Chemical weathering silicate minerals

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. 

Some minerals of silicate class and oxide and hydroxide class are stable in the environment prevailing at or near die Earth s surface, and they are resistant to chemical weathering. These minerals, when pulverised to very fine grain size by natural processes, classified as in Table 2.3, can be present as associated minerals in the clays. These minerals are not plastic when wet or harden by drying or firing neither do they impart these properties to the clay. On the contrary, the plasticity of clay decreases as the proportion of associated minerals increases in them. Some associated minerals, especially the oxides and hydroxides of iron, imparts to clay some red or reddish-brown colour, which even persists after firing of those clays. Thus the increase of associated minerals decreases the economic value of clay. Some common associated minerals are listed below. 

Blum, A. E. and Stillings, L. L. (1995). Feldspar dissolution kinetics. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. 

White, A. F., 1995, Chemical weathering rates of silicate minerals in soils. Reviews in Mineralogy 31,407 161. 

Blum, A.E. Stillings, L.L. 1995. Felsdpar dissolution kinetics. In White, A.F. Brantley, S.L. (ed.), Reviews in mineralogy, 31 Chemical weathering rates of silicate minerals, Mineralogical Society of America, USA, 291-352. 

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,... 

Other examples of weathering reactions involving igneous silicates are provided in Table 14.1. In some cases, chemical weathering proceeds in a stepwise feshion in which one clay mineral can be transformed into another given fevorable environmental conditions. 

The amount and type of chemical weathering that occms depends on the chemical composition of the parent rock and the weathering solution, as well as the environmental conditions. As shown in Figure 14.2, the high-temperatme igneous silicates are more prone to chemical weathering than the low-temperature minerals. Climate is important because it determines temperature and water availability, both of which control... 

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. 

As the rock cycle continues, the calcium silicate minerals are eventually uplifted onto land where they imdergo chemical weathering. This reaction involves acid hydrolysis driven by carbonic acid. The latter is derived from the dissolution of the magmatic CO2 in rainwater ... 

This is a very sketchy depiction of the deep carbon cycle because it illustrates only the behaviors of calcium and silica. In reality, a wide variety of other cations are present in the silicate minerals, such as in the plagioclase feldspars (Table 13.2). Furthermore, not all of the limestone is converted into siUcate minerals some remains as limestone. Uplift of the limestone onto land, followed by chemical and biological weathering, is another sink for atmospheric CO2, via... 

The rates of chemical weathering reactions increase with temperature as aU chemical processes. This effect has been confirmed in laboratory experiments and field studies involving silicate rocks [12]. Weathering reactions occur at mineral... 

Bums, R.G. (1993) Rates and mechanisms of chemical weathering of ferromagnesian silicate minerals on Mars. Geochim. Cosmochim. Acta 57 4555-4574... 

Hochella, M. F. Banfield, J. F. 1985. Chemical weathering of silicates in nature A microscopic perspective with theoretical considerations. In White, A. F. Brantely, S. L. (eds) Chemical Weathering Rates of Silicate Minerals. 31, Mineralogical Society of America, Washington, DC, 353-406. 

Velbel, M. A. 1993. Constancy of silicate-mineral weathering-rate ratios between natural and experimental weathering Implications for hydrologic control of differences in absolute rates. Chemical Geology, 105, 89-99. 

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