Solutions, congruent incongruent

The isothermal evaporation of a sat. soln. of two salts with a common ion leads to the separation of both salts, but with sat. soln. of three salts, this may or may not occur. The isothermal evaporation of solution II furnishes as solid phases the same three salts as are present as solutes. These soln. were called by W. Meyer-hofer congruent solutions, because, on isothermal evaporation, they furnish as solid phases the same salts as are present as solutes in soln. On the other hand, with solution I the salts which separate as solid phases are different from those present in soln., for sodium chloride appears in place of ammonium hydrocarbonate. In contrast with congruent soln., W. Meyerhofer styled them incongrucnt solutions here the sat. soln. contains a different salt from those which are present as solid phases. The gradual addition of sodium chloride to solution I changes the composition of the soln. until the incongruent solution I passes into II ammonium hydrocarbonate does not change when added in a similar manner to solution II. 

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... 

We also note that at the moment deposition of AI began, congruent solution of the original rocks would have been replaced by incongruent selective solution, manifested more completely in submarine conditions. As the acid ocean was neutralized, the residual products were enriched first in Si02, then in Al, and in neutral environments Fe and part of Mg were added. 

Nesbitt H. W., Macrae N. D., and Shotyk W. (1991) Congruent and incongruent dissolution of labradorite in dilute acidic salt solutions. J. Geol. 99, 429-442. 

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... 

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. 

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. 

A congruently saturated solution is one from which the solid phases are continuously deposited during isothermal evaporation to dryness, whereas in the case of incongruently saturated solutions, at least one of the solid phases disappears during the process of evaporation. 

Pure hydrates are crystalline ice-like solids usually prepared from water or ice and an appropriate guest species. Hydrates of water-soluble guests have well-defined phase diagrams characteristic of two-phase systems with compound formation."." If the hydrate melts congruently, it is simply a matter of freezing a solution of the correct composition to obtain the hydrate. For systems that melt incongruently, the aqueous solution must be quenched below the peritectic temperature and conditioned so that the crystalline hydrate can form on... 

The solute and solvent of a binary system may, and frequently do, combine to form one or more different compounds. In aqueous solutions these compounds are called hydrates for non-aqueous systems the term solvate is sometimes used. Two types of compound can be considered one can coexist in stable equilibrium with a liquid of the same composition, and the other cannot behave in this manner. In the former case the compound is said to have a congruent melting point in the latter, to have an incongruent melting point. 

Figure 4.32. Jdnecke projections for aqueous solutions of a reciprocal salt pair, showing a) two congruent points, fb) congruent and incongruent points...

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