What is a mineral type defined by the combination of chemical composition and crystal structure called?

• A. A species
• B. A variety
• C. An element
• D. A compound

Answer: (A)

In mineral naming, a mineral species is defined by an exact chemical composition and a specific crystal structure. This means that both what the mineral is made of and how its atoms are arranged in space together determine its identity. If you change either the composition or the crystal arrangement, you end up with a different mineral species. For example, SiO2 forms quartz, cristobalite, and tridymite, which are distinct species due to their different crystal structures, even though their chemical composition is the same. Variations within the same species that don’t alter the crystal structure—such as color differences caused by trace elements—are called varieties, like amethyst being a purple variety of quartz. So the term that best fits a mineral type defined by the combination of chemical composition and crystal structure is species.