Mineral weathering mechanisms

The importance of "parabolic kinetics" in laboratory studies of mineral dissolution has varied as interpretations of the underlying rate-controlling mechanism have changed. Much of the research on silicate mineral weathering undertaken in the past decade or so served to test various hypotheses for the origin of parabolic kinetics. 

Common minerals in Earth s crust, soils, and atmosphere, weathering mechanisms and products, and less common minerals that contain or adsorb environmental contaminants... 

However, as Velbel (1985) stated so well, more research is needed to see whether the above mechanism is operational under field conditions. We know that rates of mineral weathering are often lower in the field than in the laboratory. Natural feldspar weathering may be one to three times slower in the field (Paces, 1973). Velbel (1985) found that the rate of... 

Schalscha, E.B., Appelt, H. and Schatz, A., 1967. Chelation as a weathering mechanism-1. Effect of complexing agents on the solubilization of iron from minerals and grano-diorite. Geochim. Cosmochim. Acta, 31 587—596. 

Mineral weathering generally occurs whenever water comes into contact with primary mineral particles. The overall processes of weathering are not reversible even though individual reaction steps are often treated as reversible equilibria. The main chemical mechanisms of weathering are ... 

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. 

The decomposition of several common soil minerals has been examined in the laboratory and probably represents the mechanism of mineral weathering in soils as well. The decomposition rate of montmorillonite is proportional to the H+ concentration of the attacking solution ... 

Under natural conditions the rates of dissolution of most minerals are too slow to depend on mass transfer of the reactants or products in the aqueous phase. This restricts the case to one either of weathering reactions where the rate-controlling mechanism is the mass transfer of reactants and products in the soHd phase, or of reactions controlled by a surface process and the related detachment process of reactants. 

The weathering process which eventually reduces the rock of the parent material to the inorganic constituents of soil comprises both physical and chemical changes. Size reduction from rocks to the colloidal state depends not only upon the mechanical action of natural forces but also on chemical solubilisation of certain minerals, action of plant roots, and the effects of organic substances formed by biological activity. 

Natural mobilization includes chemical, mechanical, and biological weathering and volcanic activity. In chemical weathering, the elements are altered to forms that are more easily transported. For example, when basic rocks are neutralized by acidic fluids (such as rainwater acidified by absorption of CO2), the minerals contained in the rocks can dissolve, releasing metals to aqueous solution. Several examples are listed below of chemical reactions that involve atmospheric gases and that lead to the mobilization of metals ... 

Biological and volcanic activities also have roles in the natural mobilization of elements. Plants can play multiple roles in this process. Root growth breaks down rocks mechanically to expose new surfaces to chenaical weathering, while chemical interactions between plants and the soil solution affect solution pFF and the concentration of salts, in turn affecting the solution-mineral interactions. Plants also aid in decreasing the rate of mechanical erosion by increasing land stability. These factors are discussed more fully in Chapters 6 and 7. 

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