Multiple miscibility gaps

The existence of multiple miscibility gaps should be detected by following the procedure outlined by Heidemann and Mandhane. This includes seeking an alternate set of NRTL parameters that do not predict multiple sets of mutual solubilities. 

Consideration of the actual extension of miscibility gaps in natural systems led Will and Powell (1992) to establish systematic relationships between the actual free energy of pure components and their Active potentials in the phases of interest, as listed in table 5.51. Note that, amphiboles being multisite phases, their ideal activity in a chemically complex phase is expressed in terms of multiple product of site ionic fractions (see section 3.8.7). For anthophyllite ( Mg2Mg3Mg2Si4Si4022(OH)2), for instance, we have... 

Calculation of the excess Gibbs free energy of mixing (third term on right side of eq. 6.78) involves only binary interactions. Although there is no multiple interaction model that can be reduced to the simple summation of binary interactions used here (cf Acree, 1984 see also section 3.10), this choice is more than adequate for the state of the art, which does not allow precise location of the miscibility gap in the chemical space of interest. 

As the core is likely to contain more than one light element (e.g., Stevenson, 1981), initial efforts have been undertaken to investigate the compatibility of multiple light elements in a liquid. Wang et al. (1991) found a vast miscibility gap in the Fe-S-C ternary system at 1 bar, which shrinks with increasing temperature (Figure 9). A similar miscibility gap was observed in Fe-S-Si system at 1 bar, which widens as temperature increases (Raghavan, 1988). The effect of pressure on miscibility has not been studied. 

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