Clay Minerals, (2009) 44, 135–155

GEORGE BROWN LECTURE 2008

Allophane and imogolite: role in soil biogeochemical processes

R. L. PARFITT*

Landcare Research, PB 11052, Palmerston North, New Zealand

(Received 18 November 2008; revised 18 March 2009; Editor: Balwant Singh)

ABSTRACT: The literature on the formation, structure and properties of allophane and imogolite is reviewed, with particular emphasis on the seminal contributions by Colin Farmer. Allophane and imogolite occur not only in volcanic-ash soils but also in other environments. The conditions required for the precipitation of allophane and imogolite are discussed. These include pH, availability of Al and Si, rainfall, leaching regime, and reactions with organic matter. Because of their excellent water storage and physical properties, allophanic soils can accumulate large amounts of biomass. In areas of high rainfall, these soils often occur under rain forest, and the soil organic matter derived from the forest biomass is stabilized by allophane and aluminium ions. Thus the turnover of soil organicmatter in allophanicsoils is slower than that in non-allophanicsoils. The organicmatter appears to be derived from the microbial by-products of the plant material rather than from the plant material itself. The growth of young forests may be limited by nitrogen supply but growth of older forests tends to be P limited. Phosphorus is recycled through both inorganic and organic pathways, but it is also strongly sorbed by Al compounds including allophane. When crops are grown in allophanic soils, large amounts of labile P are required and, accordingly, these soils have to be managed to counteract the large P sorption capacity of allophane and other Al compounds, and to ensure an adequate supply of labile P. Because of their physical and chemical properties, allophanic soils are excellent filters of heavy metals and pathogens.

KEYWORDS: brown soils, rain forest soils, allophane, imogolite, carbon, organic matter, phosphate, podzols, Spodosols.

Allophane and imogolite are clay-size minerals drains. Ming et al. (2006) reported that allophane commonly associated with volcanic ash soils where also occurs on Mars. There are three main types of there is sufficient moisture for leaching of silica to allophane, namely, Al-rich soil allophanes, Si-rich take place. They also occur in some non-tephric soil allophanes, and stream-deposit allophanes. soils and sediments, as well as in stream beds and Ross and Kerr (1934) showed that allophane was an X-ray amorphous material commonly associated with the clay mineral halloysite. They suggested that ‘‘the name allophane should be restricted to * E-mail: [email protected] mutual solid solutions of silica, alumina, water and DOI: 10.1180/claymin.2009.044.1.135 minor amounts of bases but should include all such

# 2009 The Mineralogical Society 136 R.L. Parfitt materials, even though the proportions of these structure. Guimara˜es et al. (2007) recently studied constituents may vary’’. Other definitions were the stability and the electronic and mechanical given by van Olphen (1971), Wada (1977), and properties of ‘zigzag’ and ‘armchair’ imogolite Farmer & Russell (1990). In light of work in the nanotubes using the density-functional tight- 1970s, Ross and Kerr’s definition needs to be binding method. The (12,0) imogolite tube had the modified to exclude imogolite (Farmer & Russell, greatest stability of all tubes studied. Uniquely for 1990) and allow for broad peaks in the X-ray nanotubes, imogolite had a minimum in the strain diffraction (XRD) patterns of allophane (Parfitt, energy for the optimum structure. This was in 1990a) as well as syntheticallophanes that may not agreement with experimental data, as shown by contain bases at low pH. The definition given by comparison with the simulated XRD spectrum. Parfitt (1990a) is ‘‘Allophane is the name of a Allophane is made up of hollow spherules with group of clay-size minerals with short-range order an outer diameter of 4À5 nm (Henmi & Wada, which contain silica, alumina and water in chemical 1976) (Fig. 2). Al-rich allophane has an imogolite- combination’’. Imogolite, a tubular clay mineral like structure with an Al:Si ratio of ~2, while Si- found in association with allophane, is excluded rich allophane contains polymerized silicate and has from this definition because it has long-range order an Al:Si ratio of ~1. Stream-deposit allophanes in one dimension (Farmer & Russell, 1990). have Al:Si ratios of 0.9À1.8, with Al substituting for some Si in the polymerictetrahedra. STRUCTURES Al-rich soil allophanes The unit particle of imogolite is a hollow tubule with an outer diameter of ~2.1 nm and a wall Infrared (IR) and nuclear magnetic resonance thickness of ~0.7 nm (Fig. 1). The wall consists of (NMR) spectroscopic studies have shown that Al- an outer gibbsite-like curved sheet, while the inner rich soil allophanes have the imogolite structure surface consists of O3SiOH, with oxygens replacing over a short range (Farmer et al., 1977a; Tait et al., the inner hydroxyls of the gibbsite sheet (Cradwick 1978; Parfitt & Henmi, 1980; Barron et al., 1982; et al., 1972; Farmer & Fraser, 1979). The structural Wilson et al., 1984). The 27Al NMR spectrum of formula of imogolite is (OH)3Al2O3SiOH, giving an allophane from Andisols showed that the Al was Al:Si atomicratio of 2.0. Levard et al. (2008) largely in octahedral coordination (Wilson et al., reported on the synthesis of a single-walled 1984). The 29Si NMR spectra of Al-rich allophanes aluminogermanate nanotube with the imogolite from Spodosols and Andisols (Barron et al., 1982;

FIG. 1. Electron micrograph of imogolite (external diameter of the tubes is ~2.1 nm and internal diameter FIG. 2. Electron micrograph of an Al-rich allophane is ~0.7 nm). (external diameter of the spherules is 4À5 nm). Allophane and imogolite 137

