Allophane, Imogolite, and Poorly Crystalline Materials
Further Reading
Wada, K., 1989. Allophane and Imogolite. Ch. 21, p.1051-1087. In: J.B. Dixon and S.B. Weed (ed.), Minerals in Soil Environments, 2nd Edition.
Poorly Crystalline Materials
There are a variety of states or degrees of structural order that are less than fully crystalline. For example, paracrystalline materials have regular compositions and repeating structures either on a molecular scale, such as in the fullerenes, or in one or two dimensions, as in chrysotile or halloysite. Due to various factors, however, the repeating structure cannot be translated in three dimensions to form larger crystals. Examples include chrysotile, tubular and spherical halloysite, and the smectites.
Amorphous materials are at the opposite extreme from crystalline materials: they do not have a regularly repeating structure, even on a molecular scale. Their compositions may be regular or, as is more commonly the case, they may have a large degree of variability. They do not produce XRD peaks. Amorphous materials are not limited to clay-sized particles, but can also be present in the silt- and sand-size fractions of the soil. Examples include amorphous silica, and volcanic glasses and ashes. The term amorphous is presently being discouraged. Unless the materials are truly amorphous, like volcanic glass, then they should be called x-ray amorphous or simply non-crystalline.
Short-range ordered materials have a repeating structure on a molecular scale and a more or less regular composition. They do not have a repeating structure on a larger scale. They are sometimes referred to as microcrystalline or cryptocrystalline, depending on the size of the individual crystal domains. Short-range ordered materials are not restricted to particles of the clay-size fraction, either. Examples include Al, Fe, and Mn (hydr)oxides, many of the sulfide minerals, opals, and cherts.
Most short-range ordered materials form by very rapid crystallization processes, where the nucleation of "seed" crystals occurs readily and numerous seeds are formed. The large number of seeds causes the formation of microcrystals that have dimensions in the range of a few tens to a few thousands of Ångstroms.
The scale below gives some idea of the relationship between these classes of disordered materials.
| Degree of Crystallinity |
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| One should note, however, that more than one type of structural disorder is represented along the x-axis of this figure. |
Many types and "species" of paracrystalline, amorphous, and short-range ordered components are present in soils. They are particularly common in soils derived from highly siliceous volcanic materials, such as volcanic ash, cinders, or glasses. Recent research has shown that some of the disordered materials commonly thought to occur only in weathered volcanic ashes are present in soils derived from crystalline rock parent materials.
The physical and chemical properties of disordered materials make their identification very difficult, particularly by normal XRD means. Little is known about their forms, quantities, or occurrence in most soils. Quantification is even more difficult.
Short-range ordered and paracrystalline materials can sometimes be identified by XRD or selective dissolution in conjunction with differential XRD. Many can be identified from their morphologies under the scanning or transmission electron microscope, but this is too time consuming and expensive to be practical for routine analyses.
Truly amorphous materials cannot be identified by XRD methods as they produce no peaks at all. They are most often identified by one or more alternate techniques such as infra-red spectroscopy, thermal methods, optical methods, or selective dissolution in conjunction with chemical analyses.
Non-crystalline materials may dominate the clay fraction of some soils. Even in soils dominated by crystalline materials, small quantities of non-crystalline materials may strongly influence the behavior and properties of soils due to their small particle sizes and high surface areas.
Allophane
Allophane is a series name used to describe clay-sized, short-range ordered aluminosilicates associated with the weathering of volcanic ashes and glasses.
Allophane commonly occurs as very small rings or spheres having diameters of approximately 35 - 50 Å. This morphology is characteristic of allophane, and can be used in its identification.
Allophanes have a composition of approximately Al2Si2O5·nH2O. Some degree of variability in the Si:Al ratios is present: Wada reports Si:Al ratios varying from about 1:1 to 2:1. Because of the exceedingly small particle size of allophane and the intimate contact between allophane and other clays (such as smectites, imogolite, or non-crystalline Fe and Al [hydr]oxides and silica) in the soil, it has proven very difficult to accurately determine its composition. Consequently, there is always some potential error associated with the compositional ratios reported.
