Chemical weathering of minerals

Groundwater environments can be represented as a simple flow-through system. For the situation where chemical weathering of mineral grains is transport controlled, the weathering rate of a mineral should be directly dependent on the rate of throughput of water. For the situation where rates are controlled by surface... 

Chemical weathering of minerals during pedogenesis can be enhanced by microbial activity by a factor as high as 106 (Kurek 2002). Microorganisms can dissolve minerals by direct and indirect actions under aerobic and anaerobic conditions (Robert and Berthelin 1986 Ehrlich 2002 Kurek 2002). In some cases of attack, the microorganisms may be dispersed in the soil solution in others, they may grow in biofilms on the surface of susceptible minerals. 

In this chapter, we build on applications in the previous chapter (Chapter 26), where we considered the kinetics of mineral dissolution and precipitation. Here, we construct simple reactive transport models of the chemical weathering of minerals, as it might occur in shallow aquifers and soils. 

The rate of chemical weathering of minerals in the subsurface depends on a number of factors, including mineralogy, temperature, flow rate, surface area, presence of ligands and CO, and H+ concentrations in the subsurface water (Stumm et al. 1985). Figure 2.3 shows the rate-limiting steps in mineral dissolution consisting of... 

Chemical weathering of minerals results not only in the introduction of solutes to the aqueous phase but often in the formation of new solid phases. Dissolution is described as congruent, where aqueous phase solutes are the only products, or incongruent, where new solid phase(s) in addition to aqueous phase solutes are the products. These reactions... 

Stage 1 corresponds to a steady state where base cation concentrations in runoff are relatively constant, averaged over periods longer than an annual cycle. BS of soils is also relatively constant. The rate of export of base cations is equal to the rate of chemical weathering of minerals... 

Jackson, M. L. (1969). "Soil Chemical Analysis Advanced Course," 2nd edn. 7th printing, 1973. Department of Soil Science, University of Wisconsin, Madison, Wisconsin. Jackson, M. L. and G. D. Sherman (1953). Chemical weathering of minerals in soils. Adv. Agron. 5, 219-318. Jackson, M. L., S. Y. Lee, F. C. Ugolini, and P. A. Heimke (1976). Age and uranium content of soil micas from Antarctica by the fission track replica method. Soil Sci. 123, 241-248. 

Equation 19.5 demonstrates why the presence of bicarbonate ions and molecular silicic acid in the streams of Taylor Valley is evidence that chemical weathering of feldspar and other silicate minerals is occurring in spite of the harsh climatic conditions. The only prerequisite to chemical weathering of minerals on the surface of the Earth as well as in the subsurface is the presence of liquid water. The work of Nezat et al. (2001) has demonstrated that silicic acid and bicarbonate ions are present in all meltwater streams in the watersheds of Lake Bonney, Lake Hoare, and Lake Fryxell. 

The importance of oxalic acid in the chemical weathering of minerals and rocks was emphasized by several early workers, Salter (1856) suggested... 

Jackson, M. L., and D. Sherman, 1953. Chemical weathering of minerals in soils. Advan. Agron. 5 219. 

Another major process at the Earth s surface not involving rapid exchange is the chemical weathering of rocks and dissolution of exposed minerals. In some instances the key weathering reactant is H30 in rainwater (often associated with the atmospheric sulfur cycle), while in other cases H30" comes from high concentrations of CO2, e.g., in vegetated soils. 

Clayton, J. L. (1986). An estimate of plagioclase weathering rate in the Idaho batholith based upon geochemical transport rates. In "Rates of Chemical Weathering of Rocks and Minerals" (S. M. Coleman and D. P. Dethier, eds). Chap. 19, pp. 453-466. Academic Press, New York. 

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. 

As noted earlier, diverse forms of organomercury are released into the environment as a consequence of human activity. Methyl mercury presents a particular case. As a product of the chemical industry, it may be released directly into the environment, or it may be synthesized in the environment from inorganic mercury which, in turn, is released into the environment as a consequence of both natural processes (e.g., weathering of minerals) and human activity (mining, factory effluents, etc.). 

Section 4.3 sets out the principles underlying the structure of the silicate mineral family. Natural clay deposits are formed by the chemical weathering of rocks -largely as a result of the attack by slightly acidic surface waters. Rainwater,... 

Biogeochemists use the terms dissolved silica (DSi) or dissolved silicate to collectively refer to all of the dissolved silicon. Silicic acid exhibits tetrahedral geometry with the silicon atom at the center and a hydroxyl group occupying each of the four corners. This structure is similar that of the mineral silicate tetrahedra (Figure 14.3c). Chemical weathering of the silicate minerals is the major source of DSi to the ocean, giving rise to the term dissolved silicate, which is usually abbreviated to just silicate. ... 

The chemical weathering of crustal rock was discussed in Chapter 14 from the perspective of clay mineral formation. It was shown that acid attack of igneous silicates produces dissolved ions and a weathered solid residue, called a clay mineral. Examples of these weathering reactions were shown in Table 14.1 using CO2 + H2O as the acid (carbonic acid). Other minerals that undergo terrestrial weathering include the evaporites, biogenic carbonates, and sulfides. Their contributions to the major ion content of river water are shown in Table 21.1. 

---