Goodman et al., 1985) show a signal with a 1980). The data, however, are subject to various chemical shift of À78 ppm. Being identical to uncertainties because of the porous nature of that of imogolite, this signal is indicative of the allophane (Paterson, 1977), the presence of impu- presence of tri-substituted O3SiOH tetrahedra. The rities, and assumptions about monolayer coverage. resonance line width for allophane, however, is One model for allophane suggests that the unit larger than that for imogolite, suggesting the particle of allophane is a hollow spherule with structure of allophane is relatively disordered. defects in the wall structure, giving rise to Soil allophanes with Al:Si ratios close to 2:1 micropores of 0.3À2.0 nm diameter (Paterson, would therefore appear to be made up of fragments 1977; Wada & Wada, 1977). The external surface having the imogolite atomicstructure over a short area of allophane calculated for this model is range (Parfitt & Henmi, 1980). The fragments link 800 m2 gÀ1. The defects in the structure may arise to form a porous, hollow spherule with water when the proto-imogolite fragments make edge-to- molecules occupying the spherule interior space as edge contacts to form spherules. These conclusions well as being adsorbed to the outer AlOH surface are supported by recent data from Creton et al. (Henmi & Wada, 1976). Accordingly, Al-rich (2008) who used molecular dynamics calculations allophanes have been referred to as proto-imogolite to propose a structure for allophane. Their model allophanes (Farmer & Russell, 1990), or imogolite- structure was characterized by a Si/Al ratio of 0.49, like allophanes (Parfitt & Wilson, 1985). had external and internal diameters of 5.0 and Proto-imogolite allophane has been used as a 4.0 nm, respectively, and contains six holes of reference point in the allophane group of minerals. ~0.7 nm size. The calculated XRD pattern and Relatively pure samples can be obtained from Raman spectrum of the structure were in a good within lapilli, whereas imogolite is found outside agreement with the corresponding experimental pumice granules (Yoshinaga & Aomine, 1962; data. Their calculations for hydrated allophane Parfitt & Henmi, 1980). showed that the model reproduced the main Some Al-rich soil allophanes have Al:Si ratios as experimental findings on the adsorption of water high as 4:1 (Farmer & Fraser, 1982; Farmer & molecules in the void of allophane and the Russell, 1990; Parfitt & Kimble, 1989) where up to exchange between the ‘‘internal’’ and ‘‘external’’ half the O3SiOH tetrahedra on the inner surface of water. The adsorption of water molecules occurred the spherules are, presumably, replaced by hydro- mostly in the regions with negative electrostatic xyls. Alternatively, fragments of spherules may be potential, where the molecules find an environment present with no Si attached to edge sites (Farmer, close to that in bulk liquid water that explains the 1986). These allophanes are found in the B and C significant water retention by allophane. horizons of some podzolized soils. Electron paramagnetic resonance (EPR) spectro- The XRD patterns of Al-rich allophane have scopy indicates that Fe3+ in natural, Al-rich broad lines at 1.2, 0.43, 0.34, 0.22, 0.19, 0.17 and allophanes occupies two distinct sites: a regular 0.14 nm. The XRD patterns of air-dry allophane and a distorted octahedral site (Parfitt & Henmi, films deposited on glass slides have an additional 1980). Some Fe may substitute for Al in the proto- line at ~2.7 nm arising from X-ray scattering by imogolite structure. In natural samples with an spherical particles. Using these data, van der Gaast Fe:Al atomicratio >1:10, the Fe may be present as et al. (1985) calculated that the diameter of a separate phase such as ferrihydrite (McBride et allophane spherules is ~4.0 nm. al., 1984). Electron micrographs of Al-rich allophane show globular aggregates of varying size and shape. Each Si-rich soil allophanes aggregate is composed of many hollow spherules with external diameters of 4.0À5.5 nm (Wada & The IR spectrum of Si-rich soil allophane with Wada, 1977). Small-angle neutron-scattering Al:Si ratio close to 1:1 differs from that of proto- measurements by Hall et al. (1985) gave an imogolite allophane in that the intense SiÀO average spherule diameter of 5.6 nm for Al-rich stretching band occurs at a greater frequency allophane, and a specific surface area of (1020 cmÀl) and the band at 348 cmÀl is much 638 m2 gÀ1 estimated from ethylene glycol reten- weaker, indicating that some of the silicate is tion. A similar value (581 m2 gÀ1) was obtained by polymerized (Parfitt et al., 1980). The presence of adsorption of nitrogen at 77 K (Vandickelen et al., polymerized silicate (together with some ortho- 138 R.L. Parfitt silicate) is also indicated by the 29Si NMR spectrum This is consistent with the stream-deposit allophane (Goodman et al., 1985). Although some Al may having less active AlOH groups than soil occupy tetrahedral sites, the bulk of the Al is allophanes. apparently present in octahedral sites (Kirkman, 1975; Goodman et al., 1985). Thus, the (gibbsitic) IDENTIFICATION AND Al octahedral sheet also provides the structural ESTIMATION framework for Si-rich soil allophanes but this is now linked to silicate polymers (Parfitt et al., Allophane deposits in the field may be identified by 1980). The Al in tetrahedral sites is probably their characteristic greasy feel. At large water associated with the silicate polymers. The band at contents, the consistence ratings for allophane are À78 ppm in the 29Si NMR spectrum indicates the strongly smeary, slightly plastic, slightly sticky, and presence of some proto-imogolite structures. slightly fluid (Wells & Childs, 1988). As little as Thermal analysis of Si-rich allophanes (Na 2% allophane in soil can be detected in this way. saturated) shows exotherms at a lower temperature Allophane in the field may also be chemically (950ºC) than that of Al-rich allophanes (975ºC). identified by the NaF (Fieldes & Perrott, 1966) or The temperature at which mullite begins to form toluidine blue test (Wada & Kakuto, 1985). The when Si-rich allophanes are heated is >1000ºC fluoride test, however, is not specific for allophane whereas, for Al-rich allophanes, this transformation because fluoride also reacts with Al in Al-humus occurs at 900ºC, suggesting that the two types of complexes. allophane have different structures (Henmi, 1980). In the laboratory, allophane in soil samples may Alternative names for Si-rich allophanes are be identified by dissolution in boiling NaOH defect kaolin allophane or halloysite-like allophane (Hashimoto & Jackson, 1960), hot Na2CO3 (Yoshinaga, 1986). (Mitchell & Farmer, 1962), HCl/NaOH (Segalen, 1968), acid oxalate (Higashi & Ikeda, 1974), and from selective dissolution (Wada & Greenland, Stream-deposit allophanes 1970). Differential XRD shows that acid oxalate is Stream-deposit allophanes have Al:Si ratios of effective at dissolving allophane (Campbell & 0.9À1.8. The unit particles are hollow spherules Schwertmann, 1985). Dissolution in acid oxalate with a diameter of <3 nm, forming globular reagent, and measuring the concentrations of Al and aggregates (Wells et al., 1977). Their structure, Si in solution, are routinely used for the quantitative however, differs from that of soil allophanes in that analysis of allophane (Farmer et al., 1983; Parfitt & the polymerized SiO4 tetrahedra form the frame- Wilson, 1985: Parfitt, 1990a; Parfitt & Childs, work. The IR spectrum shows some substitution of 1988). However, if the sample contains >~0.5% Al for Si in tetrahedral sites. The NMR spectro- carbon (C), the contribution of Al from dissolution scopy and elemental analysis indicate that the Al:Si of Al-humus complexes must be estimated sepa- ratio of the outer tetrahedral sheet is close to 1:3 rately (Parfitt, 1990a). Imogolite, gibbsite, and (Childs et al., 1990). The remaining Al forms an hydroxy-Al interlayers also dissolve in acid incomplete (fragmented) octahedral sheet, the oxalate, but their concentrations are small, at least positive charge of which balances the negative in New Zealand soils (unpublished data). charge of the outer tetrahedral sheet (Goodman et The values of Si and Al extracted by acid oxalate al., 1985). are used in conjunction with the value of Al Stream-deposit allophanes have been called extracted from Al-humus complexes by pyropho- hydrous feldspathoids (Farmer & Russell, 1990; sphate (Alp) (Parfitt & Wilson, 1985). The Al:Si IV Childs et al., 1990). As Al is the dominant Al ratio of the allophane, calculated from (AloÀAlp)/ species in solution at pH > 6, allophanes that form Sio, ranges from 0.4 to 4.0 with a median value of in soils at relatively high pH values may have 1.9, close to that for proto-imogolite allophane similar structures to hydrous feldspathoids (Childs (Parfitt & Kimble 1989). The amount of allophane et al., 1990). in the sample is estimated by multiplying Si by the One sample of a stream-deposit allophane relevant factor that depends on the Al:Si ratio developed less charge than soil allophane, and (Parfitt & Wilson, 1985; Parfitt, 1990a). Allophane adsorbed less phosphate at a low solution concen- contents of up to 60% (w/w) have been measured tration (Theng et al., 1982; Parfitt & Henmi, 1980). for B horizons of soil profiles, and deeper layers of Allophane and imogolite 139 tephra (Stevens & Vucetich, 1985). The limit of Lindner et al. (1998) synthesized allophane using detection is 0.5% for allophane with an Al:Si ratio organicreactants(aluminium trialkylates and of 2. tetraalkyl orthosilicates) and alkanes as solvents. Allophane in the clay fraction of soils can also be Slow addition of water via the gaseous phase, identified by IR spectroscopy, provided the bands followed by hydrolysis and condensation, produced from layer silicates do not obscure those of allophane with a structure similar to that of stream- allophane (Farmer et al., 1977a; Parfitt & Henmi, deposit allophanes. The syntheticallophane 1982), while imogolite may be estimated quantita- consisted of hollow spherules with an outer tively by electron microscopy and differential diameter of 3À4 nm and wall perforations of thermal analysis (Parfitt, 1990b). ~0.35 nm in diameter. The wall was made up of a tetrahedral silicon oxide/hydroxide sheet with partial replacement of Si by Al. Octahedrally PROCESSES OF FORMATION coordinated Al species acted as counter-ions on Laboratory synthesis the inner surface of the spheres. Aluminosilicate particles of short-range order Our understanding of the processes underlying have also been synthesized from solutions the formation of allophane and imogolite has been containing decimolar concentrations of AlCl3 salts greatly improved by the work of Colin Farmer and and tetraethyl-orthosilicate with varying Al:Si and colleagues at the Macaulay Institute (Farmer et al., OH:Al molar ratios (Montarges-Pelletier et al., 1979; Farmer & Fraser, 1982; Farmer, 1986). They 2005). Transmission electron microscopy (TEM) showed that proto-imogolite could be synthesized showed condensed aggregates with a homogeneous using acid solutions of Al and silicic acid (Farmer elemental composition, and an Al:Si molar ratio et al., 1977b; Farmer & Fraser, 1979). If the ranging from 0.2 to 1.7. As the Al:Si ratio gradually concentration of free silicic acid in solution is increased up to 1.3, allophane-like materials with between 3 and 15 mg Si lÀ1, the Al:Si ratios of the macropores, mesopores and micropores developed product range from 1.8 to 2.2; but if this successively. When the Al:Si ratio exceeded 1.3, concentration is between 1 and 3 mg Si lÀ1, the materials with a small surface area were formed. Al:Si ratios range from 2.2 to 4. The formation of The results indicated a gradual increase in particle imogolite and proto-imogolite in soil requires that condensation as the Al:Si molar ratios increased the soil solution is supersaturated with respect to from 0.8 upwards. imogolite. For typical Si concentrations of Abidin et al. (2007) combined experiments with ~3 mg Si lÀ1, the concentration of Al3+, AlOH2+ molecular orbital calculations to show that certain + and Al(OH)2 in equilibrium with proto-imogolite metal ions in solution facilitate allophane formation must be close to that in equilibrium with gibbsite but inhibit the formation of imogolite in the order: (Farmer & Fraser, 1982). The magnitude of this Na, K < Ca, Mg. This is because metals affect the concentration is predicted to fall rapidly from dissociation of the Si-OH group of orthosilicic acid. À4 À6 10 M at pH 4 to <10 M at pH 5. If the Structure optimization of the proto-imogolite model concentration of Al in the soil solution is showed that when the Si-OH was undissociated, the À5 <3610 M, proto-imogolite does not form at pH proto-imogolite adopted an asymmetrical configura- <4 and may not form at pH <4.5 (Farmer, 1986). tion, which then curled to form an imogolite tubule. The formation of allophane and imogolite is also On the other hand, when the Si-OH was dissociated, inhibited by the presence of organic ligands and the proto-imogolite model had a symmetrical humicsubstances(Inoue & Huang, 1986, 1990). configuration which curved to make a hollow The laboratory synthesis of allophane containing allophane spherule with the orthosilicic acid inside. some tetrahedral Al requires neutral or alkaline Lumsdon & Farmer (1995) have shown that the conditions (Farmer et al., 1979; Wada & Wada, activity of Al species in the soil solution is greatly À 2À 1981; Wada et al., 1988). At pH >5.5, Al(OH)4 modified as SiO4 concentrations increase. Bi et al. begins to form, becoming dominant at pH >6. (2001) have used theoretical modelling and Precipitates formed at pH >7 with Al:Si <1 have computer simulation of Al in soil solution, in almost all of the Al in a tetrahedral network. These equilibrium with the mineral phase imogolite, to precipitates have been called hydrous feldspathoids explain the distribution of Al species in the and are similar to stream-deposit allophanes. presence of silicic acid. Increasing concentrations 140 R.L. Parfitt of silicic acid may effectively inhibit the formation following weathering sequence: (1) dissolution of of polymericalumino-hydroxo species, and remove Al from volcanic glass accompanied by the Al toxicity. transformation of AlIV to AlVI; (2) formation of a White et al. (2008) found that allophane gibbsite-like sheet resulting from the hydrolysis of precipitated inside the digestive gland of freshwater the dissolved Al; (3) dissolution of silica gel-like snails. In view of the environmental abundance of polymer in volcanic glass resulting in the both Si and Al, the authors suggested that formation of monosilicic acid; and (4) formation precipitation of Al and Si in living cells provides of an imogolite-like structure as a result of the a defence against Al toxicity. reaction between the gibbsite-like sheet and monosilicic acid. These precipitation reactions could occur in solution as well as on the surface Precipitation and solubility in soils formed in of volcanic glass. tephra In old soils (170,000 y) of Hawaii, Chadwick et Allophane formation and weathering in tephra al. (2003) found that weathering and soil properties and Andisols have been reviewed by Lowe (1986) changed in a non-linear fashion with increasing and Dahlgren et al. (2004). The rate of formation is rainfall. In areas where rainfall was >1400 mm, controlled chiefly by macro- and micro-environ- most of the base cations and Si, but only 60% of mental factors together with the mineralogical and Al, were lost from the old basaltictephra and lava physicochemical composition of the parent deposits. by leaching. At all sites, the secondary clay mineral The effect of time is subordinate to these factors. assemblage was dominated by metastable non- The important environmental factors are the activity crystalline weathering products. of silicic acid in the soil solution, the availability of Allophane can form rapidly from fine-grained Al species, the opportunity for co-precipitation, glass particles (Kirkman, 1980). On the face of leaching regime, soil organic matter, and pH. In open soil pits containing rhyolitic tephra, allophane turn, these factors are influenced by climate, has been observed to precipitate in a matter of drainage, vegetation, tephra thickness, depth of months (Parfitt, unpublished data). Allophane is burial, and additions of fresh tephra. Generally, relatively stable in soils. Indeed, under favourable allophane forms from tephra at pH(H2O) values conditions, it can persist in tephra beds for at least between 5 and 7, and a pH of at least 4.8 is 250,000 y (Stevens & Vucetich, 1985). required for allophane to precipitate (Parfitt & Kimble, 1989). Allophane is generally formed in Precipitation and solubility in soils in the situ in Andisols with very little translocation presence of soil organic matter (Dahlgren et al., 2004). The presence of Al in both octahedral and Soil organic matter can affect the formation of tetrahedral coordination was observed in the coarse allophane in soils. Working in Iceland, Sigfusson et particle-size fractions, which contained both al. (2008) showed that soil solutions from field allophane and rhyoliticglass (Campbell et al., soils, soil cores and repacked soils with an 1977). Tetrahedrally coordinated Al is presumably allophane content of 2À22% and a C content of present in the glass. Hiradate & Wada (2005) and 11À42% were all highly undersaturated with Hiradate et al. (2006) studied the weathering of respect to basaltic glass. Field samples were volcanic glass to allophane using solid-state 29Si supersaturated with respect to allophane and and 27Al magic-angle-spinning NMR. The volcanic imogolite, while samples from the repacked soils glass showed a broad 29Si NMR signal between were often undersaturated with respect to allophane À80 and À120 ppm, with the centre at À104 ppm. and imogolite. The dissolution rate was dictated by This signal was indicative of Si-O-Si bridging the (H+)3/(Al3+) activity ratio in the soil solution; structures. The Al in volcanic glass was present in the rate was slower by up to 20% in field soils tetrahedral coordination. On the other hand, compared with repacked soils, because of octahedral Al (3 ppm by 27Al NMR) and decreasing undersaturation. Dissolution rates, imogolite-like Si (À78 ppm by 29Si NMR) were predicted by the (H+)3/(Al3+) activity ratio, the major components in both Al- and Si-rich increased up to seven-fold in the field, and by allophanes. Based on the NMR spectra of size- 37-fold in the repacked soils by speciating Al3+ fractionated soil samples, they proposed the with oxalate. Speciation of Al3+ with oxalate, which Allophane and imogolite 141 represents the maximum effect of the dissolved however, are less stable than kaolinite. The stability organicC (DOC) on dissolution rates, generally had lines of these minerals are not dependent on either more effect near the surface than at deep levels in pH or basiccations. The stability line for proto- the soil profile. This study showed that for a given imogolite allophane is parallel to that of imogolite temperature, reactive surface area, and volcanic but may lie either above or below the line for glass composition, the chemical weathering rates of imogolite (Percival, 1985). If the allophane line lies Andisols were dictated by: (1) aeolian deposition above that of imogolite, imogolite will be more rates and drainage both of which affect the stable than allophane, and kaolinite will be more saturation state and (H+)3/(Al3+) activity ratio; stable than imogolite. (2) the production of organic anions in the soil; Since halloysite often occurs in older tephras, and (3) the external supply of anions capable of allophane may weather to halloysite when the Si complexing Al3+. activity in the soil solution is high. Thus, this Mineral dissolution kinetics were used by transformation is more likely to occur in rhyolitic Yagasaki et al. (2006) to investigate the mechan- tephra than in basalticand andesitictephra isms controlling the activity of free Al in soil (Kirkman & McHardy, 1980; Dahlgren et al., solutions, and their implications for pedogenesis in 2004). For proto-imogolite allophane to weather to two Japanese volcanic-ash soils. The A and AB halloysite would require a rearrangement of crystal horizons of the soils were rich in organic matter, structures which can only take place by dissolution organically-bound Al, and imogolite and proto- and reprecipitation. imogolite. Nevertheless, the A horizons were Halloysite can also form directly from volcanic undersaturated with respect to imogolite and glass when the Si activity in the soil solution is proto-imogolite. This suggested that the dissolution high (Parfitt et al., 1984; Parfitt & Wilson, 1985). rate of these minerals was low, possibly due to their Singleton et al. (1989) showed that halloysite forms interactions with organicmatter. Organicacids, in preference to allophane when the Si activity however, can also dissolve allophane; thus the exceeds 10 mg lÀ1 while allophane forms in accumulation of organic matter combined with the preference to halloysite when this activity is depletion of imogolite and proto-imogolite are <10 mg lÀ1. Halloysite formation may occur in important long-term processes in the formation microsites within an allophane matrix (Aomine & and development of soils from volcanic-ash. Wada, 1962). In New Zealand aeolian soils, Al-humus complexes are the dominant constitu- halloysite (and no allophane) is found where the ents in some old volcanic-ash soils of Japan which rainfall is 1000 mm, while only allophane is found account for 30% of these Andisols (Dahlgren et al., where the rainfall is 1600 mm. Kaolinite may be 2004). The availability of Al3+ appears to be a present in A horizons of Andisols that also contain major factor in the formation of Andisols with a pH halloysite and where seasonal desiccation occurs, < 5 and large amounts of C. Under these conditions, but it is absent in B horizons (Dahlgren et al., organicacidsare the main proton donors, lowering 2004). pH and Al activity through the formation of Al- Since Si activity in the soil solution is controlled humus complexes. In allophanic Andisols, where by drainage, climate, vegetation and tephra the capacity of soil organic matter for complexing composition, volcanic-ash soils may contain Al becomes saturated, the excess Al can react with various proportions of allophane and halloysite. Si to form allophane (Dahlgren et al., 2004). Where allophane predominates, the soils are usually Andisols, but where halloysite predominates, the soils range from Inceptisols to Alfisols, Ultisols and Precipitation of imogolite and halloysite Oxisols. Where tephra, drainage, parent material Imogolite is often found together with allophane, and/or climate are the dominant controlling factors, but classical pure imogolite in Japan occurs as gel the soil pattern in the landscape can be predicted films over the surface of lapilli (Yoshinaga & (Lowe, 1986; Singleton et al., 1989; Lowe & Aomine, 1962). Stability diagrams show that Palmer, 2005). Where Si in the soil solution is close imogolite is more stable than halloysite over a to 10 mg lÀ1, either halloysite or allophane may wide range of Si activity (Percival, 1985). Above a predominate, and the pattern of soils in the certain level of Si activity, halloysite is more stable landscape is difficult to predict (Parfitt et al., than imogolite. Both halloysite and imogolite, 1984; Parfitt & Wilson, 1985). 142 R.L. Parfitt

