Aqueous-solid solution (Aq-SS) systems are ubiquitous both in natural and anthropogenic environments. In most nuclear waste repositories, solid solutions are intrinsic constituents of the various components of the repository system: from the waste source to the geosphere/biosphere interface.

The complexity and richness of natural minerals and multicomponent ceramics (like UO2 spent nuclear fuel) arise not only from the large number of phases and crystal structures, but also from the variety of solid solutions, complex elemental substitutions, order-disorder relations, and exsolution phenomena one encounters. Indeed, a pure mineral of end-member composition is the exception rather than the rule. Elemental substitutions in minerals may be classified in terms of the crystallographic sites on which they occur and the formal charges of the chemical elements (ions) which participate.