The term allophane has been applied to numerous other materials in the past, including imogolite, any non-crystalline aluminosilicate, or any clay-sized material exhibiting structural randomness. Current usage is limited to short-range ordered aluminosilicates having Si:Al ratios between 1:2 to 1:1 possessing a spherical or ring-shaped morphology. Allophane usually gives weak XRD peaks at 2.25 and 3.3 Å. Identification is commonly made by infrared analyses or based on transmission electron morphology.
A limited amount of isomorphous substitution occurs in allophane. The most common type is the substitution of Fe for Al. Little permanent charge is assumed to be present. The majority of the charge is variable charge, and both cation and anion exchange capacities exist, with the relative amounts depending on the pH and ionic strength of the soil chemical environment. Wada reports CEC values of 10 - 40 cmol kg-1 at pH 7.0 and AEC values of 5 - 30 cmol kg-1 at pH 4.0. Other studies have measured CEC values as low as 10 and as high as 135 cmol kg-1 at pH 7.0.
The surface area of allophane has been calculated to be about 1000 m2 g-1, while values measured with ethylene glycol monoethyl ether are in the range of 700 - 900 m2 g-1.
Figure 1. Total positive charge, total negative charge, and net charge of allophane as a function of pH.
Imogolite
Imogolite is a paracrystalline aluminosilicate having an ideal end-member composition of SiAl2O5·2.5 H2O. Wada reports that measured Si:Al ratios fall within a narrow range, from 1.05:1 to 1.15:1, but this is in direct conflict with the structural formula he provides, which predicts Si:Al ratios near 1:2.
Imogolite occurs as very small tubes having inside diameters of 10 Å and outside diameters of 20 Å. These tubes may be several µm in length, and often form bundles of two to several hundred tubes. Occasional branching of tubes may occur.
The imogolite structure has Si in tetrahedral coordination and Al in octahedral coordination, though not in sheets as in the phyllosilicates.
Imogolite has several characteristic, though rather broad, low XRD peaks. Unless imogolite is dominant in a soil, however, these peaks may be overshadowed by peaks associated with other clay minerals. Semi-conclusive identification can be made by infra-red spectroscopy. Morphological identification is conclusive, but again requires a transmission electron microscope.
Like allophane, imogolite is most commonly associated with weathering of non-crystalline volcanic materials. Recent research also finds it to be a common component of spodic horizons, and it may very well be associated with other soils and sediments; more research is required to determine its geographical extent.
Imogolite can readily be formed via precipitation reactions. It is commonly formed in the laboratory to provide pure substrates for experiments.
Little is known about the amount of permanent charge in imogolite, but it is presumed to be fairly small due to the consistency of composition. Variable charge is present, and Wada cites a CEC value of 17 cmol kg-1 at pH 7.0 and an AEC value of 40 cmol kg-1 at pH 4.0.
The surface area of imogolite has been calculated to be about 1000 m2 g-1, while measured surface areas are on the order of 900 - 1100 m2 g-1.
Soils containing imogolite and allophane form very complex associations with organic matter in soil. These complexes are very stable, and appear to protect the organic fraction from degradation by soil microbes. This stability has been formerly ascribed to the formation of allophane-organic and imogolite-organic complexes. Recent studies show that stable organic matter associations also occur in soils (particularly volcanic soils) where allophane and imogolite are virtually absent. It appears now that these complexes are with free Al, and not necessarily with imogolite or allophane.
Author: Ed Nater
Department of Soil, Water, and Climate
Copyright: Ed Nater and the Regents of the University of Minnesota
Copyright for mineral models held by the Minerals & Molecules
Project
The opinions expressed herein are those of the authors and do not
necessarily represent those of their respective universities or their Regents.
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