What is the smallest group of atoms with the characteristic chemical composition and the basic crystal structure of a mineral?

• A. The largest repeating pattern in a crystal.
• B. The interface between crystal faces.
• C. The smallest group of atoms with the characteristic chemical composition and the basic crystal structure of a mineral.
• D. A single molecule in a crystal.

Answer: (C)

In crystallography, the smallest group of atoms that defines both the mineral’s chemical composition and its basic crystal structure is the unit cell. This tiny, repeating building block contains the arrangement and types of atoms or ions in a defined geometry. When the unit cell is repeated in three directions, it constructs the entire crystal lattice, preserving the overall composition and structure.

This concept matters because the unit cell encapsulates how atoms are bonded and arranged, which determines the mineral’s properties and symmetry. The other ideas don’t fit as the fundamental building block: the largest repeating pattern isn’t the smallest unit that builds the crystal, the interface between crystal faces relates to morphology rather than internal structure, and crystals are typically extended networks rather than a single discrete molecule.