Miscibility of Natural Melts Ghiorso-Carmichael Model

7 Miscibility of Natural Melts Ghiorso-Carmichael Model 

The existence of miscibility gaps in silicate melts has been experimentally known for decades (Greig, 1927 Bowen, 1928 Bowen and Schairer, 1938 Schairer and Bowen, 1938 Roedder, 1951,1956 Holgate, 1954) and has been directly observed both in terrestrial rocks (Anderson and Gottfried, 1971 Ferguson and Currie, 1971 De, 1974 Gelinas et al., 1976 Philpotts, 1979 Coltorti et al., 1987) and in lunar basalts (Roedder and Wieblen, 1971). 

Immiscibility phenomena in silicate melts imply positive deviations from ideality in the mixing process. Ghiorso et al. (1983) developed a mixing model applicable to natural magmas adopting the components listed in table 6.12. Because all components have the same standard state (i.e., pure melt component at the T and P of interest) and the interaction parameters used do not vary with T, we are dealing with a regular mixture of the Zeroth principle (cf sections 2.1 and 3.8.4)  

In applying the model above (which is an extension of the previous version of Ghiorso and Carmichael, 1980, integrated with new experimental observations), some caution must be adopted, because of the following considerations  

If the heat capacity of a chemically complex melt can be obtained by a linear summation of the specific heat of the dissolved oxide constituents at all T (i.e., Stebbins-Carmichael model), the melt is by definition ideal. The addition of excess Gibbs free energy terms thus implies that the Stebbins-Carmichael model calculates only the ideal contribution to the Gibbs free energy of mixing. 

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