An important aspect of many minerals is the ability to form solid solutions.
In solid solutions, the chemical composition varies, but fundamental properties such as crystal structure remain the same.
Most often, a solid solution is formed when cations of similar ionic radius are available to replace one another in a crystal lattice. For example:
The behavior of solid solutions is more complicated than homogenous minerals. For example, the two end-member compositions could have different melting points. Then, at a specific temperature, we would have a mixture of solid and liquid, each with a different composition.