Incongruent solubility

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. 

P. P. Fedotieff showed that the change of Px with temp, can be represented by the movement of the point H and this in turn depends on the variation of the solubilities of ammonium hydrocarbonate and chloride. When the temp, approaches 32°, Px lies on the ammonium chloride axis, and the incongruent sat. soln. becomes congruent because the same salts now appear in the solid phase as are present in soln.—viz., sodium and ammonium hydrocarbonates and ammonium chloride. 

Although aqueous hydrolytic equilibria such as equation (1) provide an overall framework for the discussion of isopolyanions, several species have been synthesized in nonaqueous media, and are unstable in water. Other isopolyanions that are crystallized from aqueous solution, may be incongruently soluble and have polymeric structures. It will, however, prove to be convenient to treat all isopolyanions as though they were derived by equation (1), and characterize each by its acidity , Z (=p/q). Note that Z for a specific anion is not necessarily equal to the value of p/q for the solution from which the anion was crystallized.10... 

Some elements in aquatic systems exist only at low concentrations (pg/L range) in spite of readily soluble minerals. This phenomenon is not always caused by a generally small distribution of the concerned element in the earth crust mineral as for instance with uranium. Possible limiting factors are the formation of new minerals, co-precipitation, incongruent solutions, and the formation of solid-solution minerals (i.e. mixed minerals). 

Fig. 50 Calcite solubility and incongruent solution calcite-dolomite (calcite -precipitation and dolomite dissolution)...

Reactions M-O represent incongruent dissolution of Ca(OH)2s because the solubility of CaCOjS is much smaller than the solubility of Ca(OH)2s (Table 2.7). Therefore, introduction of Ca(OH)2s to water in equilibrium with atmospheric C02 leads to spontaneous formation of CaC03s. The well-known incongruent dissolution phenomena are those representing the dissolution of aluminosilicate minerals. For example, K-feldspars (orthoclase) undergo incongruent dissolution when exposed to water and carbonic acid to form kaolinite ... 

Initially in this study, it was planned to critically evaluate AG data for complex clays, including chlorite, illite, and the smectites. However, there is much evidence that these clays dissolve Incongruently so that the apparent equilibria in solution are determined by secondary phases, such as gibbsite, boehmite, amorphous silica, and ferric oxyhydroxldes. The smectites are frequently the dominant clays in the colloidal size fraction in natural sediments. They have very large exchange capacities, and exhibit wide chemical variations. Usually, one or more of these factors have not been considered in the experimental solubility work. Even if appropriate corrections could be made, it is uncertain whether a AG value so obtained would have applicability to natural systems. 

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. 

Numerous potentiometric titration-based chemical models have been proposed, a small number of which has been verified by H, and Al NMR and electrospray mass spectroscopic techniques. The models range in character relative to the number and nature of species considered, from simple (a small number of mononuclear monocitrate species) to inordinately complex (several mono- to trinuclear and mono- to tricitrate species). The models are incongruous, in that no two models are composed of the same species, nor do they predict similar Fe(III)-or Al-citrate aqueous speciation. Similarly, the models differ relative to the predicted impact of citrate on the solubility of Fe(III)- or Al-bearing accessory minerals. 

In addition to ion exchange with rock surfaces, alkali can react directly with specific rock minerals. When divalents, Ca and Mg ", exist, alkali will react with them and precipitation can occur. One example is the incongruent dissolution of anhydrite or gypsum in the rock to produce the less soluble calcium hydroxide (CaS04(s) -F NaOH Ca(OH)2(s) + Na2S04). Another simple example is Ca -F COs " CaC03(s). Alkali can also dissolve other minerals from a rock, for example, silica. These reactions could cause plugging. 

In the case of salts which can form crystalline hydrates which melt incongruently, the solubility curve must exhibit a break at the incon-gruent melting-point where the solid undergoes a change (p. II3). 

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