Precipitation and solubility in other soils increased with the Alp/Cp ratio until the latter reached ~0.1. This indicated that solubility was Allophane forms not only from volcanic glass but controlled by organic complexation, at least when also from feldspar and/or biotite (Parfitt & Kimble, Alp/Cp was small. The Si dissolved slowly in most 1989). Rapid weathering of feldspar occurs at soils used in the kineticexperiments. Therefore, pH = 5, while allophane precipitates at pH = 4.8. imogolite-type materials in the upper B horizon There is, therefore, a narrow pH range where dissolved slowly because of coating with humic feldspar weathers to allophane (Parfitt & Kimble, substances or ageing or both. 1989). Giesler et al. (2000) showed that Si, Al, and Fe Allophane and imogolite formation in Spodosols within the humus layer (O horizon) play an important and related soils has been described by Tait et al. part in the biogeochemical cycling of these elements (1978), Young et al. (1980), Farmer et al. (1980, in podzolicsoils. The process of podzolization was 1983), Childs et al. (1983), Parfitt & Saigusa reviewed by Lundstro¨m et al. (2000). (1985), Parfitt & Kimble (1989), Dahlgren & Allophane has been found in soils from quartzo- Ugolini (1989), Gustafsson et al. (1995), Go¨ttlein feldspathicloess overlying stony alluvium (Parfitt & Stanjek, (1996), van Hees et al. (2000), and & Webb, 1984). These soils are not podzolized but Karltun et al. (2000). Up to 20% Al-rich allophane are Ochrepts having an ustic soil moisture regime. has been found in the Bs horizons having pH(H2O) Since there is leaching throughout the B horizon of values between 5.1 and 5.4. these soils, particularly in winter, the Si levels in Zysset et al. (1999) investigated Al extractability the soil solution may be low enough for allophane and solubility in six horizons of a Typic to form. Haplohumod from Switzerland. Pyrophosphate and Allophane can also form in drains (Farmer, pers. oxalate extractions as well as successive acid comm.), tuffs (Silber et al., 1994), lava (Jongmans et leaching indicated that in the upper horizons, al., 1995), lacustrine sediments (Warren & Rudolph, reactive Al was mainly bound to soil organic 1997), Silurian sediments (Moro et al., 2000), and matter, whereas in the lower horizons, where limestone (Farmer et al., 1977a). Allophane is also imogolite was present, Al was of inorganicnature. involved in the formation of indurated layers or pans Batch equilibrium experiments showed that in the in soils and sediments (Thompson et al., 1996; upper horizons, Al solubility was controlled by Wilson et al., 1996; Jongmans et al., 2000). complexation reactions to soil organic matter. Allophane, however, is most commonly found in Kineticstudies with samples of the lower horizons tephra layers under humid climates, where volcanic showed that ion activity products with respect to glass dissolves to produce allophane. both Al(OH)3 and imogolite reached a constant value after reaction times of 16 days. For pH >4.1, PROPERTIES the data were consistent with either Al solubility control by imogolite-type material, which dissolves Allophane and imogolite have large specific surface incongruently, or a simultaneous equilibrium with areas (700À1500 m2 gÀ1), have positive and imogolite and hydroxy-Al interlayers of clay negative charges (Parfitt, 1990a), and interact minerals. For pH <4.1, data indicated solubility strongly with anions, such as phosphate and control by poorly crystalline kaolinite. arsenate. Organicmatter is also strongly bound to Simonsson & Berggren (1998) examined the Al allophane and imogolite, decomposing only slowly solubility and dissolution kinetics of Al and Si in in allophane-rich soils. Such soils are usually very the upper B horizon of podzols and its relation to porous and have high water content. the solid phase of the soil. Oxalate and pyropho- Laboratory experiments showed that sorption of sphate extractions suggested that secondary Al was organicmatter (humicsubstances)carrying nega- mainly organically bound in most soils, and tively charged functional groups, increases the imogolite-type materials seemed to constitute negative surface charge of allophane (Perrott, much of inorganic secondary Al. No single gibbsite 1978; Yuan et al., 2000). Humicsubstancesare or imogolite equilibrium could explain Al3+ apparently adsorbed to allophane surfaces by ligand activities. In all samples, Al solubility was closely exchange (Parfitt et al., 1977; Yuan et al., 2000) related to the molar ratio of Al to C in the and up to 25% C is associated with allophanic soil pyrophosphate extracts (Alp/Cp). Solubility clay fractions (Parfitt & Henmi, 1982; Churchman Allophane and imogolite 143

