Soil Layer Silicates

How do soil layer silicates differ from pure minerals How are soil properties, such as cation exchange capacity, surface area, and swelling, expected to differ from the properties of pure layer silicates ... 

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. 

Once a layer-silicate clay forms, it does not necessarily remain in the soil forever. As conditions change it too may weather and a new mineral may form that is more in equilibrium with the new conditions. For example, it is common in young soils for the concentrations of cations such as K, Ca, or Mg in the soil solution to be high, but as primary minerals are weathered and disappear, cation concentrations will decrease. With a decrease in solution cations, a layer-silicate such as vermiculite will no longer be stable and can weather. In its place. 

Laird DA, Barriuso E, Dowdy RH, Koskinen WC (1992) Adsorption of atrazine on smectites. Soil Sci Soc Am J 56 62-67 LeBaron PC, Wang Z, Pinnavaia TJ (1999) Polymer-layered silicate nanocomposites an overview. Appl Clay Sci 15 11-29 Lee J-F, Crum JR, Boyd SA (1989) Enhanced retention of organic contaminants by soil exchanged with organic cations. Environ Sci Technol 23 1365-1372 Lee J-F, Mortland MM, Boyd SA, Chiou CT (1989a) Shape-selective adsorption of aromatic molecules from water by tetramethylammonium-smectite. J Chem Soc Faraday Trans I 8 2953-2962... 

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. 

The concentrations of the four A1 species occurring in each layer of the soil profile are expressed as a percentage of the A1 content of the specific soil layer. While Alpstot increased with depth of the profile, the contribution of the operationally defined A1 species towards speciation however decreased with depth (Figure 3a-d). The maximum differentiation of Alpstotas A1, A1, A1, and A1 was 8.51% which was observed in the surface layer while the mean contribution was 5.43 1.60%. On the contrary, in 37-57 cm depth which had the most abundant Alpstot, the mean share of the A1 species (2.72 0.75%) was the lowest contribution to A1 in all the layers of the soil profile. This implied that pseudo total A1 in these depths are predominantly bound as silicates and hence are not available for speciation under the experimental conditions. 

The clay fraction, which has long been considered as a very important and chemically active component of most solid surfaces (i.e., soil, sediment, and suspended matter) has both textural and mineral definitions [22]. In its textural definition, clay generally is the mineral fraction of the solids which is smaller than about 0.002 mm in diameter. The small size of clay particles imparts a large surface area for a given mass of material. This large surface area of the clay textural fraction in the solids defines its importance in processes involving interfacial phenomena such as sorption/desorption or surface catalysis [ 17,23]. In its mineral definition, clay is composed of secondary minerals such as layered silicates with various oxides. Layer silicates are perhaps the most important component of the clay mineral fraction. Figure 2 shows structural examples of the common clay solid phase minerals. 

In real soils where oxides, layer silicates, organic matter and other materials are present in intimate mixtures, with the oxides and organic matter often coating the surfaces of the other materials, the different functional groups interact with... 

Sims, J.R. Bingham, F.T (1968) Retention of boron by layer silicates, sesquioxides, and soil materials. 11. Sesquioxides. Soil Sci. Soc. Am. 

Currently, the only biological barrier registered as a biochemical pest control agent is kaolin, a clay mineral [47]. Kaolin is a ubiquitous clay substance found in soils worldwide and consists of a layered silicate mineral, with one tetrahedral sheet linked through oxygen atoms to one octahedral sheet of alumina octahedra [Al2Si205(0H)4]. 

JACKSON (M.L.), HZEUNG (Y.), COREY (R.B.), EVANS (E.J.) and VANDEN HEUVEL (R.C.), 1952. Weathering sequence of clay-size minerals in soils and sediments II. chemical weathering of layer silicates. Proc. Soil. 

Whereas we now begin to understand solid state kinetics in orthosilicates, this understanding is still unsatisfactory for other silicates with interlinked tetrahedra. Let us turn to the discussion of chemical kinetics in layered silicates since they play a prominent role in soil chemistry. For illustration we will concentrate on transport... 

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