Incongruent release

Table 3 Incongruent release of isotopes from strongly mechanically weathered continental rocks ...

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. 

Smdies of FAP and CFA dissolution indicate initial incongruence, with preferential release of Ca over P and F dissolution then becomes congruent as equilibrium is approached (Jahnke, 1984 Tribble et al., 1995 Guidry and Mackenzie, 2003). Simultaneous increases in solution pH indicate the formation of a hydrogen-rich, cation-depleted surface layer during dissolution (Guidry and Mackenzie, 2003). Rates of dissolution of FAP and CFA have been... 

Silicate mineral dissolution is usually incongruent, with precipitation of relatively amorphous metastable products that may crystallize with time to form minerals such as gibbsite, kaolinite, illite, and montmorillonite (Helgeson et al. 1984). The incongruency means that the net release rates of individual components from a silicate mineral into the water may not be equal (cf. White and Claassen 1979 Helgeson et al. 1984). 

More common minerals in such a setting are jarosite and alunite, which often dissolve incongruently to form their oxyhydroxides, releasing protons. The jarosite reaction produces significant acidity, whereas alunite is a very weak acid. For the reaction... 

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. 

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. 

Steps 1, 2, and 3 account for the initially high rate of dissolution of alkali and alkaline earth metals (Na", K, Ca ) from feldspars, and the relative lack of silica and aluminum dissolution. This is termed incongruent (nonstoichiometric) dissolution, meaning that a portion of the mineral structure is dissolving selectively, leaving a residue enriched in silica and alumina. Step 4 accounts for the later stage of congruent (stoichiometric) dissolution, in which the elements are released into solution in proportion to their mole fractions in the structure. 

The glass network modifiers (Na, Ca, Mg) show incongruent dissolution in acidic and neutral solutions (RX>1) and stoichiometric release in alkaline solutions (Rx= 1). 

These incongruent dissolutions result in the synthesis of clay minerals, release of monomeric silica and cations, and formation of bicarbonate. 

The authors favouring the hypothesis of the incongruent dissolntion are of the opinion that only the calciinn ions are released into solution and the sohd transforms into the sihcate skeleton with low calciinn content [2, 4, 5]. This prodnct is absorbing calcium ions from the solution and the C-S-H is produced, as a result of topochemical reaction. The proof of this mechanism was based by these authors on the measured positive values of potential [4]. 

The importance of chemical weathering in the geochemistry of the ice-free valleys of southern Victoria Land is also indicated by the presence of bicarbonate ions and of molecular silicic acid (H SiO ) in the streams and lakes of this area. These constituents are released into solution in surface water by the incongru-ent dissolution of alumino-silicate minerals such as microcline ... 

Minerals usually dissolve incongruently during the initial stages of the dissolution process. In this case, Ca is released from wollastonite faster than Si. In addition, the rate of both Ca and Si declines with increasing extent of reaction. Eventually the difference between release rates, as well as the change in the rates, from one sample time to the next becomes smaller than the resolution of the rate measurements. When both of these conditions exist the rates are assumed to represent the dissolution rate that is appropriate for long-term geochemical processes. 

Apprehension with respect to possible competitive disadvantages or of revelation of state secrets may speak in favor of restraint in public reports of arguments or in the release of available data and facts. Thus, under certain circumstances, there are bound to be incongruous demarcations, i.e., misevaluations, unless it is possible to change the procedure by... 

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