Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Mineralogists are scientists who study minerals. They divide minerals into groups based on chemical composition. Even though there are over 4,000 minerals, most minerals fit into one of eight mineral groups. Minerals with similar crystal structures are grouped together. Answer: minerals
Similarity of crystal structures is used to categorize what non-living materials?