Weathering of silicate rocks

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. 

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] =... 

Drever J. I. and Zobrist J. (1992) Chemical weathering of silicate rocks as a function of elevation in the southern Swiss Alps. Geochim. Cosmochim. Acta 56, 3209-3216. 

Controls on C02-drawdown In the modern Earth the principal agent of C02-drawdown is through the weathering of silicate rocks as part of what is known as the "Urey Cycle," after its identification by Urey (1952). The chemical reaction which summaries this process is... 

It has long been known that the long-term carbon cycle is linked to the weathering of silicate rocks (for example, see Berner 1992a, 1995). The influence of silicate weathering on the global carbon cycle has been the focus of intense discussion because of its potential impact on the concentration of atmospheric CO2 through the reaction ... 

Figure 5. Typical differentiation of chemical elements between waters and soils occurring during weathering of silicate rocks (after Nesbitt and Young (1982), data from this paper) the open circles correspond to the initial rock compositions, the arrows pointing towards AI2O3 show the chemical evolution of the soil horizons lying on top of the parent rock (molecular proportions of oxides). They become more and more enriched in AI2O3. For each rock type, the typical evolution of water is shown in the left comer of the triangle the squares correspond to the rain water and the tip of the arrows correspond to the spring waters (molecular proportions of cations), points in between correspond to the soil percolation water. They evolve towards proportionally Al-K-poor, but Ca-Na-rich compositions.

This has caused an increase in the normalized calcium concentration as well as the normalized alkalinity in the Baltic Sea. Weathering of silicate rocks is... 

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. 

Clays are layer silicates (phyllosilicates) of particle size less than about 4 pm, produced by the weathering of aluminosilicate rocks. Clay minerals fall roughly into two structural classes the kaolinite type, based on paired sheets of tetrahedral (SiC>44-) and octahedral [A10n(0H) g " or... 

Silicates Many complex aluminosilicates or silicoaluminates are found in nature. Of these, clay in more or less pure fomi, pure day, kaolintte, kaolin, china clay TLSvA102G or Al203.2,Si02.2TT20 is of great importance. Clay is formed by the weathering of igneous rocks, and is used in the manufacture of bricks, pottery, procelain, and Portland cement. 

The weathering of surface rocks has had a critical role in the chemical evolution of the continental crust for most of the Earth s history. In the presence of air and water, mafic minerals tend to rapidly weather into iron (oxy)(hydr)oxides, clays, and other silicate minerals, and at least partially water-soluble salts of alkalis (sodium and potassium) and alkaline earths (calcium and magnesium). In contrast, quartz in felsic and intermediate igneous rocks is very stable in the presence of surface air and water, which explains why the mineral readily accumulates in sands and other sediments. 

Experimentally determined dissolution kinetics are applicable to natural weathering processes of silicate rocks. Mass transfer from the mineral to the aqueous phase was determined to be incon-gruent under a range of experimental conditions. Transfer rates of individual species (Q) at times (t) can usually be described by one of two rate expressions ... 

The role of chemical weathering has long been recognized in economic geology. Tropical bauxites, which account for most of world s aluminum ores, are typical examples of residual concentration of silicate rocks by chemical weathering over long time periods (Samma, 1986). Weathering of ultramafic silicates such as peridotites forms residual lateritic deposits that contain... 

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