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III. Structure of Allophane Soils

III. Structure of Allophane Soils

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The structure of imogolite in the 10- to 100-A range is known from electron

micrographs supplemented by X-ray diffraction, electron diffraction, and salt

absorption measurements (Wada et aZ., 1970; Wada and Henmi, 1972). The unit

is a hollow tube with inside and outside diameters of 7-10 and 17-21 A,

respectively. A number of tubes lying parallel form threads of 100 to 300 A in

diameter. Three different classes of pores would be present in such a material:

intraunit pores of about 10 A in diameter, intrathread (interunit) pores of the

same or slightly smaller diameter, and interthread pores of several hundred

Angstrom (A) units diameter.

The “unit particle” of allophane has been defined (Kitagawa, 1971) as a

spherical particle with a diameter of about 55 A (Birrell and Fieldes, 1952). The

density of this particle is about 1.9 g ~ m - Gtagawa


(1971) has assumed that

these unit particle spheres exist in close packing in the air-dry state. This forms

the “microaggregates” commonly found. On heating, the layer of adsorbed

water is lost, and the spheres come closer together. Grinding distorts the unit

particle spheres allowing closer packing, which, in the electron micrographs,

gives the appearance of sheets rather than the individual spherical particles of the

dried sample.

The close packing of these 55-A diameter units would again result in voids about

10 A in diameter, so the water absorption properties would be simdar t o

imogolite. Irregular packing would produce some larger pores.

These models, therefore, explain many observed properties. The decrease in

surface area on grinding is due to collapse of some of the small pores. Kitagawa

(1971) has shown that calculation of particle size from surface area and density

also gives values of about 55 A diameter. He found that phosphate adsorption

was not decreased by grinding on drying, which argues against any chemical

change in the surface, specifically in the number of hydroxyls at the surface.

Interparticle bonding is then by physical forces that would be weaker than if

chemical bonding were involved.

These measurements were made on a series of allophane soils from Japan. It is

not known whether all allophanes have this same “unit particle.” Measurements

by Rousseaux and Warkentin (1976) of water vapor absorption show a maximum in pore volume at diameters of 7-10 A for allophanes from the Caribbean

and from Japan. The Caribbean samples show a narrower size distribution that

could result from closer and more regular packing of unit particles. Fujiwara and

Baba (1973) calculated effective pore size from nitrogen absorption measurements, and found a maximum at 25 A. The volume of these 25-A pores was

about one-twentieth of the volume of 10-A pores found from water absorption

by Rousseaux and Warkentin (1976). These measurements confirm the unit

particle in allophane soils.

Measurements of water vapor absorption are of necessity made on dried

samples. It is reasonable to assume that undried samples consist of the same unit



particles, but that they are further apart and more irregular. Such a model would

explain water retention, plasticity, etc. It is more difficult to relate differences in

these properties among soils, wet or dry, to different structures in the model.

Some allophane soils become nonplastic on air drying, others do not. This could

be due to differences in arrangement of unit particles, or to different nonallophane components in the soils. Also, it is not known how differences in chemical

composition such as the AI/Si ratio affect the structure, although these differences are correlated to differences in physical properties (Rousseaux and Warkentin, 1976). The Caribbean samples, for example, have lower Si02/A1203ratios

than the Japanese samples.

Ito (1 964) postulated that allophane structure consisted of individual particles

and massive particles (structure units), with only the latter affected by drying.

The individual particles retain their undried properties, and different phases of

shrinkage are due to different effects of individual particles and structure units.

The irreversible changes in physical properties of allophane soils on drying set

them apart as a separate group of soils. In the extreme, allophane soils change

from highly plastic when wet, to sandy when dry. This has been described by

many workers, e.g., Sherman et al. (1964), who suggest research on changing the

colloidal properties of allophane soils by promoting dehydration. The particles

become cemented together to form units of sand size that are sufficiently strong

to withstand the usual manipulation in field or laboratory. Neither the forces

involved in this cementing nor the fabric of the units have been adequately

described. A tentative model for structure is described in Section 111, B.

The observed pore structure of allophane soils has been described by a number

of workers in relation to permeability. Tabuchi et al. (1963) described the

channels for water flow that they observed in thin sections of surface and

subsoils. Interaggregate pores conducted water in surface soils, tubelike pores

were found to conduct water in subsoils. Takenaka et al. (1963) report similar

results. Nagata (1963) found that plots of log air permeability against air

porosity were linear functions which could be related to the kind of structure in

the soil sample. For some soils log K, versus va consisted of two straight-line

portions, depending upon changes in structure with void size.

