Silicate minerals complex, weathering

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

Iron occurs in two oxidation states, the divalent (Fe ) ion or trivalent (Fe ) ion, and sedimentary rocks contain iron in these various forms with ferric oxides being the most common. When iron is weathered out of the rocks, it is not retained in solution but, depending upon conditions, it is redeposited as oxides or hydroxides. In addition, Fe can replace aluminium in some silicate minerals. An important chemical feature of iron (in solution) is its tendency to form complexes with organic materials. Such complexes are considerably more stable and consequently survive in solution or in the soil for longer periods of time. Specific examples of Fe-organic complexes will be discussed in later sections. 

However, in the presence of carbonic acid, Fe2+ is released into the weathering system by hydrolysis, and may then be oxidised to Fe203 (Krauskopf, 1967). In fact, reactions involving C02-rich rain- and groundwaters are likely to be the more common. Again, as an example, the effect of this acid upon a silicate such as fayalite can be used, but the principle also holds for Fe liberation from more complex silicates such as pyroxenes and amphi-boles. As far as these silicate minerals are concerned, the two-stage weathering reaction can be summarised as ... 

Silicate mineral weathering is more complex than the dissolution of oxides and hydroxides. The primary silicates, those silicates of most interest in weathering studies, are not stable in aqueous environments at earth surface temperatures. For these minerals, solution equilibrium with respect to soluble mineral components is not a factor in determining stability, and only the forward dissolution rate needs to be considered. 

Clay minerals are a complex group of finely crystalline to amorphous hydrated silicates, mainly alumino-silicates, which were largely formed by alteration, or weathering of silicate minerals. 

Carbohydrates and proteins may also play an important role in weathering of soils. As much as 30% of the DOC in soil solutions can occur as saccharides, although only a small portion has been accounted for as polysaccharides. Polysaccharides extracted from soil usually contain Al, Fe, and Si an indication of their ability to complex polyvalent ions. Amino acids, peptides, and proteins are also capable of forming complexes with metal ions. These substances have a strong affinity for binding to silicate minerals and are able to perturb the hydrolytic reactions of Al. 

Sorption depends on Sorption Sites. The sorption of alkaline and earth-alkaline cations on expandable three layer clays - smectites (montmorillonites) - can usually be interpreted as stoichiometric exchange of interlayer ions. Heavy metals however are sorbed by surface complex formation to the OH-functional groups of the outer surface (the so-called broken bonds). The non-swellable three-layer silicates, micas such as illite, can usually not exchange their interlayer ions but the outside of these minerals and the weathered crystal edges ("frayed edges") participate in ion exchange reactions. 

These reactions, however, are complex and generally proceed through a series of reaction steps. The rate of weathering of silicates may vary considerably depending on the arrangement of the silicon tetrahedra in the mineral and on the nature of the cations. 

Clay minerals and clay colloids are the products of the advanced weathering of primary silicates. They are comprised mainly of silica and alumina, often with appreciable amounts of alkali and alkaline earth metals and iron. Most also have varying amounts of water bound to their surfaces and can take on a variety of different chemical and physical properties depending on the amount of water adsorbed. They have the ability to exchange or bind cations and anions and are capable of complex formation with a wide variety of organic molecules. 

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