On the End-member Composition of Solid Solutions

Abstract

The term end-member is here defined as a component that can be independently added to or subtracted from a solid soln. An end-member cannot be expressed by the sum of other end-members of the solid soln., and in this respect the definition used here differs from that given by perry (1967). In clinopyroxene and calciferous amphiboles that are used as examples, a solid soln. cannot generally be expressed by the sum of the independent end-members, which obey structural constraint. In such a system, the mole fraction of an end-member is not uniquely detd., and hence the mole fraction cannot be equal to the activity. Even the most idealized pyroxene and calciferous amphibole solid solns. are not ideal. The bearing of the non-ideality in phase equil. consideration of petrological systems, such as the max. soly. under given phys. conditions and the partition of cations between coexisting phases, is discussed in some detail.