Clay fraction of soils

Wilding, L.P., and Drees, L.R., 1974. Contributions of forest opal and associated crystalline phases to fine silt and clay fractions of soils. Clays Clay Miner., 22 295—306. 

Layer Silicates. Although the common primary minerals include island, chain, sheet, and framework silicates, the most stable and persistent silicates, which occur as weathering products (secondary minerals) in the clay fraction of soils, are sheet silicates. Figure 2.9a depicts the structure of the tetrahedral sheet in these minerals, which is comparable to the tetrahedral structure of mica. For the layer silicate clays, however, numerous structural combinations of the tetrahedral sheet with octahe-drally coordinated metal cations are possible. 

Solutes, electrolytes, and nonelectrolytes in the soil solution are the immediate sources of the elements required by plants for growth. This supply can be continuously renewed by the many mechanisms of ion-soil interaction that remove and add ions in the soil solution (1) mineral weathering, (2) organic matter decay, (3) rain, (4) irrigation waters containing salts, (5) fertilization, and (6) release of ions retained by the colloid or clay fraction of soils. 

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. 

Minerals of the clay fraction of soils are largely secondary, that is, formed by low-temperature reactions and either inherited from sedimentary rocks or formed directly in the soil by weathering. These secondary (authigenic) minerals in soils commonly... 

Layered aluminosilicates are the most important secondary minerals in the clay fraction of soils. When layer silicate minerals are clay or colloidal size (<2 gm effective diameter), their large surface area greatly influences soil properties. Most of the important clay minerals have similar silicate structures. Inasmuch as clay minerals are such important clay components, and as different clay minerals can change sail properties greatly, an understanding of soil properties begins with an understanding of silicate structures. 

All of the clay minerals given in Table I may occur in the clay fractions of soils. They often occur in mixtures, but frequently one mineral predominates. Kaolinite and vermiculite generally characterize acid soils of the Southeastern United States. Illite and other clay-sized micas are common in the Northeast and the Midwest. Montmorillonite occurs commonly in the neutral and alkaline soils of the Midwest and Western states. However, any and all of the clays can and do occur throughout soils of the United States, depending on the natural parent material and the formation conditions. Other details concerning clay minerals are described in the works of Grim (28), Marshall (31), and Bear (32). 

Table 1.1. Ionic radii of elements commonly found in clay fractions of soils...

A more pronounced structural disorder often exists in freshly precipitated silica or metal hydroxides in soils, since these compounds typically are amorphous. Structurally disordered aluminosilicates, known collectively as allophane and imogolite, are common in the clay fractions of soils formed on volcanic ash parent material. ... 

In assessing whether a particular polymorph e.g., 2Mi muscovite) is likely to be detrital when found in clay fractions of soils, it would help considerably to know the pressure-temperature stability fields in which different polymorphs are stable, and the ease with which metastable polymorphs are formed. This problem has been studied for the IMd IM 2M sequence of muscovite polymorphs by Yoder and EugSter [1955] and Velde [1965]. Two main problems of study exist— firstly, the reactions can be unexpectedly slow, so that experiments are incomplete in laboratory times e.g., Yoder and Eugster), or secondly a fast reaction may not conveniently be reversible (Velde [1965]) so as to establish stability fields unambiguously. 

Mica is a very common constituent of soils other than laterites. The earth s crust contains 1.4% muscovite and 3.8% trioctahedral micas (Ahrens [1965]). On the average, soil clays and silts probably contain more mica. The percentage of mica in the silt and clay fraction of the average soil is probably closer to 10%, and this is largely dioctahedral mica. Soils are derived more from sedimentary rocks than from other rock types. Since mica is a common component of shale, and shale is a common sedimentary rock, natural rock weathering and soil-forming processes have tended to favor the concentration of mica in the silt and clay fractions of soils. 

Antipow-Karatajew and Tsyurupa [1964], reviewing the results of Russian investigations on the composition of clay fractions of soils derived from igneous rocks in the different climatic zones of the USSR and parts of South East Asia, pointed out that hydrous mica predominates in almost all soils investigated, except in soils from humid subtropical or tropical zones. 

Soil allophane, like geological allophane, is amorphous to X-rays, and different samples may have different compositions. In the clay fractions of soils, allophane occurs in different states of aggregation and though largely of random structure, infrared evidence suggests that allophane samples of different origin may have different types of ultimate arrangements. Certain aspects of the similarity of and difference between different samples of soil allophane are made clearer by appreciation of possible fundamental differences in modes of formation. 

Electron microscopy can also detect amorphous material in soils containing crystalline clay minerals. Fieldes and Williamson [1955] detected small quantities of amorphous matter in the electron micrographs of most of the soil clays they described. Similar features probably would be visible in soils such as those described by Mitchell and Farmer [1962] from Scotland. The electron microscope is a very useful tool for examining the fine detail of the clay fraction of soils, but care should be taken because of the changes that can take place with different treatments, as has been shown above. 

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