Crystal Chemistry Crystal Chemistry III Isomorphous Substitution Compositional variations in minerals - Purity of composition in a mineral is more the exception than the rule.
Most minerals display some variability in composition.
Compositional variation in minerals is a result of the substitution of one ion or ionic compound for another ion or ionic compound. Conditions for solid-solution Similarity of ion size. The charges of ions involved must be the same. If not, then there must be a compensating substitution in another site so as to maintain electric neutrality. In general, the higher the temperature during crystallization, the greater the chance for substitution.