The use of thermodynamic excess functions in the Nernst distribution law: discussion

Holst (1978) has used thermodynamic theory to derive a Nernst distribution equation in which nonideal behavior of coexisting phases is represented by excess functions rather than activity coefficients. This equation was employed in a thermodynamic and geothermometric evaluation of30 kbar orthoenstatite-diopside solvus data to illustrate a practical application of the theory. However, Holst did not include crystal structure as a factor in his thermodynamic analysis. This paper presents thermodynamic theory which illustrates some of the complexities and constraints in applications of the Nernst distribution law to crystalline solutions. It is concluded that a strict accounting for the effects of crystal structure is required for a rigorous thermodynamic assessment of equilibria between coexisting, mutually-soluble crystalline solutions with di-fferent structures.