Semiquantitative determination of imogolite and Al-rich allophane (Si / Al ≃ 1 : 2) in some volcanic ash soils in San'in region by a combination of thermogravimetry-differential thermal analysis and trimethylsilylation analysis of soil clays

Abstract

A method to determine the contents of imogolite and Al-rich allophane (Sil Al ? 1 : 2) in volcanic ash soils was presented. The method is based on the (1) assessment of the presence of Al-rich allophane in clays by successsive extraction with dithionite-citrate and oxalate-oxalic acid, (2) trimethylsilylation of soil clay with a mixture of hexamethyldisiloxane, HCl, and isopropyl alcohol, and determination of the content of monomeric Si based on the trimethylsilyl derivative of monomeric orthosilicate anion by gas / liquid chromatography, (3) determination of the total content of imogolite and Al-rich allophane based on the content of monomeric Si from imogolite, (4) determination of the imogolite content by Thermogravimetry (TG )-Differential Thermal Analysis (DTA) based on the weight loss due to endothermic dehydroxylation with maximum values at ca. 386°C, (5) calculation of the Al-rich allophane content by subtracting the imogolite content from the total content of these minerals, and (6) evaluation of the imogolite and Al-rich allophane content of soil by multiplying clay content of soil and the two mineral content of clay. The trimethylsilylation analysis was found to be reproducible, and the estimated total amounts of two minerals in clays by this method were adequately approximated to those evaluated from the amount of Si (= Si_o) extracted with oxalate-oxalic acid after extraction with dithionite-citrate. The variation in the abmldance of two minerals in the soil horizons of volcanic ash soils from the San'in region indicated that this method is suitable for the profile-study of volcanic ash soils.     

ASH SOILS IN WESTERN SUDAN

The ash soils of the Jebel Marra region of Western Sudan are developed on a relatively recently deposited felspathic ash and do not contain allophane. Though they show some specific characteristics of ash soils elsewhere, many features are common to other soils of entirely different origin in the same region. Because of their immaturity, variation due to altitude and rainfall, which may differ considerably over short distances in the Jebel Marra, Massif, is minimal.     

Mineralogical changes with depth in a layered Andosol near Bandung, Java (Indonesia)

The mineralogy and some physical and chemical properties of an Andosol developed in four distinct layers of volcanic ash have been investigated. Analytical methods used include X-ray diffraction, transmission electron microscopy, particle-size analyses, infrared spectroscopy, pyrophosphate extracts of C, Al and Fe, and acid-oxalate extracts of Al, Fe and Si. Three buried A horizons in the ash were also dated by the ¹⁴C method. The dates obtained indicate burial of A horizons c. 17700, 14500 and 8600 years B.P., respectively. Halloysite dominates the weathered fraction in the ash older than 17700 B.P., whereas allophane and imogolite with molar A1/Si ratios of 1.7–2.0 are predominant in the younger ash layers. Some kaolinite also occurs in the upper two layers of ash. It is considered possible that the halloysite in the deepest ash layer formed under different climatic conditions from those prevailing at present, rather than due to resilication of allophane under the overburden of ash. The results also demonstrate the mineralogical complexity of soil developed in more than one layer of volcanic ash.     

A climosequence of soils on late quaternary volcanic ash in highland Papua New Guinea

Some chemical, physical and mineralogical properties of three soils developed in volcanic ash at altitudes of 1040 m, 1720 m, and 2350 m in Papua New Guinea's Enga Province are presented. Silt-fraction mineralogy and total chemical analyses show that fresh ash occurs in the upper approximately 30 cm of profile at each site. This fresh ash is probably only a few thousand years old and overlies an older weathered ash (Tomba Tephra;more than 50000 years old). At the lowest site the majority of the primary minerals (predominantly amphiboles, volcanic glass and felspar, and some pyroxenes) have been decomposed by weathering to produce a high clay content. With increasing altitude, greater proportions of silt-sized, unweathered and partially altered primary minerals are recognised and molar ratios of calcium, magnesium, sodium and potassium to aluminum increase accordingly. In the clay fractions, allophane with Al/Si ratios of approximately 2.0 is dominant at the highest site, whereas with decreasing altitude lower proportions of allophane occur, Al/Si ratios decline and halloysite becomes dominant. Gibbsite is found in all three profiles     

