Kaolinite, dissolution

Huertas, F. J., Chou, L. Wollast, R. 1998. Mechanism of kaolinite dissolution at room temperature and pressure Part I. Surface speciation. Geochimica et Cosmochimica Acta, 62, 417-731. 

Carroll-Webb S. A. and Walther J. V. (1988) A surface complex model for the pH-dependence of corundum and kaolinite dissolution rates. Geochim. Cosmochim. Acta 52, 2609-2623. 

Chin P. K. F. and Mills G. L. (1991) Kinetics and mechanism of kaolinite dissolution—effects of organic ligands. Chem. Geol. 90, 307-317. 

Oelkers E. H., Schott J., and Devidal J.-L. (2001) On the interpretation of closed system mineral dissolution experiments Comment on Mechanism of kaolinite dissolution at room temperature and pressure Part II. Kinetic study by Huertas et al. (1999). Geochim. Cosmochim. Acta 65, 4429-4432. 

Effects of Humic Substances on Mineral Dissolution. Although humic substances appear to adsorb to oxide surfaces at least in part through surface complex formation, they have only slight effects on the dissolution of oxide and silicate minerals in laboratory studies. Both inhibition (at pH 4) and acceleration (at pH 3) of aluminum oxide dissolution have been observed 61), but in neither case was the effect dramatic (Table I). Kaolinite dissolution at pH 4.2 was also observed to be only minimally affected by humic substances (62). 

In our weathering example (Table 7,2) kaolinite, rather than the aluminum oxyhydroxides, was the chief weathering product of the feldspars. This reflects the fact that the silica present in soil moisture and natural waters, generally, is high enough to stabilize kaolinite relative to the aluminum oxyhydroxides. This observation is better understood if we write the kaolinite dissolution reaction ... 

For the preceding kaolinite dissolution/precipitation reactions, the congruent dissolution is... 

The pH-dependent rates of kaolinite dissolution are presented in Figure 13. Within the experimental time range (t= 10-15 days), the proton-promoted dissolution of kaolinite occurs nonstoichiometrically, that is, the detachment of Si is faster than the release of A1 from the kaolinite surface (jRh.s h.ai)-... 

The pH dependence of kaolinite dissolution can be discussed by expressing proton-promoted rates RH in terms of pH ... 

Then the kaolinite dissolution reaction gas the final appearance... 

There are no unequivocal weathering reactions for the siUcate minerals. Depending on the nature of parent rocks and hydrauhc regimes, various secondary minerals like gibbsite, kaolinite, smectites, and iUites are formed as reaction products. Some important dissolution processes of siUcates are given, for example, by the following reactions (19). 

During caustic waterflooding the alkali can be consumed by the dissolution of clays and is lost in this way. The amount lost depends on the kinetics of the particular reaction. Several studies have been performed with kaolinite, using quartz as a yardstick, because the kinetic data are documented in the literature. The initial reaction rate has been found pH independent in the pH range of 11 to 13 [517]. The kinetics of silica dissolution could be quantitatively described in terms of pH, salinity, ion-exchange properties, temperature, and contact time [1549]. 

In summary, the model proposed on the basis of acid-base characteristics of kaolinite platelets explains the pH-dependent charge primarily to the protonation of the hydroxyl groups at the basal gibbsite and the edge surface. We will later illustrate how this charge characteristics (surface protonation) influences the reactivity (dissolution characteristics) of kaolinite. 

Wieland, E., and W. Stumm (1991), "Dissolution Kinetics of Kaolinite in Add Aqueous Solutions at 25° C", submitted to Geochim. Cosmochim. Acta. 

Ca (aq), Mg (aq), and HCOjCaq). Silicate weathering is an incongruent process. The most important of these reactions involves the weathering of the feldspar minerals, ortho-clase, albite, and anorthite. The dissolved products are K (aq), Na (aq), and Ca (aq), and the solid products are the clay minerals, illite, kaolinite, and montmorillonite. The weathering of kaolinite to gibbsite and the partial dissolution of quartz and chert also produces some DSi,... 

Muller, J. Joubert, J.C. (1974) Synthese en milieu hydrothermal et caracterisation de Voxyhydroxyde de vanadium V OOH et d une nouvelle variete allotropic du dioxide VO2. J. Solid State Chem. 11 79—87 Muller, J.P. Bocquier, G. (1986) Dissolution of kaolinites and accumulation of iron oxides in lateritic-ferruginous nodules Mineralogical and microstructural transformations. Geoderma 37 113-136... 

Nagy, K. L. Blum, A. E. Lasaga, A. C. 1991. Dissolution and precipitation kinetics of kaolinite at 80°C and pH 3. The dependence on solution saturation state. American Journal of Science, 291, 649-686. 

Wieland, E. Stumm, W. 1992. Dissolution kinetics of kaolinite in acidic aqueous solutions at 25 °C. Geochimica et Cosmochimica Acta, 56, 3339-3355. 

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