A more detailed description is available for the pores in pumice (Borchardt et

al., 1968; Maeda et al., 1970; Tsujinaka et aZ., 1970). On the basis of waterretention measurements carried out in different ways, Maeda et al. (1970)

distinguished dead, active, semiactive, secondary active, and semidead pores. The

active and semiactive pores usually occupy the largest volume, but semidead and

dead pore space may be large in some samples of pumice. The pumice materials

have been described by Sasalu (1 957).

Some information on structure, especially on the fabric component, can be

obtained from measurements of rheotropy or thixotropy of allophane soils. The

behavior of allophane on mechanical manipuiation is usually not true thixo-



tropy, i.e., a reversible sol/gel transformation. The changes on remolding described by Takenaka and Yasutomi (1965) are discussed in Section V, B.

A number of papers have reported experiments with soil conditioning chemicals; the general impression is that these chemicals are only marginally effective

in promoting aggregation in allophane soils. This may be partly due t o mixing

the chemicals with soils at water contents which are not optimum for aggregation. Sudo and Suzuki (1963) found that sodium alginate increased stable

aggregates in an allophane soil but the polyelectrolyte CMC did not. They

report, in their literature review, that additives are generally considered to have

little effect on aggregation of allophane soils. Kawaguchi et al. (1963) report

that bentonite added to polyvinyl alcohol is effective in aggregation. Fujioka et

d. (1965) found an effect of soil conditioners on allophane soils, but Terasawa

(1 967) found very little effect of synthetic polymers on aggregate formation.


What is required is a model for structure, including fabric, of allophane soils.

This model must explain the known physical properties of allophane and their

changes on drying. The model should predict other physical properties. It should

also be related t o important differences in chemical composition and properties,

for example the Si/Al ratio. The model must describe structure in the 0.01- to

100-pm range, where physical properties can be explained.

Measurements to date on allophane soils allow only a general specification of

this model. The unit particle of about 50 A is well established and can be taken

as the starting point. This particle has an “internal” water content. The unit

particle has been established in dried samples and is assumed to be present also

in wet samples. These unit particles are then weakly bonded together to form

domains in the diameter range of 0.01 to 1 pm. The modal size may be 0.05 to

0.1 pm. The voids between particles in these domains account for the large water

retention above 15-bar suction. Drying brings the particles closer together,

increasing the bulk density of the domains and decreasing water retention above

15 bar. Irreversible changes occur when the unit particles come sufficiently close

together to allow strong bonding between unit particles. These domain units

may be the clay-size grains measured in a grain size determination. They are not

broken up on remolding and account for the high water content at the plastic

limit. They are the units that move on plastic readjustment. Organic molecules

are held within the domains.

The domains are arranged in clusters in the size range of 1-100 pm. These

units have weaker bonding than the domains. Remolding breaks up the clusters,

releasing water held within the cluster. Drying causes shrinkage of the domains

and rearrangement into clusters of highei bulk density. In some allophane soils



the bonds holding the clusters together after drying are strong, and the clusters

become the units of dried soils. Bonding within clusters would then aIso

contribute to irreversibility. This may be true only in soils that are composed

entirely of allophane and do not have crystalline minerals or oxides mixed in.

Water in the plant-available range is held withm the clusters.

The water-retention curves for allophane soils show an approximately linear

change of water content between 0.03 and 1 bar, with decreasing amounts of

water retained below and above these values. The break at 1 bar indicates a

change at void diameters of 30 pm; the upper boundary of cluster size could be

drawn there. This would put much of the plant available water between clusters.

Kubota (1971) has studied the formation of units in the sand- and silt-size

range. Drying is necessary for pedogenic formation of these units. These units

fall within the range defined here as “clusters.” These units are stable against

dispersion if the allophane soil has been dried. Dehydration alone can cause

irreversible binding. The formation of these clusters is best studied on subsoil

samples of allophane soils that have been continually moist. Presumably the

clusters are already present in surface soil horizons that have been previously


Kubota (1971) divides the clay-size grains into three types on the basis of their

potential for forming sand- and silt-size grains on drying. The clay content was

measured on sonic-dispersed samples of subsoils of wet allophanes. Type I is

active free clay which can form large grains on air drying. This is measured as the

difference between clay content of moist and air-dry soils on shaking. Type I1 is

the aggregated clay that is released from moist soils by sonic dispersion. These

are pedogenically formed aggregates. Type 111 is inactive free clay not affected

by air-drying. Ttus is the measured clay content of an air-dry soil. Type I clay

content increases from the Ap to the B2 horizon, while Type 111 decreases. Type

I1 remains approximately constant with depth.

Monolayer adsorption of water vapor on allophane is strong but multilayer

adsorption of more than two water layers is weak compared with layer silicate

minerals. Therefore unit particles of allophane can come in close contact.