Phosphate reactions with natural allophane, ferrihydrite and goethite

The reactions of phosphate with natural samples of allophane, ferrihydrite, hematite and goethite were measured for up to 30 d. The amount of phosphate sorbed on allophane showed the biggest increase with time whereas the amount sorbed on goethite showed the least increase with time. The total amount of phosphate sorbed either at high levels of phosphate addition or after 10 d followed the order hematite < goethite < ferrihydrite < allophane and was probably related to the specific surface. Si was desorbed as phosphate was adsorbed on the minerals.
The reactions of phosphate on allophane involved rapid, strong adsorption, probably at defect sites, followed by weaker adsorption, followed, probably, by disruption of the allophane structure together with precipitation of aluminium phosphates. Previous suggestions either of diffusive penetration of phosphate into surfaces or about the formation of aluminium phosphate coatings, are unlikely to hold for allophane, if all the Al is at the surface and if the structure can be ruptured.
The reactions of phosphate with iron oxides involved a rapid, strong ligand exchange, followed by weaker ligand exchange, and, probably, by a relatively slow penetration at defect sites and pores. Highly crystalline goethite has virtually no slow reaction and therefore solid-state diffusion of phosphate does not readily occur. The extent of phosphate uptake during the slow penetration reactions probably depends on the degree of crystallinity or porosity of iron oxides.
The most reactive adsorbents, such as allophane, ferrihydrite and Al-humus complexes do not have planar surfaces, and this needs to be considered when modelling phosphate reactions.     

Implications of clay mineralogy to the weathering and chemistry of AP horizons of ando soils in Japan

The clay mineralogy of 22 samples of the Ap horizons of Ando soils was determined by a combination of methods. Of these samples, 15 did and 7 did not contain allophane and imogolite. Opaline silica was found in 4 samples, whereas aluminum—humus complexes, iron oxides and layer silicates were found in all samples. The presence of allophane and imogolite and the absence of opaline silica in a few Ap horizons was related to mixing of A₁ horizons and subsoils by cultivation and to lower supplies of organic matter relative to the amounts of aluminum released from volcanic ash by weathering. The contents of 2:1 and 2:1:1 layer silicates and their intergrades were larger in soils in which quartz predominated in fine fractions. It was inferred that aluminum bound with humus and in allophane-like constituents, rather than aluminum in allophane and imogolite, is important in reactions with phosphate and fluoride.     

Allophane in Some Ando Soils

Allophane has been known to occur widely in volcanic ash soils in Japan and New Zealand. However, exact knowledge of its nature has not been well established, mainly because of extreme difficulty to separate it in pure state and of its x-amorphous nature. In the course of the studies on soil allophane, it was noticed that certain Ando soils contained two different mineral colloids together, in addition to crystalline clay minerals and free sesquioxides. X-ray examination revealed that one was x-amorphous colloid which would be called allophane, and the other an unknown colloid of low crystallinity. Imogolite*** Imogolite will be described in a subsequent paper by the present authors. was proposed as the name of the latter colloid by the present authors after imogo in which imogolite was first found. Imogo is a brownish yellow, volcanic ash soil. in the Kuma basin in the Kumamoto Prefecture³⁾. When deferration treatment is applied to the soils, allophane disperses both in an acid and alkaline media, whereas imogolite disperses in an acid medium and flocculates in an alkaline one.     

Demerium exchange of hydroxyl groups in allophane

The nature of amorphous silicates, allophane,ö The name allophane is used in this paper in a dual sense; the one stands for a group of amorphous or nearly amorphous aluminum silicates associated with minor amounts of other metals, and the other in some restricted sense for amorphous silicates other than imogolite. The latter term, imogolite, is used for convenience to denote a component that shows broad x-ray diffractions at 14 (strong), 7.6 (medium) and 5.6 (weak) Å, gives an endotherm at 430°C, and appears as thread-like particles of diameter 100 to 200 Å in electronmicrographs (21). as a main clay constituent determines many of the physical and chemical properties of volcanic ash soils. Recent developments in the study of allophane together with other amorphous inorganic materials in relation to their nature and occurrence in soils have been reviewed by MITCHELL et al. (12).     

