Back to Research Scroll over the image at right to see the crystal structure of apatite (viewed down the [0001] crystallographic axis) in two different formats. The structure consists of (PO4)3+ groups (pink tetrahedra), monovalent anions like F- and (OH)- (purple spheres) in the "tunnels" parallel to [0001], and Ca2+ cations in two coordinations: seven- and nine-fold. The CaO6(F,OH) and CaO9 polyhedra are colored yellow and green, respectively. A large number of natural and an even greater variety of synthetic compounds adopt this type of structure, or derivative structures arising from ordering of large cations in the Ca sites, small cations in the P sites, or anions in the "tunnels".