Silicate minerals: dissolution rates, 75-78 relationship

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...

Figure 2.10 Schematic relationship between pH and dissolution rate for silicate and aluminosilicate minerals. Reprinted from Geochim. et Cosmochim. Acta, 58(10), J.T. Drever, The effect of land plants on weathering rates of silicate materials, 2325-32, 1994, with permission from Elsevier Science, Ltd, The Boulevard, Langford Lane, Kidlington, 0X5 1GB, U K.

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... 

In this chapter we discuss the rates of adsorption, paying special attention to those few cases where information on the rate of specific adsorption (reaction of an adsorbate in the adsorption layer) is available. Furthermore, we elaborate on the chemical processes involved in the dissolution of minerals and concentrate on the dissolution of oxides, silicates, and carbonates, which play an enormous rx)le in the chemical weathering and erosion. We try to demonstrate that in most cases the rate-determining step in the dissolution is a chemical reaction at the surface of the mineral. Thus we have here an excellent example of the relationship between surface stracture and reactivity. Surface chemistry plays an equally important role in the formation of the solid phase (precipitation, nucleation, and crystal growth). Nature s selectivity is reflected in the creation of a crystal and its growth. 

Note that this relationship is similar to the van t Hoff equation. If the Arrhenius expression is obeyed, then a plot of log k versus l/T is a straight line, with a slope of-EJ2303 R. Such plots are shown in Figs. 2.5(a) and (b), which describe the effect of increasing temperature on the rates of dissolution of some silicate rocks and minerals. 

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