Calcium fluoride crystal, molecular model. Calcium ions are purple, fluoride ions are gold. The unit cell of this crystal is outlined with black lines. A calcium ion sits at each vertex of a cube, with another at the centre of each of the cube's faces. This is an arrangement known as a face centred cubic (FCC) crystal. Each calcium ion is linked by two fluoride ions. Calcium fluoride (CaF2) occurs naturally as fluorite, used as the industrial source of fluorine.