Micas fine-grained

Clays composed of layers are called layered silicates. The most common sheets are tetrahedral silicon and octahedral aluminum (see Figure 3.2, Figure 3.3, and Figure 3.4). Three common representative clays in soil are 1 1 kalo-inite, 2 1 fine-grained micas, and 2 1 smectites that is, kaolinites have one sheet of silicon tetrahedra and one sheet of aluminum octahedra. The finegrained mica and smectites have two sheets of silicon tetrahedra and one sheet... 

In terms of soil development and the development of soil horizons, the smectites and fine-grained micas are found in younger, less weathered soils. Kaolinite and amorphous clays are found in highly weathered soils. Considering a time sequence, at the beginning of formation, soil will contain more complex clays that weather to simpler forms over time. However, it is convenient to start with a description of the simpler layer silicate clays and then describe the more complex clays. 

An additional important characteristic of clays is their surfaces, which are distinguished as being either external or internal. Internal surfaces occur, for example, in nonexpanding 2 1 clays, such as the fine-grained micas, and are generally not available for adsorption, chemical, or exchange reactions. 

The crystals of 2 1 swelling clays are typically smaller than either kaolinite or fine-grained mica and thus have higher adsorption capacity and cation... 

The surface separation in the slit-shaped pore is determined by the crystal thickness. For an illite [a fine-grained mica with a surface area of 1.6 X 10 m per kg], the slit-shaped pores have a median size of about 5nm and in the overlap pores the surface separation is about Inm. The stability of day domains within a soil is a crucial feature for agricultural production because the permeability of a soil to aqueous electrolyte solutions depends on this stability. Swelling of these domains reduces permeability. 

Pyrophyllite (aluminium silicate hydroxide) Sericite (fine grained mica)... 

Fine-grained micas are present in nearly all soils. Frequently, they are the most abundant component of the clay fraction. In many soils, micas are the principal source of native potassium for plants. On weathering, micas are altered to vermiculite and sometimes to smectitelike minerals and, as such, significantly increase the cation exchange capacity and affect the relative selectivity by soils for various exchangeable cations. 

The fine-grained micas in soils present problems of identification and characterization. One reason for this difficulty is that soil micas are probably always partially altered. Each particle is nonhomogeneous and, in a strict sense, not a single mineral. Micas present a real challenge in characterization to those interested in soil mineralogy and chemistry. 

Soil micas exhibit a wide variety of compositional, structural, and morphological features. This is also reflected by the nomenclature used in soil mineralogy. Hydrous mica, micaceous clay, sericite, illite, and degraded illite are some of the terms used, partly synonymously for the general description of fine-grained micas in soils, and partly with special reference to particular deviations from the ideal mica structure. 

The most abundant fine-grained mica in soils is aluminous as well as dioctahedral and, thus, is more similar to muscovite than to any other large-grained specimen-type mica. Biotite and phlogopite, like fine-grained micas, are probably next in abundance, but these minerals are present only in slightly weathered soils. 

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) of fine-grained micas from soils indicate (Figures 1, 2) that some compromise between the early frayed-edge idea and that of interstratification of expansible layers may best express the present concept of the morphology of fine-grained micas in soils. 

Illite is thick compared to the smectites (Kahn [1959]), but fine-grained micas in soils probably vary greatly in thickness (Mackie ei al. [1948]) because of the ease with which mica particles are cleaved. The particles containing mica probably decrease in thickness as weathering to vermiculite proceeds, but thin particles may be more resistant to weathering than thick particles of the same diameter (Ross and Rich [1973]). 

Although these morphological features are also present in coarse-grained micas, the specific surface for fine-grained micas is greater, so the importance of these features in affecting diffusion and selectivity of exchangeable ions is more important for the finer particles. 

Topography would be expected to affect the occurrence of fine-grained micas in soils in that topography affects the extent and rate of weathering and the occurrence of soil material moved by water, air, and gravity. Water affects the hydrolysis, and removal of cations, particularly potassium, in mica and physical breakdown of micas are increased by the hydration of replacing interlayer cations. Eluviation of fine clays and aeration, particularly as it affects the oxidation state of iron in micas, would also be an indirect function of topography. 

The cation exchange capacity (c.e.c.) of fine-grained micas should be a function of particle size. Cations should be readily exchanged on exposed planar surfaces, at positions where potassium is normally located, and also at pH-dependent sites at particle edges. 

Experimentally fine-grained micas, naturally occurring or ground from large crystals, exhibit a high selectivity for potassium compared to calcium (Schwertmann [1962b], Rich and Black [1964] Knibbe and Thomas [1972]). 

Fine-grained micas in clays tend to be concentrated in the coarser fraction (0.2 to 2 /tm). Many physical properties of illite are intermediate between those of smectites and kaolinite, but somewhat closer to kaolinite than to smectites (Grim [1962], Gillot [1968]). 

Fine-grained micas may contribute to the increased sensitivity of soils. Sensitivity is defined as the ratio of the strength of the soil in an undisturbed state to the strength of the remolded material at the same moisture content (Grim [1962], p. 236). 

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