SOLID SOLUTION

solid analogue of liquid solutions. As with liquid phases, a tendency for mutual solubility exists between any two coexisting solid phases; depending on the chemical similarities of the solid phases, mutual solubility of two mineral species may be 100 percent, just as between water and alcohol, or it may be near zero, as between water and kerosene. The higher the temperature of equilibration, the greater the mutual solubility. A solid-solution series is the continuous sequence of substances with compositions intermediate between two distinct mineral phases, called end members. In a solid-solution series, the components may be thought to substitute for one another on a molecular level in the crystal structure; the intermediate members have properties that vary continuously with composition from those of one end member to those of another. Solid-solution series are said to be complete if examples of complete variation exist in nature or have been synthesized in the laboratory; partial solid-solution series exhibit a compositional range that is theoretically possible but that has not been observed. An example of a continuous solid-solution series is the forsterite-fayalite series of olivine minerals. In this series the members vary chemically from forsterite, a magnesium silicate (Mg2SiO4), to fayalite, an iron silicate (Fe2SiO4); the structure of the intermediate members is the same as that of the end members, and the physical properties show complete gradation from those of forsterite to those of fayalite.