Crystalline clay minerals of the soils derived from recent volcanic ashes in Hokkaido,Japan

For the past ten years much work has been carried out on clay minerals of volcanic ash soils. Most investigators have reported that allophane is dominant among clay minerals of volcanic ash soils and crystallizes to halloysite or meta-halloysite with the advance of weathering (1–8). On the other hand, UCHIYAMA, MASUI and ONIKURA (1960) found that montmorillonite predominates in the clay fraction of volcanic ash soil in Kawatabi (9). Furthermore, MASUI, SHOJI and UCHIYAMA (1966) showed that the major crystalline clay minerals of volcanic ash soils in the Tohoku district are montmorillonite, vermiculite, intergradient montmorillonite-vermiculite and chlorite (10). They also showed that these minerals increase with the advance of weathering and that kaolin minerals are minor constituents.     

Stability of allophane,allophanic clay,and allophane-halloysite floc in aqueous solutions of an anionic exocellular heteropolysaccharide (Gum Xanthan) from Xanthomonas campestris

Stability of allophane with positive and/or negative charges in the presence of gum xanthan (GX) was mainly investigated at three pH levels. At pH 4.5, allophane showed polymeric iocculation (PFL) at low GX concentrations, and polymeric stabilization (PST) at medium GX concentrations. Flocculated allophane (i.e.p.) at pH 6.5 showed polymeric weak-stabilization (PWST) at extremely low GX concentrations, besides PFL and PST described above. At pH 8.5, allophane did not exhibit any change in stability with GX addition. The PFL and PWST can be regarded as charge neutralization between allophane and GX. Allophanic clay with i.e.p. at pH 10 exhibited both PFL and PST but no PWST, even at pH 8.5. The PWST can be explained by the disintegration of the floc when interparticle attraction was weakened by the adsorption of GX. The lower PWST and PST of allophane-halloysite mutual floc strongly suggested the presence of a more stable arrangement among these dissimilar particles. Stability of crystalline clays was also studied for comparison. The above behavior was analyzed on the basis of (i) the suspended microaggregate of allophane, and (ii) the double-stranded wormlike structure of GX, which was taken in this study as a model substance of anionic microbial polysaccharide.     

Volcanic ash and its clay mineralogy at Cape Hoskins,New Britain,Papua New Guinea

Volcanic ash falls, the oldest laid down approximately 2500 years ago, are described and correlated wherever possible.Most of these ash deposits are thought to have been derived from the Garbuna volcano, located about 40–50 km from Cape Hoskins.Clay mineral data show with increasing age a weathering sequence in the beds in which allophane changes to halloysite. Charcoal data indicate that large amounts of halloysite are present in fossil soil horizons dated between 300 and 2000 years B.P. This compares with a date of 8000–9000 years from Japan,whereas data from St. Vincent indicate that halloysite can be formed under humid tropical conditions within 4000 years.     

Nippon Dojohiryogaku Zasshi (Journal of the Science of Soil and Manure,Japan)

Some allophanic soils in Japan, developed from deposits of volcanic ashes have aggregates, the sizes of which are mostly those of silt and sand, and which are very stable against the ordinary soil dispersion treatment such as decomposition of organic matter with hydrogen peroxide and addition of deflocculants to the suspension, but can be broken to smaller particles by mechanical forces. Oba and Kono (1, 2) reported that there were aggregates stable even against deferration treatment in the soils, especially plenty in ones developed from basic volcanic ashes, and these aggregates could be broken up into clay of allophane mainly by a sonic-wave vibration technique. Miyazawa (3) recognized that the aggregates were only found in Andosols developed under warm-temperate climates, being concentrated in B horizon, but not so much in A horizon and few in gleyey subhorizon, and assumed that they were formed under a specific weathering like seasonal wetting and drying. Yasuo et al. recognized a high correlation between the degree of aggregation and free iron oxide content of the soils in Kanto Plain and suggested that sesquioxide and structure of allophane might associate with the aggregate fonnation. On the other hand, airdrying of volcanic ash soils sometimes reduces their dispersibilities through irreversible formation of aggregates. Kishita (5) found that this effect of drying was remarkable at subsoiLs, and similar results were obtained by TAPA et al. (6). The poor dispersibility and the irreversible change by drying are known also about volcanic ash soils of New Zealand (7, 8), and Latin America (10, 11), Hawaii (9).     

