Estimating Margules Parameters Symmetrical Solvi

Recalling that H = U + PV and that Wh = VVu + PWv, we can also write the approximation Wu Wh because the PV term for solids is generally negligible at low pressure (1 bar) relative to U and H. Note that this requires that our solvus, hence Wg and Wh were all measured initially at a sufficiently low P, such as 1 bar. We now have estimates for Wq, Ws, Wh, and Wu, based on the variation of solvus composition with temperature. If we also need to know the properties of the system as a function of pressure, several options are available. With luck, there might be data for the solvus at different pressures. We could then find Wq as a function of P, and by analogy with (15.47), calculate Wy  

Alternatively, we could estimate l/ (= Vreai Vueai) from X-ray data on the single-mineral solid solution just above the solvus (more on this below). Knowing 

The above procedure required that we have a symmetrical solvus to work with. We have already observed that the feldspars, as most minerals, show asymmetrical behavior and we need to extend the same methods to this more general case. This follows exactly the same reasoning as the above section, but the equations turn out to be much less convenient to use. 

This applies to all steps such as (15.48), (15.49) and so on, which are used to calculate one Margules parameter from another. Finally, a general equation of state such as (15.50) could be derived to describe the mineral system, using the same assumptions and with the same warnings about accuracy. 

Calculating Solvi, Spinodal Curves, and Consolute Points 

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