Fluorite has a very simple structure. Calcium (green) atoms in a face-centered pattern contain a cube of fluorine atoms (purple). Darker shades are used to portray calcium atoms toward the rear of the unit cell. We can also view the structure as a simple cubic array of fluorine atoms with a calcium atom in the center of alternate cubes. Considered that way, there are obviously diagonal planes of cubes containing no cations. These planes will obviously be planes of weakness, accounting for fluorite's excellent octahedral cleavage.