Final answer:
Mineral structures that combine in the same proportions but in different structural arrangements are known as polymorphs. These differ in atomic arrangement and crystal structure, distinguishing them from allotropes, which are different forms of the same element.
Step-by-step explanation:
When elements or compounds combine in the same proportions but in more than one structural arrangement relative to each other, those mineral structures are called polymorphs. Polymorphs are two or more minerals that have the same chemical composition but differ in their internal atomic arrangement and crystal structure. An example of polymorphs is carbon, which can exist as graphite or as diamond; each has a different crystal structure but the same chemical formula. On the other hand, allotropes are two or more forms of the same element in the same physical state that display different chemical structures. The terms polymorph and allotrope both describe instances of a substance that occurs in different forms; however, allotropes pertain specifically to different forms of the same element, whereas polymorphs can include compounds.