Silicate reactions

We have presented spinel synthesis because these systems were studied first and can be understood in a simple manner. Various silicate systems have also been studied and it has been determined that they are more representative of the general case involving solid state synthesis reactions than spinel. Let us examine the following simple silicate reaction ... 

While trifluoro and other halosilanes function by increased electrophilicity at silicon, nucleophilic reactivity of allylic silanes can be enhanced by formation of anionic adducts (silicates). Reaction of allylic silanes with aldehydes and ketones can... 

The generated OH" will probably attack silicates in an analogous way. Thus it is likely that silicate reactions tend to neutralize the formed hydroxyl ions by the following or some analogous process ... 

The budget can be balanced by consideration of a variety of silicate reactions producing different ratios of cations HCC>3 SiC>2. The simplest approach was that of Garrels and Mackenzie (1971a), in which they considered the reactions of feldspars with C02-charged water to produce kaolinite and a montmorillonite-type mineral ... 

White A. F., Blum A. E., Schulz M. S., Bullen T. D., Harden J. W., and Peterson M. L. (1996) Chemical weathering rates of a soil chronosequence on granitic alluvium I. Quantification of mineralogical and surface area changes and calculation of primary silicate reaction rates. Geochim. Cosmochim. Acta 60, 2533-2550. 

Another class of models relevant to chemical weathering is based on the reaction path approach originally developed by Helgeson et al. (1970). EQ3/EQ6 (Wolery et al. (1990), PHREEQC (Parkhurst and Plummer, 1993), and PATHARC.94 (Gunter et al., 2000) are some codes currently used to describe the progressive reaction of primary silicates and the precipitation of secondary phases as a function of time and mass. These codes are discussed in Nordstrom (see Chapter 5.02). They commonly permit the introduction of user-defined silicate reaction rates. Such models also commonly consider solubility controls on reaction kinetics as defined by... 

In deeper systems dominated by calcium-rich saline fluids, it has been shown that both solubility constraints and silicate reactions act to further remove bicarbonate ions as precipitated calcium and magnesium carbonates, often adjusting pH to levels greater than 9 (Barnes and O Neil, 1971 Fritz et al., 1987a Clauer et al., 1989). For example, during closed-system dissolution of magnesium olivine (forsterite), a major component of many ultramafic rocks, as the silicate water reaction proceeds water breaks down, H" " ions are consumed, carbonates precipitate, and hydroxyl ions force the pH to rise (Barnes and O Neil, 1971 Drever, 1988). 

The silicate in the injected reagent cannot react in the sample stream because the oxalic acid added upstream of the injector destroys the excess molybdate. However, the quenching of the silicate reaction before injection of the reagent is not essential. Automatic refractive index matching also works well in the phosphate analysis (Figures 4 and 5). Any phosphate in the injected reductant is reactive in this case. The additional phosphate will only make the peaks proportionately larger. Standards must be analyzed by the same procedure used for the samples. 

In the Abo Arroyo zone, the two ground-water samples with As concentrations greater than 3 pg/L have relatively high pH values and Si02 and K concentrations relative to the other two samples Cl concentrations for these two samples are the lowest. It appears possible, although not certain, that a silicate reaction may have increased pH enough to allow some As to desorb from aquifer materials. 

HCO3 and CO3 to form CO2 in hydrothermal solutions, resulting in a much lower pH than in normal seawater. H4Si04 is released to solution hy high-temperatme silicate reactions, and SO4 is reduced to hy oxidation of Fe + to Fe within iron-containing silicates. 

The sodium aluminate/sodium silicate reaction is temperature sensitive and a rise of approximately 8°C will halve the gelation time. 

Adu-Wusu, K. and Wilcox, W.R., Kinetics of silicate reaction with gibbsite, J. Colloid Interf. Sci., 143, 127, 1991. 

The lA Pliocene, Ranzano and Antognola formations were cemented by meteoric water the Bismantova Formation was cemented in part by water with a meteoric component the Loiano and the Borello formations were cemented by slightly modified marine pore water and all the foreland basin units (except the Borello) were cemented by water variably enriched in 0 (8 0 = -2 to +8) generated from silicate reactions. The most 0-enriched values for S 0, a,er are compatible with depths and temperatures of cementation of the three deepest formations, but not for the less deeply buried Loiano and upper part of the Mamoso-arenacea formations. 0-enriched fluids in these latter formations were more probably derived from underlying, more deeply buried rocks apd expelled by compaction. Possibly, the calcite in the deepest buried formations re-equilibrated with hot water after precipitation. 

In this chapter we will first discuss briefly the basic theory behind ab nitio methods. Then a discussion of transition state theory, in light of the ab initio capabilities, will be taken up. This discussion will be followed by an analysis of the molecular mechanisms in water-silicate reactions and the ab initio elucidation of the adsorption and kinetic barriers involved in the bulk chemical reactions occurring at mineral-water interfaces. 

Despite all these safeguards to extend the service life of the antifreeze, fluid replacement is required periodically. Typically, fluids are replaced because of irreversible damage caused by one of four conditions contamination, gel formation because of glycol/silicate reaction, extensive glycol degradation caused by overheating or excessive oxygen exposure, or inhibitor depletion. 

When the mass of CaC03 in soils exceeds several percent, it controls both soil pH and soil solution Ca2+ concentrations. Silicate reactions with H+ and Ca2+ iii this case are relatively insignificant. Hie pH and Ca2+ can then be calculated from Eqs. 7.12 and 7.13, the Pco and the solubility product of CaC03 ... 

Figure 7.5 is approximate because the AG values of formation of Ca- and Na-montmorillonites are educated guesses. Figure 7.5 also ignores the slow kinetics of silicate reactions. In addition, die activities of Mg2+, K+, andNa+ affect the stability of other soil minerals and their igneous minerals, which direcdy and indirecdy affect the stability of Ca minerals. 

To assess the behavior of some common silicate minerals in the weathering environment and the eflFect of water-silicate reactions on both the minerals and the resulting solution composition, we investigated the geochemical balance of a small wooded watershed underlain by silicate bedrock. 

The small magnitude and short duration of the compositional variation of the stream in flood stage and the constancy of base flow composition suggest that the water-silicate reactions are rapid. The rapidity with which water equilibrates with the fresh and weathered bedrock was tested in the laboratory (term project of Thomas Dunne and Terrance Smith). Two columns of soil were collected from the side slopes of the watershed. Plexiglass tubes 6 inches inside diameter by 3% feet long were forced into the soil, then dug up. 

Gifford, P.M., and Gillot, J. (1996) Alkali-silicate reaction (ASR) and alkali-carbonate reaction (ACR) in activated blast fiunace cement (ABFSC). Cement and Concrete Research 26,21-26. 

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