Modelling competitive anion adsorption on oxide minerals and an allophane-containing soil

Can surface complexation constants for anions, drawn from the literature for reference oxides, be combined to describe competitive adsorption in a spodic B horizon sample containing the important adsorbent minerals proto‐imogolite allophane and ferrihydrite? To answer this and to derive complexation constants for the corresponding reference oxides, a CD‐MUSIC model was used, with arsenate as the sorbing ion. To minimize the interference from competing organic substances, a sample containing little organic matter was used. To describe the adsorption of added arsenate correctly, it was found that competitive interactions from sulphate, silicic acid and phosphate had to be considered. In the model, the specific surface area of singly coordinated AlOH groups of allophane, the sulphate surface complexation constant on allophane, and the total concentration of reactive silicic acid were fitted. All other parameters were fixed using reference oxide values. The results indicated that arsenate, phosphate and silicic acid formed stronger surface complexes on ferrihydrite than on gibbsite or allophane, whereas the reverse was true for sulphate. I conclude that the approach used should provide significant qualitative information on the competitive adsorptive interactions in soils. However, the approach may be impractical for routine simulations and predictions. This is partly due to the uncertainty of the assumption that the properties of allophane and ferrihydrite in real soils can be approximated by those of gibbsite and ferrihydrite synthesized in the laboratory. Another difficulty is that the adsorption of arsenate and phosphate might not reach equilibrium within the limited time of most experiments.     

Interactions between Escherichia coli and allophane—II. Metabolism

A purified allophane clay fraction increased the respiration of Escherichia coli (ATCC 11303B) by buffering the suspension against decreases in pH. Allophane had no significant effect on respiration in the presence of 0.02 M phosphate buffer. When E. coli was exposed to pH values below 5.0 in the presence of allophane, allophane supernatant (which contains soluble Al), or soluble Al. toxicity was evident even after raising the pH to 7.0.     

Mineralogy of gel-like substance in the pumice bed in Kanuma and Kitakami Districts

A characteristic gel-like substance has been noticed around weathered pumice grains in the pumice beds of Kanuma near Utsunomiya and of Kitakami, Iwate prefecture. This substance was first studied by SHIOIRI (6) in 1934, and reported as allophane according to its chemical composition, refractive index, and dye-adsorbing nature similar to the colloid of volcanic ash soils of the Onji-type. Recently, KUWANO and MATSUI (5) remarked that the colloidal film in the Kanuma and Imaichi pumice beds diffracted x-rays at about 8 and 33 Å, and they presumed that this substance might be an early transitional material from allophane to some crystalline clay minerals. KANNO (2) and KANNO et al. (3) examined this gel-like substance from Imaichi and Kitakami districts precisely by the x-ray diffraction, differential thermal, infrared spectroscopical, electron microscopical, and chemical methods, and they concluded that the substance was a mixture of poorly crystallized montmorillonite, allophane in various weathering stages, and free sesquioxide, although there was no positive evidence of montmorillonite. YOSHINAGA and AOMINE (7) noticed that the properties of imogolite designated by themselves bore a striking likeness to those of the gel-like substance reported by KANNO et al. (3), and they considered that both substances were essentially of the same kind irrespective of occurrence.     

Flocculation and Stabilization of Allophane in the Presence of an Anionic Microbial Polysaccharide (Gum Xanthan) and Several of Its Hydrolytic Intermediates

The polymeric effect of gum xanthan (GX) on the stability of allophane at pH 4.5 and 6.5 was compared with the effect of several of its hydrolytic intermediates (hGX). The polymeric effect of GX was manifested by (i) the effective flocculation at low GX concentrations, (ii) the higher stabilizing ability above the flocculation concentration (FC), and (iii) the appreciable ability of deflocculation of (a) flocculated allophane and (b) allophane-halloysite floc, at extremely low concentrations of GX solution at pH 6.5. In contrast to GX, in the presence of sodium polyacrylate, there was a complete lack of deflocculation of the flocculated allophane at pH 6.5. This fact suggests that effective deflocculation is due to the double-stranded wormlike structure of the GX chain with a small but finite flexibility. The fact that flocculated allophane by GX at pH 4.5 still exhibited a positive electrophoretic mobility can be ascribed to the flocculation of allophane particles by bridging with this biopolymer. The relationship between FC of GX and the allophane content in soil clays was also examined.     

