Clay minerals isomorphic substitution

In the case of layered clay minerals, isomorphic substitutions occur not only in the tetrahedral layers but in the octahedral layers, too. When Al3+ is in the central... 

Fluorine occurs exclusively as the fluoride anion, F , in soils, where it complexes strongly with metals such as and Fe ". It is found in structures of hydrous minerals, isomorphously substituting for structural OH . Thus, F can be found in micas, amphiboles, layer silicate clays, apatite (rock phosphate), and numerous other minerals. Because it is associated with clay structures, the natural concentrations of fluorine in fine-textured mineral soils and sedimentary rocks can be high. 

Since adsorption at a mineral surface is a replacement process, we would expect mineral surfaces with weak affinity for water to have the strongest affinity for hydrophobic solutes. Infrared spectroscopy shows that siloxane surfaces on clays with little isomorphic substitution form weaker hydrogen bonds than water forms with itself (64), which corresponds to one of the definitions of a hydrophobic surface offered by Texter et al. (65) Therefore,... 

As shown in Figure 14.4, each clay mineral exhibits a large range in the type and degree of isomorphic substitution. The central silicon atom in the tetrahedral layers can be replaced by aluminum, alkali, alkaline earth, and trace metal atoms. In the octahedral layers, the central Al and Mg atoms can be similarly replaced. The large range in composition within each mineral type reflects variability in the environmental conditions under which crystallization and chemical weathering occur. Thus, the... 

After delivery to the ocean, clay minerals react with seawater. The processes that alter the chemical composition of the terrigenous clay minerals during the first few months of exposure are termed halmyrolysis. These include (1) cation exchange, (2) fixation of ions into inaccessible sites, and (3) some isomorphic substitutions. Another important transfiarmation is flocculation of very small (colloidal-size) clay particles into larger ones. 

Clay minerals have a permanent negative charge due to isomorphous substitutions or vacancies in their structure. This charge can vary from zero to >200cmol kg" (centimoles/kg) and must be balanced by cations (counter-ions) at or near the mineral surface (Table 5.1), which greatly affect the interfacial properties. Low counter-ion charge, low electrolyte concentration, or high dielectric constant of the solvent lead to an increase in interparticle electrostatic repulsion forces, which in turn stabilize colloidal suspensions. An opposite situation supports interparticle... 

Mixed solid (or solid solution) formation can occur, for example, when secondary minerals, such as carbonates, metal oxides, or clay minerals, precipitate from the soil solution during weathering. These solids often are characterized by a wide range of isomorphic substitutions, in which both cations... 

The clay minerals usually are classified into layer types, distinguished by the number of tetrahedral and octahedral sheets combined to form a layer, and groups, differentiated by the kinds of isomorphic cation substitutions that occur [2]. Layer types are shown in Fig. 1 and some of the groups are described... 

The net permanent structural surface charge density, denoted gq and measured in coulombs per square meter (C/m2), is created by isomorphic substitutions in minerals [4]. These substitutions in clay minerals produce significant surface charge only in the 2 1 layer types. In these minerals, Co < 0 invariably because of structural cation substitutions. The relation between gq and the layer charge jc is [3]... 

The intrinsic surface charge density reflects particle charge developed from either isomorphic substitutions or adsorption involving H+ or OH-. A widely used technique for measuring intrinsic surface charge density is the Schofield method. In this method [3], clay mineral particles are reacted with an electrolyte solution (e.g., NaCl) at a given pH value and ionic strength the specific surface excess of the cation and the anion adsorbed from the electrolyte is determined and the value of is calculated with the equation... 

Variations on this overview conceptualization occur because of differing isomorphic substitution patterns in 2 1 clay minerals. For example, both Li- and Na-vermiculite can form mono- and bilayer hydrates, whereas K-, Rb-, and Cs-vermiculite cannot. For the latter, inner-sphere surface complexes are stable against solvation of the cations because of the softer Lewis acid character of the latter and a favorable siloxane-surface stereochemistry [23]. 

Serves as an excellent example for understanding the structure of 2 1 soil clay minerals because of no isomorphous substitution... 

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