¹ and SO₄ content >200 µmol L-¹), Al-sulphates such as jurbanite also could exert some control over Al solubility. In adition to these minerals, a possible role of organo-Al complexes or the influence of adsorption reactions of sulphate is considered, especially for samples with very low Al³⁺ content (<0.5 µmol L-¹).     

Factors and processes of gibbsite formation in Northern Thailand

Gibbsite is usually considered as end product of weathering in tropical environments with potentially high leaching rates. However, there are also hints towards gibbsite formation in initial stages of weathering in different climates. This study reports on a systematic approach based on soil forming factors in order to research the conditions of gibbsite formation in northern Thailand highlands. Therefore, three major study sites were chosen, which differ with respect to parent rock, relief, climate and vegetation. The results show that gibbsite is common in soils of the area. Reasons for its occurrence in soils are manifold. It can be a heritage of the parent rock, a result of initial weathering under free draining conditions or an accumulation under intense chemical weathering caused by high rainfall. Especially the investigation in granite and gneiss areas with a high share of primary minerals indicates that gibbsite can be an early and direct transformation without intermediates from micas and feldspars if free drainage is assured. With progressing soil genesis clay formation reduces drainage and favours kaolinite formation. Only sites with extremely high rainfall and low evaporation (high elevations in northern Thailand) show again a dominance of gibbsite in the clay fraction throughout the whole soil profile.     

Prolonged leaching of mineral forest soils with dilute HCl solutions: the solubility of Al and soil organic matter

Long-term acidification has been shown to result in a considerable decrease in the amount of organically bound soil Al and in a gradual decrease in the solubility of Al. We examined the solubility of soil organic matter (SOM) and Al in four acid mineral soils (one Arenosol Ah, two Podzol Bh, and one Podzol Bs) as they were leached sequentially using a solution containing 0.001 m HCl and 0.01 m KCl. The acid leaching resulted in relative decreases in Al that were 2–6 times greater than for organic C. The organic C and Al dissolved by the acid leaching originated mainly in the pyrophosphate-extractable fraction of the elements. Protonation seems to be a major mechanism in stabilizing the residual SOM, as indicated by small changes in effective cation exchange capacity with the degree of acid leaching. In the samples of Podzol Bh and Arenosol Ah soils the solubility of Al (defined as log₁₀{Al³⁺} + 1.5pH) in equilibrium suspensions (0.01 m KCl) was closely related to the ratio of pyrophosphate-extractable Al to pyrophosphate-extractable organic C. The Podzol Bs sample probably contained a small amount of a surface-reactive Al(OH)₃ phase, which rapidly became depleted by the acid leaching.     

Aluminium solubility mechanisms in moderately acid Bs horizons of podzolized soils

The processes controlling the retention and release of aluminium in acid forest soils are still subject to controversy, and therefore a universal hypothesis as to what mechanisms are operating has not been firmly established. By studying the Bs horizons of Swedish and Swiss podzolized soils, and by analysing data in the literature, we have found that aluminium hydroxide, and in some cases also poorly ordered imogolite, may control Al solubility in moderately acid (pH > 4.2–4.3) Bs horizons. The strongest evidence in support of the presence of a quickly reacting Al(OH)₃ pool came from the temperature dependence of Al solubility in a Bs horizon, which was consistent with the reaction enthalpy of an Al(OH)₃ phase such as gibbsite, and from the observation that the ion activity product for Al(OH)₃ was the same regardless of whether equilibrium was reached from over‐ or undersaturation. The pool of Al(OH)₃ is commonly small and may be completely dissolved after large additions of acid. This may be explained by the continuing redissolution of reactive Al(OH)₃ to form less soluble imogolite‐type phases. By using the same methods it was found that soil suspensions did not reach equilibrium with poorly ordered imogolite even after 17 days. Thus, imogolite probably does not control Al solubility in the short term in many soils despite the common occurrence of this mineral. This is due to the relatively slow kinetics of imogolite formation and dissolution, especially at low temperatures and at small solution H₄SiO₄ concentrations.     

Developing a kinetic alternative in modeling soil aluminium

Soil chemistry models often use gibbsite solubility and similar equilibrium models to predict Al concentrations in soil solution. A kinetic alternative was developed with the goal of finding universal rate constants instead of the site- and depth-specific solubility constants usually associated with the equilibrium approach. The behavior of the two approaches was studied within the framework of the steady-state soil chemistry model PROFILE using data from Solling, Germany and Gårdsjön, Sweden, two sites with different mineralogy and land use history. The kinetic alternative uses a mass balance to predict Al concentrations. The sources of Al in soil water are deposition, weathering and mineralization. The sinks are leaching and the formation of an aluminosilicate precursor. The precursor slowly transforms into an ordinary clay mineral. Both formation and transformation of the precursor are treated as irreversible processes. The kinetic model introduces a new relationship between pH and Al and produces a systematic pattern of different apparent gibbsite equilibrium constants at different depths. Results show that the kinetic model systematically underestimates Al concentration in the upper horizons, which indicates that there may be additional sources of Al in the upper horizons not accounted for in the model. Predicted values of pH and Al concentrations are comparable with field observations.     