& Tate, 1986). Similarly, the high affinity of been converted from native forest, the amount of C 5’-adenosine monophosphate (AMP) for allophane was related to Al (extracted in pyrophosphate) may be ascribed to ligand exchange involving the rather than to the percentage of clay (Fig. 3) phosphate group of the nucleotide (Hashizume & (Percival et al., 2000). In Costa Rica, soil C Theng, 2007). On the other hand, positively charged concentrations were positively correlated with alkylammonium ions are physically adsorbed by allophane, imogolite, and ferrihydrite at high allophane (Theng, 1972), while neutral amino acids elevation sites, and to Al in organo-metal (at pH 6) adsorb as zwitterions through electrostatic complexes at low elevation sites (Powers & interactions (Hashizume & Theng, 1999). Schlesinger, 2002). Interestingly, some allophanes can apparently In measuring changes in soil C over 20 y in soils discriminate between optical isomers of alanyl- under long-term pasture in New Zealand, Schipper alanine (Hashizume et al., 2002). et al. (2007) found only small losses of dissolved organicC and bicarbonate. They also reported that 0.9Ô0.2 T C/ha/y could be lost from soils under Interaction with organic matter and organic dairy farming, possibly as a result of land-use matter stabilization intensification (MacLeod & Moller, 2006) where Allophanicsoils occupy 0.8% of the global land inputs of C were lower than losses by mineraliza- area, and contain 5% of the global soil C (Dahlgren tion (to CO2) and leaching. In the case of et al., 2004). The residence time of C in allophanic allophanicsoils, however, the loss was soils measured by C-14 is much greater than that of 0.4Ô0.3 T C/ha/y (Parfitt et al., 2007). This would other soil orders (Nierop et al., 2007). The large C suggest that a fraction of the soil C was stabilized content of allophanic soils partly account for their by allophane and Al, and C mineralization was less excellent physical and water-storage properties, and than in non-allophanicsoils (Saggar et al., 1994). high productivity (Dahlgren et al. 2004). Under Parfitt et al. (1997) reported that, under long- natural conditions with high rainfall, allophanic term pasture, an allophanicsoil (0 À20 cm) soils are usually formed under forests. Forest contained 144 T C/ha, while a Recent soil formed growth on young volcanic soils tends to be in alluvium contained 73 T C/ha (Table 1). After limited by nitrogen availability, while on old cropping for 20 y, the C content decreased by volcanic soils primary production tends to be 23 T C/ha in the Recent soil, but by only 10 T C/ha limited through lack of P (Herbert & Fownes, in the allophanicsoil where the C was more stable. 1995). The C in soil, derived from the forest Modelling with Century (Metherall et al., 1993) biomass, is stabilized both by interaction with suggested that the passive pool of the allophanic allophane and Al ions (Torn et al., 1997). In New soil contained more C in the decadal pool. The Zealand soils that are now under pasture, but had alkyl groups, present in the clay fraction of the

