Silicates, dissolution

Brady, P. V. and J. V. Walther, 1989, Controls on silicate dissolution rates in neutral... 

Pyrite Oxidation. The oxidation of Fe(ll) minerals by Fe3+ is also of importance in the oxidation of pyrite by 02. This process is mediated by the Fe(II)-Fe(III)system. Pyrite is oxidized by Fe3+ (which forms a surface complex with the pyrite (cf. formula VI in Fig. 9.1) (Luther, 1990). The rate determining step at the relatively low pH values encountered under conditions of pyrite dissolution is the oxygenation of Fe(II) to Fe(III) usually catalyzed by autotrophic bacteria (Singer and Stumm, 1970 Stumm-Zollinger, 1972). Thus, the overall rate of pyrite dissolution is insensitive to the mineral surface area concentration. Microbially catalyzed oxidation of Fe(II) to Fe(III) by oxygen could also be of some significance for oxidative silicate dissolution in certain acid environments. 

Siegel, D. I., and H. O. Pfannkuch (1984), "Silicate Dissolution Influence on Filson Creek Chemistry, Northeastern Minnesota", Geol. Soc. Am. Bull. 95,1446-1453. 

Most of the dissolved calcium in groundwater in northern Wisconsin is the result of silicate hydrolysis of the aquifer materials. The assumption of conservancy is accurate only because of the relatively slow rates of silicate dissolution. The presence of more soluble calcium-containing minerals, such as calcite or gypsum, would invalidate assumptions of conservancy and would lead to significant errors in solute budgets. 

Silicate dissolution can generally be described by one of two rate expressions ... 

One parameter necessary for modeling silicate dissolution kinetics is the influence of aqueous chemistry on the rate expression. 

Changes in pH also affect the magnitude of individual rate expressions. Wollast (28) found for K-feldspar a progressive decrease in the parabolic rate constant for silica with increases in pH over the range 4-10 at 25 C. In investigating magnesium silicate dissolution at 25 C, Luce and others ( ) found a slight... 

For minerals that dissolve incongmently, the determination of reaction rate depends upon which component released to solution is used in Equation (5). Due to preferential release of cations such as calcium and magnesium during inosUicate dissolution, for example, dissolution rates for these phases are usually calculated from observed silicon release (Brantley and Chen, 1995). Here, we report silicate dissolution rates based upon silicon release, but we normalize by the stoichiometry of the mineral and report as mol mineral per unit surface area per unit time. It is important to note that dissolution rates reported on this basis depend upon both the formula unit and the monitored solute. 

Although silicate dissolution is now not generally thought to be rate hmited by diffusion through an armoring alteration layer, much evidence has accumulated documenting the development of silicon-rich cation-depleted layers of varying... 

Despite the complications of layer formation on many silicates, dissolution is generally assumed to be interface-limited and to be accelerated by the presence of protons or hydroxyl ions. Rates are thus often described by the following empirical equation ... 

Schott J. and Berner R. A. (1983) X-ray photoelectron studies of the mechanism of iron silicate dissolution during weathering. Geochim. Cosmochim. Acta 47, 2233 - 2240. 

Xiao Y. and Lasaga A. C. (1995) Ah initio quantum mechanical studies of the kinetics and mechanisms of silicate dissolution H" " (HsO ) catalysis. Geochim. Cosmochim. Acta 58, 5379-5400. 

In addition to characterizing mass losses, textural features can be used to discern the mechanisms and chemical environment associated with silicate dissolution. It is widely accepted that pitted surfaces, such as that shown in Figure 5, indicate surface reaction whereas smooth rounded surfaces result from diffusion-controlled dissolution (Lasaga, 1998 see Chapter 5.03). 

Experimental rates of silicate dissolution decrease as solutions approach thermodynamic equilibrium (Burch et ai, 1993 Taylor et al., 2000). The saturation state fl is defined as the product of the solute activities (lAP) divided by the saturation constant of the specific mineral and is related to the net free energy of reaction AG (kJ mol ) by the relationship... 

Carbonate and Silicate Dissolution Sources of CO2 and Strong Acids... 

That nucleation and growth rate are the limiting steps in quartz cementation has no particular imphcations with respect to the ultimate source of the sihca or the mechanism of transport. Potential sources of silica for quartz cementation are numerous (McBride, 1989) and include all documented silicate dissolution reactions in sandstones and shales. 

Xiao, Y., and Lasaga, A. C. (1994) Ab-initio Quantum Mechanical Studies of the Kinetics and Mechanisms of Silicate Dissolution (H30 ) Catalysis, Geochim. 

Brady, P. V. 1992. Silica surface chemistry at elevated temperatures. Geochim. Cosmochim. Acta 56 2941-46. Brady, P. V., and J. V. Walther. 1989. Controls on silicate dissolution rates in neutral and basic solutions at 25°C. Geochim. Cosrrwchim. Acta 53 2823-30. 

The partial orders with respect to [OH ] observed for most silicate mineral dissolution reactions can be explained by the surface complexation model (Blum and Lasaga, 1988 Brady and Walther, 1989). Brady -and Walther (1989) showed that slope plots of log R vs. pH for quartz and other silicates at 25 °C is not inconsistent with a value of 0.3. Plots of the log of absorbed OH vs. pH also have slopes of about 0.3, suggesting a first-order dependence on negative charge sites created by OH adsorption. Because of the similarity of quartz with other silicates and difference with the dependence of aluminum oxides and hydroxide dissolution on solution [OH ], Brady and Walther (1989) concluded that at pH >8 the precursor site for development of the activated complex in the dissolution of silicates is Si. This conclusion is supported by the evidence that the rates (mol cm s ) at pH 8 are inversely correlated with the site potential for Si (Smyth, 1989). Thus it seems that at basic pH values, silicate dissolution is dependent on the rate of detachment of H3SiO4 from negative charge sites. 

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