Evaluation of porous ceramic cups for monitoring soil-water aluminium in acid soils: comment on a paper by Raulund-Rasmussen (1989)

Following recent observations by Raulund-Rasmussen (1989) implicating A1 contamination of soil solutions isolated by suction-cup samplers, A1 release from porous ceramic cups in acid solutions was investigated. In our studies a flush of Al, followed by a gradual decrease in leaching over successive extractions was observed. The amount of Al released was retarded by the presence of 37 μmol dm^?3 of A1 in solution. Gibbsite solubility controls were not observed; all solutions isolated by the cups were undersaturated with respect to amorphous gibbsite. The cups evaluated in this study are appropriate for sampling acidic soil solution, provided they are suitably pretreated and then equilibrated in the field before use.     

The relationships of the clay mineral suites to the parent rocks of eight soil profiles in Sarawak,Malaysia

The clay mineralogy of eight selected soil profiles developed on acid igneous, pyroclastic and sedimentary rocks was investigated. The effects on the clay mineralogy of the parent-rock mineralogy and physical conditions within each soil profile were evaluated.The results indicate that the mineralogies of the parent rocks control the type of clay minerals formed in Sarawak. In the soils derived from pyroclastic and coarse-grained acid igneous rocks without muscovite, the clay-mineral suites consist almost exclusively of kaolinite and gibbsite with small amounts of goethite. In contrast, in the soils developed from fine-grained acid igneous and sedimentary rocks with muscovite, the clays contain relatively large amounts of interstratified mica-vermiculite and chlorite-vermiculite in addition to gibbsite and/or kaolinite. The presence of gibbsite in these soils depends on the presence of plagioclase feldspar in the parent rock.     

Aluminum solubility of strongly acidified allophanic Andosols from Kagoshima Prefecture,southern Japan

Abstract

The aluminum solubility of acidified soils both from furrows and under tree canopies of a tea garden was studied using equilibrium experiments in 0.01 mol L^?1 CaCl₂ solution systems. The soils were originally classified as allophanic Andosols. The furrow soils were more severely acidified because of the heavy application of nitrogen fertilizer, especially in the upper soil horizons (pH[H₂O] of 3.6–3.8 in the A1 and 2A2 horizons). These acidified soils were characterized by the dissolution of allophanic materials (allophane, imogolite and allophane-like materials) and by an increase in Al–humus complexes. Ion activity product (IAP) values of the strongly acidified soil horizons were largely undersaturated with respect to imogolite (allophanic clay) or gibbsite. Plots of p(Al³⁺) as a function of pH strongly indicated that Al solubility of the soils was largely controlled by Al–humus complexes, especially in the A1 horizon. In the canopy soils, which were more weakly acidified (pH[H₂O] 4.9–5.0), Al solubility was close to that of gibbsite and allophanic materials, indicating that the solubility is partly controlled by these minerals.     

Reactions of nucleic acid bases with inorganic soil constituents

The adsorption at pH's 4, 6 and 8 of adenine, guanine, cytosine, thymine and uracil on clays (montmorillonite, illite and kaolinite), Fe- and Al-oxides (goethite, hematite and gibbsite), a soil, and on a laboratory-prepared fulvic acid-montmorillonite complex was investigated. Portions of the clays and soil were saturated with H⁺, Fe³⁺ and Ca²⁺.Quantitatively, the extent of adsorption of nucleic acid bases by the clays was proportional to their exchange capacities, but the nature of the dominant cation had only minor effects. By contrast, the adsorption was strongly affected by pH, tending to decrease with increase in pH. Adsorption on goethite and gibbsite was lower than that on clays, while adsorption of nucleic acid bases on soils was slightly lower than that on oxides. The fulvic acid-montmorillonite complex adsorbed substantial, although smaller amounts of purines and pyrimidines, than did montmorillonite alone. The main adsorption mechanism at pH 4 appeared to be cation exchange whereas at pH 8 complex formation between the nucleic acid bases and cations on inorganic surfaces seemed to occur.The results of this and earlier work show that both inorganic and organic soil constituents adsorb nucleic acid bases. Which adsorption reaction predominates will depend on the clay and organic matter content and on the pH.     

