Minerals crystallize from complex chemical solutions, so there is ample opportunity for the substitution of one ion for another. Practically all minerals display some variation in their composition. We call this process ionic substitution. There are several factors that determine how much variation will take place:
- The size of the ion. Ions of two elements can readily substitute for each other only if their sizes differ by
- The temperature at which the crystal is grown. The higher the temperature, the greater the amount of disorder, and the less stringent the space requirements of the crystal lattice for ionic substitution.
Usually, when the elements in a mineral vary, a series is formed. A series consists of a minerals in which one (or more) of the elements varies. Most mineral series form solid solutions. In a solid solution, there are intermediate members between the two end members; such is the case with olivine. The formula for olivine is written as (Mg, Fe)2SiO4 (If a chemical formula with two elements in parenthesis is separated by a comma, the number of those elements vary). This is the formula for common olivine.The end members (all Mg or all Fe) are forsterite: Mg2SiO4 and fayalite: Fe2SiO4. The intermediary member is chrysolite, which is a combination of the two (Mg,Fe)2SiO4.
Other examples of solid-solution series are the plagioclase feldspars, in which the end memers are albite Na(AlSi3O8) and anorthite, Ca(Al2Si2O8), and the garnets in which complete solid-solution relationships exist between most of the named varieties.