Kubota (1971) also states that hydroxyaluminum groups are the site for adsorption of water molecules; they are also the sites for bonding of unit particles in an

irreversible aggregation.

The details remain to be fdled into this model. The size ranges of the fabric

units, domains, and clusters, have been chosen mostly for convenience of

description. They will undoubtedly need refinement.

Neither the size boundaries nor the terminology of domains and clusters are

suggested as being definitive. They are both used here for convenience. The

clusters may be better described as microaggregates. However, Kitagawa (1971)

refers to the unit particles of 55 A as microaggregates, and Kubota (1971) refers

to sand- and silt-size units as aggregates.



IV. Physical Characteristics of Allophane Soils


Wet allophane soils show a large volume decrease on air-drying, and a limited

volume increase on rewetting. Most of the volume change is irreversible. This

distinguishes allophane soils from swelling mineral soils, where the volume

changes are more nearly reversible. The amount of shrinkage of an allophane soil

depends upon the initial water content and the fabric changes during drying,

both of which depend upon the allophane content. Many physical properties

change in association with this volume decrease, e.g., permeability increases.

However, volume change in the field cannot be predicted quantitatively from

shrinkage measured on small samples because the boundary conditions are


Wet allophane samples dried to intermediate water contents, suctions below

10-20 bars, will regain most of the volume on rewetting. Volume change is

approximately reversible over this range (Takenaka, 1961).

Volume change curves can be plotted as changes in measured volume, linear

dimensions, bulk density, or void ratio with change in water content. The

measurements are not precise because it is difficult to measure volume repeatedly on a sample as the water content changes. The precision is sufficient t o

characterize shrinkage on drying, but greater precision is desirable in describing

volume change for the water-retention curve (Section IV, B).

The general nature of the shrinkage curves is shown in Fig. 3 for a highly

allophanic soil (Cl) from Dominica, West Indies, and for a soil (Nl) from

Hokkaido, Japan, with low allophanic properties (Warkentin and Maeda, 1974).

There is a break in the volume change curve which can be called a shrinkage

limit, but the volume change at higher water contents is not “normal” shrinkage

where volume decrease equals water content decrease (Takenaka, 1961; Ito,


The shrinkage limit is less pronounced and occurs at a higher water content as

allophane content increases. The shrinkage limit occurs between 50 and 100%

water for allophane soils, a much higher value than for crystalline clays (Takenaka, 1965; Warkentin and Maeda, 1974; Soma and Maeda, 1974). Takenaka

(1965) found that the shrinkage limit occurred near pF 6 for a sample of Kanto

loam. He related shrinkage to soil suction values and showed that the amount of

shrinkage depended upon the rate of drying. The measured water content at the

shrinkage limit also increases with increasing organic matter content of the soil

(Takenaka, 1973). He measured a 15% increase in shrinkage limit for 10%

increase in organic matter up to 25%. Remolding does not change the value of

the shrinkage limit (Takenaka, 1965), but drying increases the shrinkage limit.

The usefulness of the shrinkage limit is in the information it gives about fabric

and structure of allophane soils. The high water content at which the slope of











Water Conieni, %

FIG. 3. Shrinkage curves for two allophane soils. Reproduced from Soil Science Society

of America Proceedings, Volume 38, Page 375, 1974 by permission of the Soil Science

Society of America. X , Field moisture; @, air dry; 0,oven dry.

the shrinkage curve changes and the high value of residual shrinkage (between

the shrinkage limit and zero water content) indicates a random arrangement of

units. Measured shrinkage is isotropic, again indicating random arrangement. For

crystalline minerals, the shrinkage limit is lowest for high swelling soils. The

shrinkage limit is, therefore, diagnostic for allophane soils (Warkentin and

Maeda, 1974).

Despite the large amount of shrinkage on drying, one would predict little

visible cracking for allophane soils in the field because the shrinkage is taken up

in small spaces between clusters (Section 111, B). Cohesion of allaphane is low

and it decreases if the samples are dried (Soma and Maeda, 1974). AUophane

soils do not form dry clods with dimensions of tenths of a meter.

The effect of remolding a soil depends upon its degree of consolidation

(Croney and Coleman, 1954). Remolding an overconsolidated soil exposes new

surfaces for water retention. AUophane soils are underconsolidated. Remolding

breaks some of the fabric bonds, decreases the soil suction, and increases the

amount of shrinkage (Takenaka, 1965).


This section will deal with soil water characteristics measured on allophane soil

samples in the laboratory; the mechanisms of water retention will be discussed.

The field water regime is described in Section IV, D.

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III. Structure of Allophane Soils

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