Solutions congruently solid

We start with compounds "A" and "B", one on each end of the pheise diagram, i.e.- 100% A or 100% B. There are two regions in between these limits where solid solutions of A B occur. More Importantly, a region exists where the composition freezes and melts congruently (that is- the melt does not decompose. 

Calcareous minerals and evaporite minerals (halides, gypsum) are very soluble and dissolve rapidly and, in general, congruently (i.e., yielding upon dissolution the same stoichiometric proportions in the solution as the proportions in the dissolving mineral and without forming new solid phases). Their contribution to the total dissolved load in rivers can be estimated by considering the mean composition of river... 

The term congruent dissolution often refers to a process by which a mineral is dissolved in stoichiometric proportions into solution without formation of a new solid phase, and this usage is convenient for isotopic studies because it constrains the phases or components that may... 

Considerable effort has been expended in growing single crystals of compositions in the solid solution, with particular emphasis on the superconducting composition. A variety of methods have been employed, all flux type, as the solid solution members are not congruently melting. There have been a few detailed reports on growth of single crystals from melts in the... 

Using the data in Example Problem 2.1 and assuming that a = 668 J, determine the composition of a mixture of Cs and Rb that will melt congruently at 282.7 K—that is, will go from a solid solution to a liquid solution without changing composition (going through phase separation). 

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. 

Fig 2 and 3 show the minimum and maximum melting points, respectively. The solid solution which corresponds to the minimum or maximum melting point behaves much like a pure component. It melts and freezes undergoing no changes in composition. In other words it melts congruently. 

Now consider the dissolution of mineral Cao.85 Mgo.15 CO3 in pure water with one atmosphere CO2. If this mineral acted like a solid solution phase, it would dissolve congruently along reaction path AA until it reached at A saturation with a calcium-rich magnesian calcite less soluble than itself. We will see later in this... 

Solubilities of annealed skeletal carbonates were estimated by Land (1967) by allowing the system to reach a steady state pH and by determining dissolved Ca2+ and Mg2+ concentrations. Dissolution of the magnesian calcites in Land s experiments appeared to be congruent in the sense that the steady state solution had a molar Ca2+/Mg2+ ratio similar to that of the solid. 

Thorstenson and Plummer (1977), in an elegant theoretical discussion (see section on The Fundamental Problems), discussed the equilibrium criteria applicable to a system composed of a two-component solid that is a member of a binary solid solution and an aqueous phase, depending on whether the solid reacts with fixed or variable composition. Because of kinetic restrictions, a solid may react with a fixed composition, even though it is a member of a continuous solid solution. Thorstenson and Plummer refer to equilibrium between such a solid and an aqueous phase as stoichiometric saturation. Because the solid reacts with fixed composition (reacts congruently), the chemical potentials of individual components cannot be equated between phases the solid reacts thermodynamically as a one-component phase. The variance of the system is reduced from two to one and, according to Thorstenson and Plummer, the only equilibrium constraint is IAP g. calcite = Keq(x>- where Keq(x) is the equilibrium constant for the solid, a function of... 

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... 

This equilibrium expression represents congruent dissolution of Na-feldspar but the solution quickly becomes saturated with respect to the solid phase Al(OH)3 (gibbsite K p = 10 ) and so the secondary mineral, gibbsite, is formed. 

In the preceding cases the solidus and liquidus curves meet tangentially to an isothermal line at a congruent point the solution freezes at this temperature without any change in composition. Au—Ni alloys exhibit the behavior depicted in Fig. 3.15.2. Inasmuch as AHC > 0 for this case, the solid solution for T < Tc is less stable than a mixture of phases this is indicated by the dotted curve at the bottom of the diagram. 

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