Mixtures subregular

Figure 3.9 Conformation of Gibbs free energy curve in various types of binary mixtures. (A) Ideal mixture of components A and B. Standard state adopted is that of pure component at T and P of interest. (B) Regular mixture with complete configurational disorder kJ/mole for 500 < r(K) < 1500. (C) Simple mixture IF = 10 - 0.01 X r(K) (kJ/ mole). (D) Subregular mixture Aq = 10 — 0.01 X T (kJ/mole) = 5 — 0.01 X F (kJ/ mole). Adopting corresponding Margules notation, an equivalent interaction is obtained with IFba = 15 - 0.02 X r(kJ/mole) Bab = 5 (kJ/mole).

The subregular model approximates several silicate mixtures with sufficient precision, as we will see in chapter 5. For a subregular mixture, we have... 

Figure 3.9D shows the form of the curve of the excess Gibbs free energy of mixing obtained with Van Laar parameters variable with T. the mixture is subregular— i.e., asymmetric over the binary compositional field. 

Figure 3,10 Solvus and spinodal decomposition fields in regular (B) and subregular (D) mixtures. Gibbs free energy of mixing curves are plotted at various T conditions in upper part of figure (A and C, respectively). The critical temperature of unmixing (or consolute temperature ) is the highest T at which unmixing takes place and, in a regular mixture (B), is reached at the point of symmetry.

If the mixture is subregular, definition of the limits of spinodal decomposition is more complex. For a subregular Margules model (figure 3. IOC and D), we have... 

The parameters of the thermodynamic model of Lindsley et al. (1981) are reported in table 5.38. The mixing properties described are those of a regular mixture for orthopyroxene and of a subregular mixture (asymmetric with Mar-... 

In pyroxenes, exsolutive processes proceed either by nucleation and growth or by spinodal decomposition (see sections 3.11, 3.12, and 3.13). Figure 5.30B shows the spinodal field calculated by Saxena (1983) for Cag sMgo sSiOj (diop-side) and MgSi03 (chnoenstatite) in a binary mixture, by application of the subregular Margules model of Lindsley et al. (1981) ... 

Table 5.60 Volumetric [Wy, J/(bar X mole)] enthalpic Wjj kJ/mole), and entropic [Ws J/(mole X K] terms of subregular Margules model for (Na,K)Al2Si3A10io(OH)2 binary mixture, according to various authors.

Newton et al. (1980) calorimetrically measured the enthalpy of the NaAl-Si30g-CaAl2Si208 mixture at P = 970 K and P = 1 bar and found a positive excess enthalpy reproduced by a subregular Margules model ... 

Saxena and Ribbe (1972) have shown that the excess Gibbs free energy of mixing of the mixture, based on the data of Orville (1972), may be reproduced by a subregular Margules model ... 

Adopting a subregular Margules model for the NaAlSi30g-KAlSi308 (Ab-Or) binary mixture and assuming that the activity coefficient of the albite component is not affected by the presence of limited amounts of the third component in the mixture (i.e., CaAljSijOg), equation 5.260 may be transformed into... 

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