Incongruent weathering

At lower temperatures, as discussed in Chapter 5.16, congruent and incongruent weathering reactions and neoformation of clays and zeolite minerals, which are common secondary mineral phases in crystalline rocks, are postulated to alter fluid and isotopic chemistry (Fritz and Frape, 1982) and increase salinity during hydration reactions (Bucher and Stober, 2000). As an example, reaction (5) represents the formation of zeolites. These well-documented fracture-filling minerals have been found by several authors at Fennoscandian sites, and at research sites in Europe (Bucher and Stober, 2000) ... 

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. 

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,... 

Chemical weathering of minerals results not only in the introduction of solutes to the aqueous phase but often in the formation of new solid phases. Dissolution is described as congruent, where aqueous phase solutes are the only products, or incongruent, where new solid phase(s) in addition to aqueous phase solutes are the products. These reactions... 

Incongruent mass transfer in the case of parabolic kinetics based on diffusion control can be readily explained by differences in the diffusion coefficients of chemical species. However, the proposed mechanisms invoked to explain linear kinetics involve the dissolution of the bulk silicate phase, which, over extended periods, should result in the chemical components being transferred to solution from the silicate in their stoichiometric ratios (25). However, many field studies have shown that silicates weather incongruently in nature (27). This is supported by our studies of ground waters associated with vitric rhyolite tuffs. 

The formula of the solid product (Al2Si205(0H)4) is that for kaolinite, an important member of the serpentine-kaolin group of clay minerals (Section 4.5.1). This reaction demonstrates incongruent dissolution of feldspar, i.e. dissolution with in situ reprecipitation of some compounds from the weathered mineral. 

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. 

Note that because olivine contains no Al, no clays result from reactions (7.1) or (7.2). Also, reaction (7.1) is congruent, whereas reactions (7.2) and (7.3) are incongruent. In other words, in the first reaction all the weathering products are soluble, whereas in the second and third reactions, weathering results in precipitation of new solid phases. Reaction (7.2) involves oxidation of Fe(II) to Fe(lII), with precipitation of the ferric iron in a mineral such as goethite. Among the three examples, only the feldspar contains Al and so can result in a clay. In reaction (7,3) we assume that all the Al moves directly from the feldspar to the kaolinite. However, if the pH is below about 4 or 5, the kaolinite itself becomes soluble and appreciable Al also goes into solution. 

Recent studies have shown the formation of altered surface layers thicker than one or two unit cell layers on feldspar surfaces, in apparent support of the leached layer theory. Thick (> 100 nm), silica-rich surface layers were detected by XPS on feldspar samples weathered in solutions having pH < 3 (Casey et at, 1988b) or >9 (Heilman et al., 1990). At these extreme pH values, the rate of release of Al and charge-balancing cations to solution is much faster than the rate of hydrolysis of silica. Under these conditions, oversaturation with respect to amorphous silica could occur, and a highly hydrated, residual leached or precipitated layer of silica could form. However, this layer is probably too porous and discontinuous to be a diffusion-limiting mechanism, but would still account for incongruence under these conditions (Hellmann et al., 1990). Whether the altered layer formed by a leached layer process or by simple precipitation from oversaturated solutions was not determined. 

Layer silicate clay minerals typically dissolve incongruently, with the octahedral sheet being more susceptible to hydrolysis and decomposition by acid attack than the tetrahedral (silica) sheet. Consequently, base cations from exchange sites and from the octahedral sheet initially dissolve out of these minerals at much faster rates than silica. This means that the residue of weathering is typically a siliceous material depleted in Ca, Mg, K, and Na. 

An incongruent dissolution of a silicate mineral reacting with H2CO3. A general weathering reaction for an alumino silicate may be represented schematically as follows (Stumm and Morgan, 1981) ... 

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