FIG. 3. Relationship between soil carbon, clay and pyrophosphate-extractable Al in New Zealand soils (0À20 cm) under pasture (after Percival et al., 2000). 144 R.L. Parfitt

TABLE 1. Carbon in allophanicand non-allophanicsoils for the allophanicsoil and 31T/ha for the gley soil, (after Parfitt et al., 1997) indicating that the mineralization of fresh C was not very different. They suggested that it was the old Pasture Crop pasture C and old forest C that was stabilized by 100 y 20 y allophane and Al in the allophanicsoil. Similarly, Buurman et al. (2007) and Paul et al. (2008) found Allophanicsoil 144 134 that recently incorporated soil organic matter in Alluvial Inceptisol 73 50 Andisols under tropical pastures was not well stabilized. In highly allophanicsoils, the organicmatter allophanicsoil, may be resistant to decomposition; appears to interact rather slowly with allophane. complexation by Al also causes the organic matter Nevertheless, allophane-organicmatter complex to be less susceptible to biological attack and formation accounts for the slower rates of C and mineralization to CO2 (Baldock & Nelson, 2000). N mineralization in these soils compared with non- In Andisols from the Canary Islands, Rodrı´guez allophanicsoils (Broadbent et al., 1964; Zunino et Rodrı´guez et al. (2004, 2006) found that allophane al., 1982; Legay & Schaefer, 1984). This slow and Fe oxides not only sequestered C but also gave mineralization was first shown by bomb C-14 some resistance to soil erosion by rain. Dahlgren et measurements on an allophanicand a non- al. (2004) suggested that organicmatter in Andisols allophanicsoil between 1960 and 1970 (Rafter & might also be stabilized by sorption to micropore Stout, 1970). These data were recently modelled by surfaces, since such pores are inaccessible to Troy Baisden, Rafter Lab, GNS Science, New microbes and their extracellular enzymes. Zealand (Fig. 4), and show that the incorporation Parfitt et al. (2002) investigated the stability of C and turnover of C is slower in allophanicsoils. The in an allophanicsoil and a gley soil by measuring process also accounts for the observation that, per stable C isotopes (delta C-13) and bomb C-14 under unit of organic C, biochemical activities, including both permanent pasture (C3 plants) and continuous most enzyme activities, are lower in allophanic than (25 y) maize (a C4 plant) on the same farm. The C non-allophanicsoils (Ross et al., 1982). contents under pasture and maize were similar but Osher et al. (2003) measured stable C isotopes in the allophanicsoil contained more C than the gley Hawaiian soils where pasture and sugar cane (C4 soil. Within the plough layer the input of maize C plants) had been grown for 100 y on land that had (and the loss of pasture C) over 25 y was 27 T/ha previously been under forest (a C3 plant). Within

