Constraints may be placed on the functional dependence of the partitioning of ore and other trace, minor, and major elements in melt (liquid)-vapor-crystal systems by relating empirical Nernst-type partition coefficients to true mass action expressions and equilibrium constants. This process is herein termed stoichiometric analysis. The resulting expressions can be used to determine the functional dependence of partition coefficients on the bulk composition of the phases in question.

In this chapter, stoichiometric constraints are used to place limits on the partitioning of europium between melt and vapor, to determine the functional dependence of the HCl concentration in the magmatic aqueous phase on the basis of the composition of the melt, and to suggest controls on the iron concentration in the magmatic aqueous phase. Based on this analysis and data available in the literature, it can be shown that the Nernst partition coefficient for europium is probably a function of the square of the chloride concentration in the magmatic aqueous phase; at the very least, it is limited to a polynomial dependence on the chloride concentration raised to the second and third power. The HCl concentration in the aqueous phase is shown to increase with the peraluminosity and chlorine concentration in the melt phase, and the aqueous iron concentration is shown to increase with increasing activity of HCl and decreasing f _O2 for a given activity of magnetite component in a magmatic system.