In a rock salt crystal structure, like NaCl, both the cation and anion have a coordination number of 6.

Here's why:

* Structure: The rock salt structure is a face-centered cubic (FCC) lattice where each ion is surrounded by six ions of the opposite charge.

* Cation: The cation (e.g., Na+) sits at the center of a cube, with six anions (e.g., Cl-) located at the corners of the cube.

* Anion: The anion (e.g., Cl-) also sits at the center of a cube, with six cations (e.g., Na+) located at the corners of the cube.

Therefore, both the cation and anion in a rock salt structure have six nearest neighbors of the opposite charge, resulting in a coordination number of 6 for each.