Incongruent dissolution—

Initial dissolution of primary silicates is typically incongruent that is, the stoichiometric ratio of elements released to solution is not the same as that found in the bulk phase of the mineral. An excellent example of incongruent dissolution (Fig. 7-13) is presented by Chou and Wollast (1984). They reacted albite with aqueous solutions in a fluidized bed reactor and maintained solution concentrations of the reaction products below saturation for potential secondary products. Even so, the molar ratio of Na/Si initially released to solution was almost an order of magnitude higher than that of the bulk albite. Dissolution incongruence has posed a particularly difficult theoretical problem for researchers working on mineral dissolution problems. 

We can use our results to predict the conditions favorable for the transformation of gaylussite to aragonite. The porous nature of the pseudomorphs and the small amounts of calcium available in the lake water (Table 24.2) suggest that the replacement occurs by the incongruent dissolution of gaylussite, according to the... 

Plagioclase dissolution is known to be incongruent, with Al and Ca being preferably dissolved at the surface (Blum... 

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. 

Initial Incongruent Dissolution. We essentially follow the explanation given by Schnoor (1990) for a representative example of an initial incongruent dissolution. 

Fig. 5.16 illustrates the schematic building of a cation depleted layer of thickness y from a hypothetical mineral with constituents A and B (stoichiometry 1 1). Initially incongruent dissolution of AB results in the rapid migration (diffusion) of constituent B from the core of the mineral grain through a layer that is depleted in B (Eq. 5.21)... 

Schematic illustration of initial incongruent dissolution. In the initial stages, a mineral grain may develop a cation depleted layer e, but eventually congruent dissolution is observed and the rate of dissolution of A (dA/dt) must equal B (dB/dt) for a 1 1 stoichiometry.

Congruent surface-controlled dissolution follows after the initial incongruent period. 

The term incongruent is generally used, if a mineral upon dissolution reacts to form a new solid or if the reversal of a dissolution process leads to a different composition. In natural environments incongruent solubility is probably more prevalent, e.g., in weathering of many clays, than congruent dissolution. 

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,... 

In very acidic solutions (pH < 2.4-3) with ionic strengths below 0.1 M and at 25 °C and 1 bar pressure, scorodite has a pK of about 25.83 0.07. The pK of amorphous Fe(III) arsenate is approximately 23.0 0.3 under the same conditions (Langmuir, Mahoney and Rowson, 2006). At higher pH values, scorodite dissolves incongruently, which means that at least one of its dissolution products precipitates as a solid. The incongruent dissolution of scorodite in water leads to the formation of Fe(III) (oxy)(hydr)oxide precipitates that is, Le(III) (hydrous) oxides, (hydrous) hydroxides and (hydrous) oxyhydroxides (Chapter 3). During the formation and precipitation of the iron(III) (oxy)(hydr)oxides, As(V) probably coprecipitates with them (Chapter 3 also see Section 2.7.6.3). The dissolution rate of scorodite at 22 °C in pH 2-6 water is slow, around 10—9 —10—10 mol m-2 s-1, which explains its presence in many mining wastes (Harvey et al., 2006). 

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