Soil organic matter chemistry in allophanic soils: a pyrolysis-GC/MS study of a Costa Rican Andosol catena

Soil organic matter (SOM) in allophanic soils is supposed to accumulate due to protection caused by binding to allophane, aluminium and iron. We investigated a catena of allophanic and non‐allophanic soils in Costa Rica to determine the effect of such binding mechanisms on SOM chemistry. These soils contain no contribution of black carbon. Molecular characterization of litter, extractable and dispersed organic matter was done by Curie‐point pyrolysis‐GC/MS. The molecular chemistry of the organic fractions indicates a strong decomposition of plant‐derived organic matter and a strong contribution of microbial sugars and N‐compounds to SOM. Both the decomposition of plant‐derived SOM – including that of relatively recalcitrant compounds – and the relative contribution of microbial SOM were greater in allophanic samples than in non‐allophanic ones. This suggests that chemical protection does not act on primary OM, although it may influence the accumulation of secondary OM in these soils. The effect of allophane on SOM contents in such perhumid soils is probably through incorporation of decomposition products and microbial SOM in very fine aggregates that – in a perhumid environment – remain saturated with water during much of the year. Greater concentrations of aliphatics are found in allophanic residues, but there is no evidence of any specific mineral‐organic binding. The results do not support the existing theory of chemical protection of plant‐derived components through binding to allophane, iron and aluminium.     

Heat of immersion of soil allophanic clays

Amorphous soil inorganic materials, specifically allophane. are believed to have a marked effect on soil properties and soil productivity, even in a relatively small amount, due to their large specific surface area, peculiar nature of surface and high chemical reactivity. They seem be present in soils, more or less, but determination of their content is generally semiquantitative or not fully reliable, because allophane includes aluminosilicates in a wide range of chemical composition and of properties, and any satisfactory, quantitative analytical method has not been established.     

Weathering and allophane neoformation in soils developed on volcanic ash in the Azores

On Faial and Pico islands (Azores), we studied two profiles on basaltic pyroclasts that contain buried horizons, and we focussed on petrography, micropedology and mineralogy. Emphasis was given to weathering of the lapilli and ashes, and the neoformation of allophane. A combination of optical studies, in situ chemical analyses, X‐ray diffraction and infrared spectroscopy of clay fractions revealed that allophane is present both in the micromass of the groundmass, in alteromorphs after lapilli or pumice, and in clay coatings. Whereas most studies describe allophane as a colloidal fraction formed by the congruent and total dissolution of the ashes, this study shows evidence for the formation of allophane alteromorphs, due to leaching of Si and cations, with preservation of the original shapes of the tephra. The allophane alteromorphs often display optical characteristics that resemble those of palagonite. Increasing alteration is observed through three steps: (i) hydration of the glass associated with strong cation and Si leaching, (ii) allophane hypocoatings, and (iii) allophane alteromorphs with development of intragrain bridges. The chemical signature of the alteromorphs varies from a pure alumino‐silicate at one extreme to an Fe(Ti) enriched alumino‐silicate at the other. Between those two extremes, the colour grades from yellow to dark orange, with microzonations. An Al‐rich allophane composition is associated with gibbsite in the EUR6‐Pico profile, whereas at the base of the EUR5‐Faial profile, Si‐rich allophane is associated with halloysite. Some variations of the palaeo‐environment are also suggested by strong iron segregation observed with various secondary phases (ferrihydrite, haematite, iddingsite).     

Clay mineralogy of some volcanic ash soils in which cristobalite predominates

The clay mineralogical composition of soils on volcanic ashes from Mashū and Kamuinupuri-dake volcanoes, Hokkaido, which are rich in cristobalite, was determined using petrological, X-ray diffraction, differential thermal, and selective dissolution and differential infrared spectroscopic methods.

The cristobalite occurred in abundance in every size of fraction from coarse sand to clay and every soli from approximately 1,700 to 8,400 years old, and was concluded to be of igneous origin. The major clay minerals were allophanelike constituents and allophane with some layer silicates as the minor clay mineral, being similar to those of andesitic ash soils and different from those of volcanic ash soils containing abundant quartz. The quartz of volcanic ashes was presumed to bederived from the groundmass-equivalent portion of the ashes which had been formed from magma at a low temperature.