Podzolization as a deferralitization process: a study of an Acrisol–Podzol sequence derived from Palaeozoic sandstones in the northern upper Amazon Basin

Morphological, geochemical and mineralogical studies were carried out in a representative soil catena of the low‐elevation plateaux of the upper Amazon Basin to interpret the steps and mechanisms involved in the podzolization of low‐activity clay soils. The soils are derived from Palaeozoic sandstones. They consist of Hydromorphic Podzols under tree savannah in the depressions of the plateaux and predominantly of Acrisols covered by evergreen forest elsewhere. Incipient podzolization in the uppermost Acrisols is related to the formation of organic‐rich A and Bhs horizons slightly depleted in fine‐size particles by both mechanical particle transfer and weathering. Weathering of secondary minerals by organic acids and formation of organo‐metallic complexes act simultaneously over short distances. Their vertical transfer is limited. Selective dissolution of aluminous goethite, then gibbsite and finally kaolinite favour the preferential cheluviation of first Fe and secondly Al. The relatively small amount of organo‐metallic complexes produced is related to the quartzitic parent materials, and the predominance of Al over Fe in the spodic horizons is due to the importance of gibbsite in these low‐activity clay soils. Morphologically well‐expressed podzols occur in strongly iron‐depleted topsoils of the depression. Mechanical transfer and weathering of gibbsite and kaolinite by organic acids is enhanced and leads to residual accumulation of sands. Organo‐metallic complexes are translocated in strongly permeable sandy horizons and impregnate at depth the macro‐voids of embedded soil and saprolite materials to form the spodic Bs and 2BCs horizons. Mechanical transfer of black particulate organic compounds devoid of metals has occurred later within the sandy horizons of the podzols. Their vertical transfer has formed well‐differentiated A and Bh horizons. Their lateral removal by groundwater favours the development of an albic E horizon. In an open and waterlogged environment, the general trend is therefore towards the removal of all the metals that have initially accumulated as a response to the ferralitization process and have temporarily been sequestrated in organic complexes in previous stages of soil podzolization.     

Effect of pH on Zinc Adsorption and Solubility in Suspensions of Different Clays and Soils

Zinc solubility in clay and soil suspensions was controlled by chemisorption at pH 4.5 – 7.0. The solubility in clay mineral suspensions was in the order palygorskite < montmorillonite « kaolinite and reflected the high affinity of zinc to palygorskite and the high CEC of montmorillonite. The solubility in soil suspensions was in the order Haplustoll < Torrifluvents and reflected the effect of high CEC and organic matter content of the first. The slopes of the pH-pZn curves, calculated zinc potential and sequential desorption data suggested that Zn⁺⁺ ? Zn(OH)₂ aqueous controlled the solubility of zinc in soil and clay mineral suspensions at pH 7.5 – 9.0. The slopes of the pH–pZn curves of two soils were, however, modified by the possible peptization of organic matter and Zn(OH)₂.     

Disintegration of the clay fraction in the eluvial horizons of some Danish podzolized soils

Abstract

Using sequential extractions, total elemental analysis, and X‐ray diffraction, we have investigated the impact of the podzolization process on component composition of the clay fraction in the eluvial horizons of eight more or less podzolized Danish soils. The results indicate that podzolization is highly aggressive towards all clay components in the eluvial horizons eventually leading to their disintegration. The 2:1 layer silicate clay minerals, illite and chlorite, are first transformed into other 2:1 layer silicate clay minerals. After passing through a microcrystalline phase high in Si but low in Al, Fe, Mg, and K, they finally disintegrate completely. Even gibbsite and kaolinite disintegrate under the aggressive conditions, caused among other things by the presence of dissolved complex forming organic molecules in these horizons. Application of lime and fertilizers seems to be able to reverse the process in case of the 2:1 layer silicate clay minerals.     

Solubility characteristics of proto-imogolite sols: how silicic acid can de-toxify aluminium solutions

