Components, rocks, soil precipitation

Smectite is the first secondary mineral to form upon rock weathering in the semi-arid to sub-humid tropics. Smectite clay retains most of the ions, notably Ca2+ and Mg2+, released from weathering primary silicates. Iron, present as Fe2+ in primary minerals, is preserved in the smectite crystal lattice as Fe3+. The smectites become unstable as weathering proceeds and basic cations and silica are removed by leaching. Fe3+-compounds however remain in the soil, lending it a reddish color aluminum is retained in kaolinite and A1-oxides. Leached soil components accumulate at poorly drained, lower terrain positions where they precipitate and form new smectitic clays that remain stable as long as the pH is above neutral. Additional circumstances for the dominance of clays are ... 

In addition to straightforward precipitation reactions, components may dissolve and react with components already present, including atoms on colloidal surfaces. For example, phosphate may dissolve from phosphate rock and react with iron present in the soil solution or on particle surfaces to form an iron phosphate that is insoluble. 

Fluoride is a natural component of most types of soil, in which it is mainly bound in complexes and not readily leached. The major source of free fluoride ion in soil is the weathering and dissolution of fluoride rich rock that depends on the natural solubility of the fluoride compound in question, pH, and the presence of other minerals and compounds and of water. The major parameters that control fluoride fixation in soil through adsorption, anion exchange, precipitation, formation of mixed solids and complexes are aluminium, calcium, iron, pH, organic matter and clay [19,20]. 

As mentioned earlier, the composition of natural groundwaters depends on the composition of the geological formations where they originate from they contain dissolved rock and soil components that were soluble under the conditions (such as temperature and pressure) of their formation. Their dissolution is governed by the law of thermodynamics that is, dissolution occurs when the solution is undersaturated with respect to components such as rocks and soils. Provided that the solid components are present in sufficient quantity and there is no kinetic barrier, this process may lead to a thermodynamic equilibrium. The reversed process of dissolution is precipitation, that is, the formation of a solid phase from the dissolved components of a supersaturated solution. The composition of the... 

On the surfaces of geological formations, different precipitation processes can be observed. The first one occurs when the concentration of some components reaches the value of the solubility product, the solution becomes oversaturated, and a new solid phase is precipitated (Section 1.2.3). The quantity of the precipitate depends only on the concentration of the solution. The precipitation takes place in a solution without the necessary presence of a solid surface. When, however, a solid phase, rocks, or soil is originally present, the precipitate is formed on it, and thus the total composition of the solid phase changes. When the precipitation forms colloidal particles, especially in diluted solutions, they can be adsorbed on the solid, if it is present. This process is governed by the so-called theory of colloid adsorption (Derjaguin and Landau 1941 Verwey and G. Overbeek 1948). 

In this chapter we give a brief overview of the structure and composition of the Earth, discuss some properties of rocks and minerals, explain the origin of minerals and the consequences of weathering, analyze the dissolution and precipitation of solids, and present key soil phenomena and components. 

In this book we considered mass transfer and elemental migration between the atmosphere, hydrosphere, soils, rocks, biosphere and humans in earth s surface environment on the basis of earth system sciences. In Chaps. 2, 3, and 4, fundamental theories (thermodynamics, kinetics, coupling model such as dissolution kinetics-fluid flow modeling, etc.) of mass transfer mechanisms (dissolution, precipitation, diffusion, fluid flow) in water-rock interaction of elements in chemical weathering, formation of hydrothermal ore deposits, hydrothermal alteration, formation of ground water quality, seawater chemistry. However, more complicated geochemical models (multi-components, multi-phases coupled reaction-fluid flow-diffusion model) and phenomenon (autocatalysis, chemical oscillation, etc.) are not considered. 

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