Fluorite grid

In fluorite crystallized, for example, calcium fluoride ( Ca F ₂ ). The calcium ions form a face-centered cubic lattice in which the fluoride ions lie in the 8 tetrahedral gaps . One fluoride ion is surrounded by 4 calcium ions and one calcium ion is surrounded by 8 fluoride ions. Alternatively, the fluorite lattice can be described as a cubic primitive packing of fluoride ions, in which half of the cubes formed by fluoride are occupied by calcium ions in such a way that each occupied cube is surrounded by unoccupied ones (as a three-dimensional analogue of chessboards).