FIG. 4. Bomb C-14 incorporation in allophanic (Egmont) and non-allophanic (Tokomaru) soils under similar climate and land-use (after Rafter & Stout (1970); data and residence times, courtesy of Troy Baisden, Rafter Lab, GNS Science, New Zealand). Allophane and imogolite 145 the pasture root zone (0À40 cm), additions of humus complexes, and Basile-Doelsch et al. (2005, pasture (C4) organicmatter were offset by losses of 2007) suggested polymerization could be limited by C3 carbon. The concentration of Fe/Al oxides the presence of organic matter. appeared to control the quantity of C stored in the Hiradate et al. (2006) investigated three repre- soils, as well as changes in the depth and magnitude sentative allophanicAndisols from Japan using of stored C that occurred with each type of land-use solid-state C-13, Al-27 and Si-29 NMR spectro- change. Growing sugar cane appeared to induce scopy and dC-13 measurements. Aliphatic, O-alkyl, both dissociation of Fe/Al oxide-humus complexes, and carbonyl C were relatively abundant in the and losses of oxide-associated organic matter from uppermost horizons, whereas aromaticC was the profile. Under pasture, these complexes were concentrated in the subsurface horizons, where it translocated to deeper soil horizons, leading to an persisted and showed resistance to degradation. The increase in C storage and apparent C turnover time contribution ratio of C4-plant-derived C (mainly below 50 cm depth. In this high-rainfall region, soil from Miscanthus sinensis) to the total C, evaluated C was apparently lost by downward movement, from dC-13 values, was 35À42%, 59À62%, and either as colloids or in solution, or by emission to 50À53% in the subsurface horizons, and 12, 23, the atmosphere as CO2. and 48% in the uppermost horizons. The lower C4/ Working in Ecuador, Lopez-Ulloa et al. (2005) total C ratio in the uppermost horizons reflected the and Paul et al. (2008) measured delta C-13 in influence of recent vegetation. The NMR results paired plots where C4 pasture was converted to C3 indicated that most of the AlIV in volcanic glass had secondary forest in Andisols and sedimentary already converted into AlVI, while large amounts of Inceptisols. Soil C contents were greater in allophane, imogolite, allophane-like constituents, Andisols than in Inceptisols. The mean residence and proto-imogolite were formed in the B horizon time of forest-derived C in pastures increased from within the past 25,000 y. The concentrations of 37 to 57 y with increasing silt + clay content in the these non-crystalline minerals, determined by Si-29 Inceptisols, but was independent of such soil NMR spectroscopy, were generally similar to those properties in Andisols. These results further indicate determined by acid-oxalate dissolution. that long-term organicC stabilization is effected Rasmussen et al. (2006, 2007) studied soils through complexation with metals and allophane in formed from three different parent rocks under Andisols, and through sorption to clay minerals in conifers in California. The andesitic soils were Inceptisols. There is good evidence to show that dominated by non-crystalline materials (allophane, recently incorporated C is not stabilized in Andisols Al-humus complexes), the granitic soils by crystal- while, in Inceptisols, part of this C is stabilized by line minerals (kaolinite, vermiculite), and the interaction with poorly crystalline oxides (Lopez- basalticsoils by a mix of crystalline and non- Ulloa et al., 2005; Paul et al., 2008). crystalline materials. Soil C mineralization rates for Basile-Doelsch et al. (2005, 2007) used DCand the soils followed the order andesitic< basaltic< D13C data, and density fractionation to study C granitic, and correlated negatively with the content sequestration by minerals in a volcanic soil of La of Fe-oxyhydroxides, Al-oxyhydroxides, and Al- Re´union. The buried horizons of the soil contained humus complexes, suggesting mineral control of C much proto-imogolite and proto-imogolite allo- mineralization. Data for respired CO2 suggested phane. These minerals stored large amounts of that soil mineralogy also controlled the increased organicmatter that turned over very slowly; the rate of C mineralization after adding C-13 labelled residence time of organic C in the most stabilized forest litter. This observation, however, may reflect buried horizon was 163,000 y. Although there may differences in the nature of the microbial commu- have been a contribution from black C (charcoal), nities among the three groups of soils. this value is comparable to that for organic C stabilized in Hawaiian volcanic soils. A large part Organic matter composition (83%) of the soil organicmatter in the 3Bw horizon (below 52 cm) was present as organo-mineral Buurman et al. (2007) investigated a catena of complexes. More organic matter was immobilized allophanicand non-allophanicsoils in perhumid by imogolite-type materials than by Fe oxides. The Costa Rica using gas chromatography-mass spectro- amounts of imogolite-type materials were found to scopy (GC-MS) and analytical pyrolysis techniques. be midway between that of allophane and Al in Al- The molecular chemistry of the organic fractions 146 R.L. Parfitt indicated intensive decomposition of plant-derived in the pyrolysates of Andisols. This might be organic matter and a major contribution of microbial because the components from which they originated sugars and N-compounds to the soil organic matter. were stabilized by interaction with short-range- Both the decomposition of plant-derived organic order minerals. Non-andicsoils contained relatively matter, including the relatively recalcitrant constitu- little stabilized C. The humicand fulvicacidsin the ents, and the relative contribution of microbial allophane-rich soils gave rise to complex pyrograms organicmatter, were greater in allophanicthan in and compound assemblages. The alkylic compounds non-allophanicsamples. Buurman et al. (2007) in the humicacids from the allophane-richsoils suggested that chemical protection did not act on were highly condensed, whereas those from primary organic matter, although it could influence Cambisols were more loosely joined fatty acids the accumulation of secondary organic matter in and other alkylicstructures. The authors suggested these soils. The effect of allophane on organic matter that microbial and plant metabolites were rapidly might be due to the incorporation of decomposition incorporated and stabilized within the allophane- products and microbial organic matter in very fine rich soils. aggregates that had large water contents for much of In investigating volcanic-ash soils under varied the year. Although large concentrations of aliphatics vegetative covers along an altitudinal transect near were found with allophane, there was no evidence the upper forest line in Ecuador, Nierop et al. for any specific mineral-organic interaction. These (2007) found that all the mineral horizons ‘‘were results, together with those of Parfitt et al. (2002), characterised by a high degree of aliphatic suggest that plant materials are not bound to compounds’’. Small differences were found within allophane, Al or Fe, as much as the microbial by- the lipid constituents, while polysaccharides and products of decomposition. lignin were virtually absent from the A and Bw Volcanic-ash soils from Atlantic volcanic islands (mineral) horizons containing <6% allophane. The were studied by Naafs & van Bergen (2002) and small abundance of polysaccharides in soils that Nierop et al. (2005), also using GC-MS and have undergone severe organicmatter decomposi- analytical pyrolysis techniques. In contrast to many tion is not unusual, but is uncommon in Andisols. non-volcanic-ash soils, the soil organic matter was The presence of lipids was ascribed to the low pH dominated by polysaccharides and protein deriva- of these soils, while the absence of polyphenols tives, while lipids and lignin were depleted. The suggested rapid decomposition (Nierop et al., 2005, polysaccharides may partly (>13%) be stabilized by 2007; Lorenz et al., 2007). interaction with allophane as suggested by Parfitt et These results and conclusions about the fractions al. (1999) in the case of podzols. of soil organicmatter and their stabilization in soils Gonzalez-Perez et al. (2007) found large amounts containing allophane are summarized in Tables 2 of both alkyl and carbohydrate-derived compounds and 3.

