Weathering of silicates

Hochella, M. F. Jr. and Banfield, J. F. (1995). Chemical weathering of silicates in nature A microscopic perspective with theoretical considerations. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), pp. 353 06. Mineralogical Society of America Washington, DC, Reviews in Mineralogy 31. 

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. 

The weathering of silicates has been investigated extensively in recent decades. It is more difficult to characterize the surface chemistry of crystalline mixed oxides. Furthermore, in many instances the dissolution of a silicate mineral is incipiently incongruent. This initial incongruent dissolution step is often followed by a congruent dissolution controlled surface reaction. The rate dependence of albite and olivine illustrates the typical enhancement of the dissolution rate by surface protonation and surface deprotonation. A zero order dependence on [H+] has often been reported near the pHpzc this is generally interpreted in terms of a hydration reaction of the surface (last term in Eq. 5.16). 

Barker, W.W., Welch, S.A. and Banfield, J.F. 1997. Biogeochemical weathering of silicate minerals. In Banfield, J.F. and Nealson, K.H. (eds) Geomicrobiology Interactions between Microbes and Minerals. Mineralogical Society of America, Washington, pp. 391-428. 

Aluminium oxide is the oldest ceramic material used in medicine. Bauxite and corundum are the main natural sources of aluminium oxide. Bauxite is a mixture of diaspore, gibbsite, iron hydroxides, clay minerals and quartz. It is formed by the tropical weathering of silicate rocks during which quartz and the elements sodium, calcium, magnesium and potassium are largely washed away. This is the reason why the remaining material becomes richer in the resistant elements titanium, iron and aluminium. The latter is extracted from this mixture in the form of aluminium hydroxide. In its turn this compound is converted into aluminium oxide by heating the mixture to 1200-1300 °C, this is called calcination. The hydroxide is thus made anhydrous. 

Fixation of Carbon Dioxide into Inorganic Carbonates The Natural and Artificial Weathering of Silicates ... 

With respect to flux Hf, in the process of weathering of silicate rocks the rate of C02 extraction from the atmosphere is negligible compared with the similar process for carbonate rocks. Therefore, let us consider the contribution of such rocks to flux Hf. Under equilibrium conditions the relationship [Ca21 ][I IC03]2//)a = const is valid. Usually, 2[Ca2+] = [F1CC)3], and therefore we have flHC03]/[HC03] =... 

In Figure 9.21 all of the carbon eventually used in weathering of minerals by CC>2-charged soil water is shown as entering the atmosphere. The difference between the flux of CO2 owing to precipitation of carbonate minerals in the ocean and the total CO2 released from the ocean is that CO2 used to weather silicate minerals on land, and agrees with the calculations of riverine source materials made earlier in this chapter, in which it was shown that 30% of the HCC>3 in river water comes from weathering of silicate minerals. 

The chemical weathering of silicates and carbonates occurs through hydrolysis... 

The ions produced through the weathering of silicates can be classified into three groups ... 

Microbially produced exopolysaccharides have been directly implicated in weathering of silicates and aluminosilicates through either complexation of cationic constituents of these minerals or, in the case of add mucopolysaccharides, through acidolysis (Barker Banfield, 1996). In some instances, however, exopolysaccharides may inhibit weathering, as in the case of plagi-oclase by gluconate at circumneutral pH (Welch Vandevivere, 1995). 

Fieldes, M. and Swindale, L. D., The chemical weathering of silicates in soil formation,... 

Correns, C. The experimental chemical weathering of silicates. Clay Mineral. Bull. 249-265 (1961). 

Barman A. K., Varadachari C., and Ghosh K. (1992) Weathering of silicate minerals by organic acids 1. Nature of cation solubilisation. Geoderma 53, 45-63. 

Bricker O. P., Godfrey A. E., and Cleaves E. T. (1968) Mineral-water interactions during the chemical weathering of silicates. In Trace Inorganics in Water, Advances in Chemistry Series 73 (ed. R. E. Gould). American Chemical Society, Washington, DC, pp. 128-142. 

White A. F. and Brantley S. L. (2003) The effect of time on the weathering of silicate minerals why to weathering rates differ in the laboratory and field Chem. Geol. (in press). 

Boyle J. R. and Voight G. K. (1973) Biological weathering of silicate minerals implications for tree nutrition and soil genesis. Plant Soil 38, 191—201. 

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