Allophane, in soil

Fielders, M. Perrott, K.W. (1966) The nature of allophane in soils. Part 3. Rapid field and laboratory... 

The occurrence of allophane in soils is favored by conditions that lead to the formation and persistence of random structures. 

Studies of Japanese soils summarized in Volcanic Ash Soils of Japan (Kanno [1964]) and studies of New Zealand soils (Fieldes [1966]) indicate that the following conditions may result in the presence of appreciable amounts of allophane in soils. 

In general, allophane in soils may be appreciably dispersed in aqueous media of low salt content provided the pH is above 10 or below 4. After dispersion, maximum flocculation again occurs in the pH range 5.5 to 6, which corresponds to the isoelectric pH range of allophane (Fieldes [1958], Fieldes and Schofield [I960]). 

The index of refraction of allophane ranges from below 1.470 to over 1.510, with a modal value about 1.485. The lack of characteristic lines given by crystals in x-ray diffraction patterns and the gradual loss of water during heating confirm the amorphous character of allophane. Allophane has been found most abundantly in soils and altered volcanic ash (101,164,165). It usually occurs in spherical form but has also been observed in fibers. 

Secondary minerals. As weathering of primary minerals proceeds, ions are released into solution, and new minerals are formed. These new minerals, called secondary minerals, include layer silicate clay minerals, carbonates, phosphates, sulfates and sulfides, different hydroxides and oxyhydroxides of Al, Fe, Mn, Ti, and Si, and non-crystalline minerals such as allophane and imogolite. Secondary minerals, such as the clay minerals, may have a specific surface area in the range of 20-800 m /g and up to 1000 m /g in the case of imogolite (Wada, 1985). Surface area is very important because most chemical reactions in soil are surface reactions occurring at the interface of solids and the soil solution. Layer-silicate clays, oxides, and carbonates are the most widespread secondary minerals. 

Amorphous Aluminosilicates. These occur in soils influenced by volcanic activity and are associated with very high moisture retention and anion fixation, and low to very high pH-dependent CEC. They may also bind organic matter tightly, protecting it against decomposition. Examples are allophane and imogolite. 

Allophane, in any abundance, is most commonly formed from volcanic material although it can presumably form from any alumino-silicate minerals and indeed is probably present as a transitory stage in the alteration of any material to a clay mineral if any significant structural re-arrangement is required. In its most pure form it occurs as veins and incrustations. Most analyses (Table LXXII) are of samples from this type, of deposit. In volcanic soils (Japan, Australia, N.W.U.S., etc.) where allophane is abundant it is usually intimately mixed with halloysite and collection of... 

Imogolite can be a major component of clays in soils developed from volcanic ash, but has also been discovered as a minor constituent of spodosols formed in northern temperate climates. The fibrous appearance of imogolite under high magnification, with tubules that are as much as several microns in length but only 21 A in diameter, distinguishes it from allophane. Its formation is favored over that of allophane in acidic environments. 

These patterns of behavior are fairly typical for acid soils of the northeastern United States and eastern Canada, although the increase in soluble SO4" in leachate seems to be a regional phenomenon. In fact, in many acid soils, the SO4" concentration actually decreases as the leachate passes through the soil profile, evidence that S04 is adsorbed on clays, particularly oxides and allophanes, in the B-horizon. 

The proof of reversibility in primary mineral weathering would be instances where primary mineral structures have formed under earth-surface conditions. There are reports that secondary quartz can slowly precipitate at room temperature from solutions supersaturated with monosilicic acid. More typically, however, precipitated silica in soils is structurally disordered, in the form of chalcedony or opal. In fact, as long as alumina is present, silica does not precipitate as a separate phase, reacting instead to form aluminosilicates (layer silicates, imogolite, or allophane). 

The exception is found in soils of variable chaige, where allophanic and oxidic minerals are able to adsorb the dissociated form of acid organic molecules, as will be described later in this section. 

Wada, K. (1977). Allophane and imogolite. In "Minerals in Soil Environments" (J. B. Dixon and S. B. Weed, eds.), pp. 603-638. Soil Science Society of America, Madison, Wisconsin. 

Secondary silicates form as clay minerals in soils after weathering of the primary silicates in igneous minerals. The secondary silicates include amorphous silica (opal) at high soluble silica concentrations and the very important aluminosilicate clay minerals kaolinite, smectite (montmorillonite), vermiculite, hydrous mica (il-lite), and others. Kaolinite tends to form at the low silicate concentrations of humid soils, whereas smectite forms at the higher silicate and Ca concentrations of arid and semiarid soils. The clay fraction of soils usually contains a mixture of these day minerals, plus considerable amorphous silicate material, such as allophane and imogolite, which may not be identifiable by x-ray diffraction. 

Other important constituents of the clay fraction are the so-called free oxides. These are Al, Fe, Mn, and Ti hydroxyoxides that accumulate in the soil as weathering removes silicon. The free oxides range from amorphous to crystalline and are often the weathered outer layer of soil particles. The hydroxyoxides, plus amorphous aluminosilicates such as allophane, are the most important clay-sized nonlayer minerals in soils. 

As soils weather and Si, Ca, Mg, Na, and K are leached away, the soil s colloidal fraction becomes enriched with Al, Fe, Mn, and Ti oxides and hydroxyoxides. The structural organization of these hydroxyoxides ranges from amorphous to crystalline. These Al, Fe, and Ti oxides and allophane are prominent nonlayer silicate minerals in most soils and then- content in soils increases with increased weathering. 

Aluminium, on the other hand, accumulates in the clay mineral fraction because it forms insoluble aluminosilicates and hydroxyoxides. The AI remains behind in the soil as other ions leach away. Iron also accumulates in soils but this is not apparent from Table 7.3 because the silicate clay minerals, with the exception of hydrous mica, are low in Fe. Iron precipitates in soils only as hydroxyoxides. Hydrous mica is altered parent material and is not reconstituted from the soil solution as are kaolinite, montmorillonite, and allophane. The <105° C water in Table 7.3 is, roughly speaking, adsorbed water the >105° C water is hydroxyl ions and water within crystal structures. 

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