Minerals silicate, dissolution rates

Fig. 5. Schematic relationship between silicate mineral dissolution rate and pH. The pH of the transition points between pH-dependent and pH-independent behavior varies from mineral to mineral...

Table 3. Effect of oxalate on silicate mineral dissolution rates...

Casey, W. H. and Ludwig, C. (1995). Silicate mineral dissolution as a ligand-exchange reaction. In "Chemical Weathering Rates of Silicate Minerals" (A. F. White and S. L. Brantley, eds), Mineralogical Society of America Washington, DC, Reviews in Mineralogy 31, 87-117. 

Experimental investigations will continue to play a critical role in understanding heterogeneous reaction kinetics, such as mineral dissolution rates in silicate melts and in aqueous solutions, the melting rates at the interfaces of two... 

Silicate mineral dissolution is usually incongruent, with precipitation of relatively amorphous metastable products that may crystallize with time to form minerals such as gibbsite, kaolinite, illite, and montmorillonite (Helgeson et al. 1984). The incongruency means that the net release rates of individual components from a silicate mineral into the water may not be equal (cf. White and Claassen 1979 Helgeson et al. 1984). 

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. 

If the effects of M-0 bond dissociation can be isolated from other reactions, such as depolymerization of a silicate chain, one expects oxide mineral dissolution rates to correlate with bond strengths. In other words, there should be correlation between dissolution rates at constant pH and the rates of water exchange about the corresponding dissolved cation, since similar dissociation of M-O bonds are involved in both solvent exchange and mineral dissolution. Well-defined conditions mean that ... 

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. 

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). 

As was mentioned in the introduction to this chapter "diffusion-controlled dissolution" may occur because a thin layer either in the liquid film surrounding the mineral or on the surface of the solid phase (that is depleted in certain cations) limits transport as a consequence of this, the dissolution reaction becomes incongruent (i.e., the constituents released are characterized by stoichiometric relations different from those of the mineral. The objective of this section is to illustrate briefly, that even if the dissolution reaction of a mineral is initially incongruent, it is often a surface reaction which will eventually control the overall dissolution rate of this mineral. This has been shown by Chou and Wollast (1984). On the basis of these arguments we may conclude that in natural environments, the steady-state surface-controlled dissolution step is the main process controlling the weathering of most oxides and silicates. 

Stumm, W., and E. Wieland (1990), "Dissolution of Oxide and Silicate Minerals Rates Depend on Surface Speciation", in W. Stumm, Ed., Aquatic Chemical Kinetics, John Wiley and Sons, New York, 367-400. 

A common phenomenon in the dissolution of silicate minerals is the formation of etch pits at the surface (90-91.,93-94). When this occurs, the overall rate of mineral dissolution is non-uniform, and dissolution occurs preferentially at dislocations or defects that intercept the crystal surface. Preferential dissolution of the mineral could explain why surface spectroscopic studies have failed... 

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. 

Bloesch, P.M. Bell, L.C. Hughes, J.D. (1987) Adsorption and desorption of boron by goethite. Aust. J. Soil Res. 25 377-390 Blomiley, E.R. Seebauer, E.G. (1999) New approach to manipulating and characterising powdered photo adsorbents. NO on Cl treated Ee20j. Langmuir 15 5970-5976 Bloom, P.R. Nater, E.A. (1991) Kinetics of dissolution of oxide and primary silicate minerals. In Sparks, D.L. Suarez, D.L. (eds.) Rates of soil chemical processes. Soil Sci. 

For the calculation of convective dissolution rate of a falling crystal in a silicate melt, the diffusion is multicomponent but is treated as effective binary diffusion of the major component. The diffusivity of the major component obtained from diffusive dissolution experiments of the same mineral in the same silicate melt is preferred. Diffusivities obtained from diffusion-couple experiments or other types of experiments may not be applicable because of both compositional effect... 

With the addition of bentonite to a crushed basalt backfill, aqueous diffusion would be the most effective mass transfer process (31). Aagaard and Helgeson (32) state that at temperatures <200°C, aqueous diffusion rates are orders of magnitude greater than rates of silicate hydrolysis even in acid solutions. Therefore, the dissolution rate of backfill phases and the overall mass transfer process could be controlled by reactions at the mineral-fluid interface. As stated earlier, dissolution of basalt phases in the sampling autoclave experiments may also be controlled by interface reactions. 

Km . 1.2. An Arrhenius plot of a zeroth-order reaction rate coefficient (normalized to unit surface itiv i atul the unit cell) for the dissolution of a variety of silicate minerals (data from B. J. Wood and I V, Walther, Rates of hydrothermal reaction. Science 222 413 (19K3). See Section 3.1 for additional discussion of rale coefficients for dissolution reactions. 

W. Stumm and E. Wieland, Dissolution of oxide and silicate minerals Rates depend on surface speciation, Chap. 13 in Aquatic Chemical Kinetics, ed. by W. Stumm, Wiley, New York, 1990. 

Since the work of Daubree in 1867 ( 2), workers have experimented to define the dissolution rates of silicate minerals. This paper will briefly review the types of reaction rates that have been observed, followed by more detailed discussion concerning the dependence of these rates on constituents in the aqueous media, and the implications for solute modeling. 

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. 

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