Final answer:
Polymorphism is the term used to describe the phenomenon where some minerals share the same chemical composition but exhibit different crystal structures.
Step-by-step explanation:
The phenomenon where some minerals have the same chemical composition but different crystal structures is called polymorphism. An example of polymorphism is when the mineral aragonite (CaCO3) sometimes changes to calcite, which is a different crystalline form of the same chemical composition. Polymorphism can occur in various materials, including minerals, metals, and polymers. In contrast, isomerism is a term used in chemistry to describe molecules that have the same number and type of atoms but are arranged differently. This is not to be confused with polymorphism, which occurs in solid materials.