Silicate mineral dissolution

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. 

The latter two assumptions are simplistic, considering the number of factors that affect pH and oxidation state in the oceans (e.g., Sillen, 1967 Holland, 1978 McDuff and Morel, 1980). Consumption and production of CO2 and O2 by plant and animal life, reactions among silicate minerals, dissolution and precipitation of carbonate minerals, solute fluxes from rivers, and reaction between convecting seawater and oceanic crust all affect these variables. Nonetheless, it will be interesting to compare the results of this simple calculation to observation. 

Siegel D. I. and Pfannkuch H. O. (1984) Silicate mineral dissolution at pH 4 and near standard temperature and pressure. Geochim. Cosmochim. Acta 48, 197-201. 

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. 

Schott, J., Pokrovsky, O.S., Spalla, O., Devreux, F., Gloter, A., Mielczarski, J.A. (2012). Formation, growth and transformation of leached layers during silicate minerals dissolution The example of wollastonite. Geochimica et Cosmochimica Acta, 98, 259-281. 

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...

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. 

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 ... 

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). 

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. 

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 ... 

Leaching of ores, i.e. the separation of Fe oxides from silicate minerals has prompted investigations into the acid dissolution behaviour of natural goethites and hematites (Surana Warren, 1969 Warren Roach, 1971). Dissolution curves... 

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. 

Examples on mineral dissolution in water and in silicate melts... 

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