Internal directions parallel to a mineral's unit cell surfaces are called?

Internal directions parallel to a mineral's unit cell surfaces are called?

• A. Crystal Shape
• B. Glide Plane
• C. Doubly Refractive
• D. Crystal Planes

Answer: (D)

Crystal planes are the flat, repeating layers that extend through a crystal and lie parallel to the faces of the unit cell. When directions run within or parallel to those faces, they are describing the orientation of these lattice planes. That's why the term that fits is crystal planes. The other options describe external shape, a symmetry operation, or an optical property, none of which capture the idea of directions aligned with the unit cell faces.

Crystal planes are the flat, repeating layers that extend through a crystal and lie parallel to the faces of the unit cell. When directions run within or parallel to those faces, they are describing the orientation of these lattice planes. That's why the term that fits is crystal planes. The other options describe external shape, a symmetry operation, or an optical property, none of which capture the idea of directions aligned with the unit cell faces.