TABLE 2. Fractions of soil organic matter (SOM) that are influenced by the presence of allophane

SOM stabilized by allophane in Podzols and Andosols References

Gonzalez-Perez et al. (2007) Nierop et al. (2005) Dominated by carbohydrate and protein Naafs & van Bergen (2002) Parfitt et al. (1999) Nierop et al. (2007) Aliphaticcompounds present Gonzalez-Perez et al. (2007) Parfitt et al. (1997) Nierop et al., (2005, 2007) Depleted in lignin, polyphenols, lipids Naafs & van Bergen (2002) Plant-derived material strongly decomposed Buurman et al. (2007) Microbial sugars and N compounds Buurman et al. (2007) Allophane and imogolite 147

TABLE 3. Recent conclusions about soil organic matter stabilization in soils containing allophane.

Conclusion References

Paul et al. (2008) Buurman et al. (2007) Fresh organicmatter is not stabilized Lopez-Ulloa et al. (2005) Parfitt et al. (2002) Buurman et al. (2007) Plant-derived material may not bind to Al or allophane Parfitt et al. (2002) Buurman et al. (2007) Microbial products stabilized slowly Parfitt et al. (2002) Basile-Doelsch et al. (2007) Carbon persists for thousands of years Basile-Doelsch et al. (2005) Chadwick et al. (2003)

Leaching of organic matter while that of DON ranged from 0.008 to 0.135 mg N/l. The concentrations of DOC and In investigating leaching of material from a non- DON were positively related to mean annual allophanicAndisol under forest in the Vosges precipitation (accounting for 64% of the variation). Mountains (France), Aran et al. (2001) found very When the soil parent material was included in the small concentrations of dissolved organic C, Al and multiple regression analysis, 79% of the variation in Fe in soil solutions collected with zero-tension plate watershed DOC could be accounted for. The ratio lysimeters over a period of 7 months. The of DOC:DON averaged 29 for waters with volcanic concentrations of water-dispersible clays were also ash and basalt geology, and 73 for waters with a small, suggesting that no significant translocation of non-volcanic geology, suggesting strong adsorption clays occurred in the soil profile. The low mobility of hydrophobicmolecules with a low C:N ratio by of DOC, together with its strong resistance against andicmaterials. mineralization, probably accounted for the large Lilienfein et al. (2004a,b) studied leaching of accumulation of C in this soil. dissolved organicmatter and the associated nutrient Nevertheless, some dissolved organiccarbon elements in andesiticsoils from California. The (DOC) does move through allophanicsoils. Neff et ability of the soils to adsorb and retain dissolved al. (2000) measured fluxes of DOC, dissolved organicmatter increased with soil age and soil organicnitrogen (DON), and dissolved organic development, reflecting an increase in allophane phosphorus (DOP) in the laboratory using samples concentrations. Analysis of the soil solutions from the O horizons of Hawaiian volcanic soils collected with suction cups during the main snow varying in age, nutrient availability and mineralogy. melt period suggested the older soils adsorbed more They found that physical desorption, dissolution, and DOP and phosphate (PO4). Stepwise multiple chemical sorption were the principal factors control- regression analyses showed that allophane concen- ling the release of dissolved organicmatter from trations controlled the adsorption of DOP and PO4 these surface horizons. The sole exception to this in these soils. Tests of preferential adsorption of pattern was at the oldest of three sites where there DOP vs. PO4 and DOC vs. DOP showed that the was a significant relationship between DOC and adsorption strength increased in the order: DOC < CO2 flux. The oldest site also contained the smallest DOP < PO4. Consequently, the tendency of DOP mineral and allophane contents, while the correlation and PO4 to leach from these andesitic soils between respiration and DOC indicated that micro- decreased as soils and ecosystems developed. bial activity influenced DOC flux at this site. Perakis & Hedin (2007) analysed water samples Phosphate and arsenate adsorption from small streams running through old growth forests in Chile and Argentina. The concentration of Most Andisols originally occurred under rain DOC in these waters ranged from 0.2 to 9.7 mg C/l, forest. In Hawaii, precipitation is the major factor 148 R.L. Parfitt determining forest productivity in forests that allophaneÀwater interface involved ligand contain N-fixing organisms. In high rainfall sites, exchange reactions between arsenate and surface- P supply and availability play important roles in coordinated water molecules and hydroxyl and controlling productivity and soil-organic matter silicate ions. This implied that dissolved tetrahedral 2À 2À dynamics (Idol et al., 2007). Although P is strongly oxyanions (e.g. H2PO4 and H2AsO4 ) are readily sorbed by Al and allophane (Parfitt et al., 1989), it retained on amorphous aluminosilicate minerals in is also recycled through both inorganic and organic waters and soils at near neutral pH. This inner- pathways (Dahlgren et al., 2004). In New Zealand sphere adsorption mechanism is probably important and Hawaiian soils, microbial demand for P may in controlling dissolved arsenate and phosphate in control its availability (Parfitt et al., 1989; Olander amorphous aluminosilicate-rich low-temperature & Vitousek 2005). Liu et al. (2006) found that tree geochemical environments. roots were less able to mobilize soil P from an Besides being adsorbed by allophanicsoils, P can allophanicsoil than from a pumicesoil. This is also be lost by leaching (McDowell & Condron, because the allophanic soil has a greater P-retention 2004). The amount leached increases with the P capacity, and a smaller concentration of plant- status (soil test value) of the soil, and decreases available P. For the same reasons, allophanicsoils with its P-retention capacity. Soils with a high P- that are used for agriculture generally require larger retention capacity can remove most of the P from amounts of fertilizer-P applications than non- leachates. Nevertheless, the P concentration in the allophanicsoils (Edmeades et al., 2006). leachate from allophanic soils may be large enough When phosphate solutions are added to allophane to support algal growth in streams and rivers at pH 5À6 in the presence of CaC12, the first (McDowell & Condron, 2004), and this is an area increments of P are rapidly and strongly adsorbed. requiring further research. As surface coverage increases, subsequent incre- ments of P are more weakly adsorbed (Parfitt, 1989; PHYSICAL PROPERTIES Parfitt et al., 1989). The amount of P which is strongly adsorbed ranges from 50 to 200 mmol gÀl, Allophanicsoils generally have a high porosity and a equivalent to 2À8 P ions per allophane spherule. low bulk density. The allophane spherules, produced The extent of adsorption increases with the Al:Si by weathering and dissolution of tephra, interact ratio of the allophane samples, and differs for strongly, particularly when dried, to form both silt- natural and syntheticallophanes (Parfitt & Henmi, size and sand-size aggregates (Wells & Theng, 1985; 1980; Clark & McBride, 1984). Phosphate is Churchman & Tate, 1987). The interaction between adsorbed by ligand exchange involving Al(OH) undried aggregates, however, is weak. Viscometry and Al(H2O) groups at defect sites on the allophane suggests interactions between individual allophane surface. This exchange is followed by a slow spherules involve both electrostatic and van der reaction by which the allophane structure is Waals attraction. The strongest interactions occur at disrupted and aluminium phosphate may precipitate pH ~6 when about equal amounts of positive and (Rajan, 1975; Parfitt, 1989). AllophanicAndisols negative charges are present on the allophane surface sorb less P than their non-allophaniccounterparts (Wells & Theng, 1985). that contain large amounts of Al-humus complexes Highly weathered and porous Andisols show and reactive AlOH groups (Dahlgren et al., 2004). pronounced shrinkage. Being more weathered and Arai et al. (2005) investigated arsenate reactivity more porous, Bw horizons shrank more than Ah and surface speciation on a synthetic allophane. The horizons (Bartoli et al., 2007). The more adsorption experiments indicated that arsenate pronounced the shrinkage, the more irreversible is adsorption was initially rapid followed by a slow the mechanical change. Samples of allophanic soils, continuous uptake, and the adsorption processes taken with a corer that is quickly pushed into the reached steady state after 720 h. X-ray absorption soil, appear to expand and have a smaller bulk spectroscopic analyses suggested that arsenate density compared with samples taken by carving predominantly formed bidentate binuclear surface the soil (Schipper et al., 2007). species on Al octahedral structures, and these Since allophanic soils do not usually have species were stable for up to 11 months. There continuous macropores, they are excellent filters, was no evidence of crystalline precipitates. They capable of removing faecal coliforms and bacter- concluded that the adsorption mechanisms at the iophages from effluents (McLeod et al., 2008). Allophane and imogolite 149

