Incongruent reaction

Another useful concept is that of congruent and incongruent reactions. These terms describe reactions involving the dissolution of minerals. If all the products of a dissolution reaction are soluble, the reaction is called congruent, as in the case of the quartz dissolution reaction (1.6) described above. Because, as written, the olivine weathering reaction leads to quartz precipitation it is an incongruent reaction. 

The initially incongruent reaction indicates that material having a composition different from that of the bulk mineral is present on the surface at steady state. The nature of this surficial material is a matter of debate, but it appears relatively certain that it is not uniformly distributed across the surface of the mineral or it would be observed by XPS. The low lateral resolution of XPS, however, prevents detection of materials that do not cover more than about 5% of the exposed surface area. 

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. 

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,... 

Figure 7,8 Gibbs free energy curves and T-X phase relations for an intermediate compound (C), totally immiscible with pure components. Column 1 Gibbs free energy relations leading to formation of two eutectic minima separated by a thermal barrier. Column 2 energy relations of a peritectic reaction (incongruent melting). To facilitate interpretation of phase stability fields, pure crystals of components 1 and 2 coexisting with crystals C are labeled y and y", respectively, in T-X diagrams same notation identifies mechanical mixtures 2-C and C-1 in G-X plots.

By comparison, melting of incongruent minerals produces not only melt but also minerals, and such a melting reaction process can be expressed as... 

Fig. 4. A macroscopic picture of an incongruently melted molybdenum sesquisulfide regulus. A melting reaction has formed molybdenite (MoS2) at the edges. The initial charge (pure M°2 06 3) was heated to approx. 1700 °C and chilled within 30 min to room temperature. Regulus size 3.5 mm diameter. Photo Udubasa...

Fig. 11.7. Isobaric phase diagram of a binary system where the following reaction takes place A + B — C resulting in (a) a congruent melting compound and (b) an incongruent melting compound.

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