Fluorite structure deserves a closer look.

The fluorite structure refers to a common motif for compounds with the formula MX2.

The fluorite structure refers to a common motif for compounds with the formula MX2. The X ions occupy the eight tetrahedral interstitial sites whereas M ions occupy the regular sites of a face-centered cubic (FCC) structure. Many compounds, notably the common mineral fluorite (CaF2), adopt this structure.