Proto-imogolite sols can be considered as highly dispersed forms of proto-imogolite allophane, the most widespread type of allophane in volcanic and non-volcanic soils world-wide. The solubility characteristics of such sols define the conditions of precipitation of allophanes in soils, and the maximum concentrations of aluminium released during acidic episodes from soils, such as podzols, that contain allophane. Direct measurement of Al, Si and pH values in equilibrium with proto-imogolite sols, approached from higher and lower pH, indicated a solubility equation: where log*K_so lay in the range 7.14 to 7.23 after equilibration for 4–24 weeks at 22 + 2°C in 17 of the 20 systems studied. The mean value of log *K_SO at 298 K was calculated as 7.02. This value indicates that proto-imogolite will be more stable than amorphous aluminium hydroxides at H₄SiO₄ concentrations above 5 × 10^?6m , but less stable than bayerite below 10^?3m H₄SiO₄, and than gibbsite below 10^?2m . Proto-imogolite is more stable than micro-crystalline gibbsite in 10^?4m H₄SiO₄, a typical minimum concentration in soil solutions and streams in landscapes where podzols are present. The rapid formation of proto-imogolite effectively prevents the formation of gibbsite seeds in soil, except in highly leached and warm environments, i.e. in older landscapes in the tropics. Although the presence of 10^?4m silicic acid has been found to eliminate the acute toxicity to fish exhibited by solutions containing 6–7 μm Al at pH 4.96, little or no proto-imogolite would form under these conditions. Silicic acid would, however, prevent the precipitation of aluminium hydroxides, and could inhibit the formation of the A1₁₃ polycation. These polymeric species are a likely cause of the increased toxicity exhibited by partially neutralized aluminium solutions.     

Aluminium solubility related to secondary solid phases in upper B horizons with spodic characteristics

We examined the aluminium solubility in the upper B horizon of podzols and its relation to the solid phase of the soil in 60 samples covering a pH range from 3.8 to 5.1. Solid phases were characterized by extractions with acid oxalate and pyrophosphate (pH 10). The solubility of Al was studied in a batch experiment in which samples were equilibrated with 1 m m NaCl at 8°C for 5 days. We also monitored the dissolution kinetics of Al and Si, in some samples. The oxalate and pyrophosphate extractions suggested that secondary Al was mainly organically bound in most soils, and imogolite-type materials seemed to constitute much of inorganic secondary Al. No single gibbsite or imogolite equilibrium could explain Al³⁺ activities. In all samples Al solubility, defined as log{Al³⁺} + 1.65pH, was closely related to the molar ratio of aluminium to carbon in the pyrophosphate extracts (Al_p/C_p). Solubility increased with the Al_p/C_p ratio until the latter reached ≈ 0.1. This indicated that solubility was controlled by organic complexation, at least when Al_p/C_p was small. Silica dissolved slowly in most soils used in the kinetic experiments. We conclude that imogolite-type materials in the upper B horizon dissolved slowly because of coating with humic substances or ageing or both.     

广西水耕人为土黏粒矿物组成及其空间分布特征

黏粒矿物影响着土壤理化性质,可指示成土因素特征和土壤发生发育过程/强度,也是中国土壤系统分类的基层单元土族矿物学类型划分的重要依据。本研究选择了广西不同纬度和成土母质的18个代表性水耕人为土的剖面,应用X射线衍射(XRD)方法分析了其典型水耕氧化还原层(Br层)的黏粒矿物组成及其空间分布特征,并确定了其中"黏质"剖面的土族控制层段矿物学类型。结果表明:①供试土壤的黏粒矿物主要包括高岭石、伊利石、三水铝石、1.42 nm过渡矿物、蒙脱石和蛭石等,依次分别出现在100%、88.9%、72.2%、61.1%、44.4%和38.9%的剖面中。②黏粒矿物组成在纬度空间分布上具有明显规律性特征。随着纬度降低,土壤黏粒中的高岭石增加,伊利石、蒙脱石、1.42nm过渡矿物逐渐减少;纬度>23°N区域内,成土母质对黏粒矿物组成影响明显。③纬度23°N是黏粒矿物组成和土族矿物学类型分界线,<23°N区域,黏粒矿物均以高岭石为主,是"黏质"剖面的土族控制层段的主要矿物学类型;>23°N区域,黏粒矿物组成以高岭石、蒙脱石、伊利石或1.42 nm过渡矿物为主,因成土母质不同而异,"黏质"剖面的...     

Solubility and standard Gibbs free energy of formation of natural imogolite at 25°c and 1 atm

We carried out a dissolution experiment to determine the solubility and the Gibbs free energy of formation of natural imogolite at 25°C and 1 atm. Purified gelatinous film of natural imogolite, synthetic gibbsite, and imogolite plus gibbsite were equilibrated in 1 mmol L^-1 HCI solutions and Si, Al concentrations and pH were monitored during 700 d. Imogolite plus gibbsite and gibbsite systems reached an equilibrium after 120 d. The logarithmic values of the equilibrium constants of dissolution reactions were 12.10±0.01 for imogolite and 8.01±0.01 for gibbsite. The calculated Gibbs free energy of formation was -2929.19±3.28 kJ mol^-1 for imogolite and -1155.11 ± 1.42 kJ mol^-1 for gibbsite. These values predict that the silicic acid concentration at the imogolite-gibbsite equilibrium would be 0.120 mmol L^-1.