Silicate mineral weathered

The importance of "parabolic kinetics" in laboratory studies of mineral dissolution has varied as interpretations of the underlying rate-controlling mechanism have changed. Much of the research on silicate mineral weathering undertaken in the past decade or so served to test various hypotheses for the origin of parabolic kinetics. 

Velbel, M.A. (1993) Formation of protective surface layers during silicate-mineral weathering under well-leached, oxidizing conditions. 

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. 

In the previous section the present-day cycle of carbon was discussed in some detail. We can now turn our attention to the sources of carbon to the ocean, and of calcium and magnesium, the major elements (other than oxygen and hydrogen) with which carbon interacts. Because carbon dioxide is the major acid gas involved in both carbonate and silicate mineral weathering reactions at the Earth s surface, it is informative to consider the sources of other elements as well. 

At present it is difficult to compare the effects of different lichen taxa, or different lichen acids, on bulk silicate mineral weathering rates under field conditions. An abundant literature depicts the physical and chemical distribution of minerals, alteration products and organic components during lichen growth, mostly provided through SEM, EDS, XRD and... 

Alexandre et al. (1997) found that the biogenic sihca input into the biogeochemical silica cycle from the dissolution of phytoliths is twice as large as silica input from primary silicate mineral weathering in the tropical Congo rainforest. Biogenic (opaline) silica dissolves faster than sihcate minerals. While most of the phytoliths dissolve rapidly with a mean residence time of 6 months (Alexandre et al., 1994), and the sihca is recycled by the forest, a small part (7.5%) does not dissolve and is preserved in the soil. 

Silicon is mobilized by the weathering of silicate minerals, mainly feldspars (see eqn. 4.14), and is transported in natural waters at near-neutral pH as undissociated silicic acid (H4Si04), an oxyanion (Fig. 5.2). Silicate minerals weather slowly, such that rates of input—and concentrations—of silicon in most freshwaters are quite low. Despite this, where silicates are the main component of bedrock or soil, H4Si04 can be a significant dissolved component of freshwater. 

The Ca—Mg—HCO3- and Ca—Mg—SO4-type groundwater from the glacial drift aquifers reflects the dissolution of calcite and dolomite by carbonic acid formed in the soil zone, and the production and leaching of secondary gypsum through oxidation of sulfide in the presence of calcite or dolomite under conditions of partial saturation. In cases where the content of carbonates is low, silicate mineral weathering potentially occurs. 

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. 

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,... 

There are no unequivocal weathering reactions for the silicate minerals. Depending on the nature of parent rocks and hydraulic regimes, various secondary minerals like gibbsite, kaolinite, smectites, and illites are formed as reaction products. Some important dissolution processes of silicates are given, for example, by the following reactions ... 

It has been shown by Berner et al. (1983) and by Berner and Lasaga (1989) that silicate weathering is more important than carbonate mineral weathering as a longterm control on atmospheric C02. The HC03 and Ca2+ ions produced by weathering of CaC03,... 

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). 

Estimate on the basis of results on experimental weathering rates and the number of functional groups (or central ions) per unit area, how long it takes to dissolve one "monomolecuar" layer of a representative silicate mineral. 

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