Allophanicsoils canalso sorb and remove heavy and molecular orbital study. Journal of Computer- metals (Adamo et al., 2003). When Na-contami- Aided Materials Design, 14,5À18. nated waste waters were applied to allophanicsoils, Adamo P., Denaix L., Terribile F. & Zampella M. the saturated hydraulic conductivity decreased, but (2003) Characterization of heavy metals in contami- nated volcanic soils of the Solofrana river valley only at high Na adsorption ratios (Menneer et al., (southern Italy). Geoderma, 117, 347À366. 2001). At this point, the concentration of DOC Aomine S. & Wada K. (1962) Differential weathering of increased in the leachate, but no dispersed clay was volcanic ash and pumice resulting in the formation of observed. Irrigation with such waste waters could hydrated halloysite. American Mineralogist, 47, cause structural deterioration in the soil matrix. At 1024À1048. high-moisture contents, however, the flow of Arai Y., Sparks D.L. & Davis J.A. (2005) Arsenate leachate in the field was sufficient to overcome adsorption mechanisms at the allophane-water inter- any adverse effects. face. Environmental Science and Technology, 39, 2537À2544. Aran D., Gury M. & Jeanroy E. (2001) Organo-metallic CONCLUDING REMARKS complexes in an Andosol: a comparative study with a Cambisol and Podzol. Geoderma, 99,65À79. This review was presented at a meeting of the Clay Baldock J.A. & Nelson P.N. (2000) Soil organic matter. Minerals Group of the Mineralogical Society of Pp. B25À84 in: Handbook of Soil Science (M.E. Great Britain & Ireland (September 2008, at the Sumner, editor). CRC press, Boca Raton, Florida, Macaulay Institute) in memory of Colin Farmer, in USA. acknowledgement of his contributions. I first met Barron P.F., Wilson M.A., Campbell. A.S. & Frost R.L. Colin in 1964 at Michigan State University where (1982) Detection of imogolite in soils using solid he helped me to enrol as a Masters student in the state 29Si NMR. Nature, 299, 616À618. Soil Science Department. I also had the privilege of Bartoli F., Begin J.C., Burtin G. & Schouller E. (2007) working with Colin at the Macaulay Institute in the Shrinkage of initially very wet soil blocks, cores and 1970s. Colin was active in research well into his clods from a range of European Andosol horizons. European Journal of Soil Science, 58, 378 392. retirement, publishing his most recent paper in 2005 À Basile-Doelsch I., Amundson R., Stone W.E.E., (Farmer et al., 2005). Dealing with phytoliths as a Masiello CA., Bottero J.Y., Colin F., Masin F., source and sink of Si in solution of some European Borschneck D. & Meunier J.D. (2005) Mineral forest soils, this publication has so far been cited 16 control of organic carbon dynamics in a volcanic times. His papers on allophane and imogolite have ash soil from La Re´union. European Journal of Soil been cited more than 100 times, and have been used Science, 56, 689À703. by many scientists. To quote the 12th century Basile-Doelsch I., Amundson R., Stone W.E.E., theologian and author John of Salisbury in Borschneck D., Bottero J.Y., Moustier S., Masin F. Metalogicon, his treatise on logic, written in Latin & Colin F. (2007) Mineral control of carbon pools in in 1159: ‘‘We are like dwarfs sitting on the a volcanic soil horizon. Geoderma, 137, 477À489. shoulders of giants. We see more, and things that Bi S.P., An S.Q., Tang W., Xue R., Wen L.X. & Liu F. (2001) Computer simulation of the distribution of are more distant, than they did, not because our aluminum speciation in soil solutions in equilibrium sight is superior or because we are taller than they, with the mineral phase imogolite. Journal of but because they raise us up, and by their great Inorganic Biochemistry, 87,97À104. stature add to ours.’’ Indeed, we stand on the Broadbent F.E., Jackman R.H. & McNicoll J. (1964) shoulders of giants. Mineralization of carbon and nitrogen in some New Zealand allophanicsoils. Soil Science, 98, 118À128. Buurman P., Peterse F. & Almendros G. (2007) Soil ACKNOWLEDGMENTS organicmatter chemistry in allophanicsoils: a pyrolysis-GC/MS study of a Costa Rican Andosol I am grateful to Professor David Lowe, Drs Des Ross, catena. European Journal of Soil Science, 58, Benny Theng, Jeff Wilson and Derek Bain for their 1330À1347. comments on this manuscript. Campbell A.S. & Schwertmann U. (1985) Evaluation of selective dissolution extractants in soil chemistry and REFERENCES mineralogy by differential X-ray diffraction. Clay Minerals, 20, 515À519. Abidin Z., Matsue N. & Henmi T. (2007) Differential Campbell A.S., Young A.W., Livingstone L.G., Wilson formation of allophane and imogolite: experimental M.A. & Walker T. W. (1977) Characterization of 150 R.L. Parfitt

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