By analogy with aqueous solution, a solid solution, sometimes called mixed crystal, is defined as an homogeneous solid-state solution of one or more solutes in a solvent. Its composition may vary in a more or less wide range of compositions, between extreme values which correspond to pure solids, known as its end-members. At variance with a mechanical mixture in which the components are mixed but not chemically bonded, mixing in a solid solution occurs at the atomic level, generally by substitution of one type of atoms by another on the same lattice. Many metal alloys, oxides, and silicates are substitutional solid solutions, but solid solutions may also be formed by insertion of foreign atoms at interstitial sites in the host crystal (interstitial solid solutions) or by loss of atoms in nonstoichiometric compounds (omissional solid solutions). The importance of solid solutions comes from the synergistic properties that they may display compared to their end-members and to their ubiquity in the natural environment. In some cases, an homogeneous SS may become unstable with respect to its components, and phase separation, also called demixing or exsolution, may occur. More precisely, exsolution is the processes of separation of an initially homogeneous solution phase (liquid or solid) into two compositionally different immiscible phases. This process may be triggered by pressure, temperature, or composition changes but mostly by the two former for solid phases which show little volume change upon pressure.